Dietary sodium intake and the risk of airway hyperreactivity in a random adult population

The impact of high-salt diet on asthma in humans and mice: Effect on specific T-cell signatures and microbiome

BACKGROUND

The rise in asthma has been linked to different environmental and lifestyle factors including dietary habits. Whether dietary salt contributes to asthma incidence, remains controversial. We aimed to investigate the impact of higher salt intake on asthma incidence in humans and to evaluate underlying mechanisms using mouse models.

METHODS

Epidemiological research was conducted using the UK Biobank Resource. Data were obtained from 42,976 participants with a history of allergies. 24-h sodium excretion was estimated from spot urine, and its association with asthma incidence was assessed by Cox regression, adjusting for relevant covariates. For mechanistic studies, a mouse model of mite-induced allergic airway inflammation (AAI) fed with high-salt diet (HSD) or normal-salt chow was used to characterize disease development. The microbiome of lung and feces (as proxy for gut) was analyzed via 16S rRNA gene based metabarcoding approach.

RESULTS

In humans, urinary sodium excretion was directly associated with asthma incidence among females but not among males. HSD-fed female mice displayed an aggravated AAI characterized by increased levels of total IgE, a TH2-TH17-biased inflammatory cell infiltration accompanied by upregulation of osmosensitive stress genes. HSD induced distinct changes in serum short chain fatty acids and in both gut and lung microbiome, with a lower Bacteroidetes to Firmicutes ratio and decreased Lactobacillus relative abundance in the gut, and enriched members of Gammaproteobacteria in the lung.

CONCLUSIONS

High dietary salt consumption correlates with asthma incidence in female adults with a history of allergies. Female mice revealed HSD-induced T-cell lung profiles accompanied by alterations of gut and lung microbiome.

Food Pyramid for Subjects with Chronic Obstructive Pulmonary Diseases

Nutritional problems are an important part of rehabilitation for chronic obstructive pulmonary disease (COPD) patients. COPD patients often present with malnutrition, sarcopenia, and osteoporosis with possible onset of cachexia, with an inadequate dietary intake and a poor quality of life. Moreover, diet plays a pivotal role in patients with COPD through three mechanisms: regulation of carbon dioxide produced/oxygen consumed, inflammation, and oxidative stress. A narrative review based on 99 eligible studies was performed to evaluate current evidence regarding optimum diet therapy for the management of COPD, and then a food pyramid was built accordingly. The food pyramid proposal will serve to guide energy and dietary intake in order to prevent and treat nutritionally related COPD complications and to manage progression and COPD-related symptoms. The nutrition pyramid described in our narrative review is hypothetical, even in light of several limitations of the present review; the main limitation is the fact that to date there are no randomized controlled trials in the literature clearly showing that improved nutrition, via the regulation of carbon dioxide produced/oxygen consumed, inflammation and oxidative stress, improves symptoms and/or progression of COPD. Even if this nutritional pyramid is hypothetical, we hope that it can serve the valuable purpose of helping researchers focus on the often-ignored possible connections between body composition, nutrition, and COPD.

Epithelial Sodium and Chloride Channels and Asthma

Objective:

To focus on the asthmatic pathogenesis and clinical manifestations related to epithelial sodium channel (ENaC)/chlorine ion channel.

Data Sources:

The data analyzed in this review were the English articles from 1980 to 2015 from journal databases, primarily PubMed and Google Scholar. The terms used in the literature search were: (1) ENaCs; cystic fibrosis (CF) transmembrane conductance regulator (CFTR); asthma/asthmatic, (2) ENaC/sodium salt; CF; asthma/asthmatic, (3) CFTR/chlorine ion channels; asthma/asthmatic, (4) ENaC/sodium channel/scnn1a/scnn1b/scnn1g/scnn1d/amiloride-sensitive/amiloride-inhibtable sodium channels/sodium salt; asthma/asthmatic, lung/pulmonary/respiratory/tracheal/alveolar, and (5) CFTR; CF; asthma/asthmatic (ti).

Study Selection:

These studies included randomized controlled trials or studies covering asthma pathogenesis and clinical manifestations related to ENaC/chlorine ion channels within the last 25 years (from 1990 to 2015). The data involving chronic obstructive pulmonary disease and CF obtained from individual studies were also reviewed by the authors.

Results:

Airway surface liquid dehydration can cause airway inflammation and obstruction. ENaC and CFTR are closely related to the airway mucociliary clearance. Ion transporters may play a critical role in pathogenesis of asthmatic exacerbations.

Conclusions:

Ion channels have been the center of many studies aiming to understand asthmatic pathophysiological mechanisms or to identify therapeutic targets for better control of the disease.

Dietary sodium manipulation and asthma

BACKGROUND

There is a wide geographical variation in the prevalence of asthma and observational studies have suggested that dietary sodium may play a role.

OBJECTIVES

To assess the effect of dietary sodium manipulation on asthma control.

SEARCH STRATEGY

We carried out a search using the Cochrane Airways Group asthma register. We searched the bibliographies of included randomised controlled trials (RCTs) for additional studies. We carried out the most recent search in November 2010.

SELECTION CRITERIA

We considered only RCTs that involved dietary sodium reduction or increased sodium intake in patients with asthma.

DATA COLLECTION AND ANALYSIS

Both review authors assessed study and extracted data. We conducted data analyses in RevMan 5 using mean differences and random effects.

MAIN RESULTS

We identified a total of nine studies in relation to sodium manipulation and asthma, of which five were in people with asthma (318 participants), and four in people with exercise-induced asthma (63 participants). There were no significant benefits of salt restriction on the control of asthma. There was some evidence from the exercise-induced asthma studies that a low sodium diet may improve lung function after exercise and possibly baseline lung function, but this is based on findings from a very small numbers of participants.

AUTHORS' CONCLUSIONS

This review did not find any evidence that dietary sodium reduction significantly improves asthma control. Although dietary sodium reduction may result in improvements in lung function in exercise-induced asthma, the clinical significance of this effect is unclear.

Salty-snack eating, television or video-game viewing, and asthma symptoms among 10- to 12-year-old children: the PANACEA study

BACKGROUND

Salty-snack consumption, as well as the amount of time children spend watching television or playing video games, have been implicated in the development of asthma; however, results are still conflicting.

OBJECTIVE

The aim of this work was to evaluate the association of salty-snack eating and television/video-game viewing with childhood asthma symptoms.

DESIGN

Cross-sectional study.

SETTINGS

Seven hundred children (323 male), 10 to 12 years old, from 18 schools located in the greater area of Athens were enrolled. Children and their parents completed questionnaires, which evaluated, among other things, dietary habits. Adherence to the Mediterranean diet was evaluated using the KIDMED (Mediterranean Diet Quality Index for Children and Adolescents) score.

STATISTICAL ANALYSIS

The association of children's characteristics with asthma symptoms was performed by calculating the odds ratios and corresponding 95% confidence intervals.

RESULTS

Overall lifetime prevalence of asthma symptoms was 23.7% (27.6% boys, 20.4% girls; P=0.03). Forty-eight percent of children reported salty-snack consumption (≥ 1 times/week). Salty-snack consumption was positively associated with the hours of television/video-game viewing (P=0.04) and inversely with the KIDMED score (P=0.02). Consumption of salty snacks (>3 times/week vs never/rare) was associated with a 4.8-times higher likelihood of having asthma symptoms (95% confidence interval: 1.50 to 15.8), irrespective of potential confounders. The associations of salty-snack eating and asthma symptoms were more prominent in children who watched television or played video games >2 hours/day. In addition, adherence to the Mediterranean diet was inversely associated with the likelihood of asthma symptoms.

CONCLUSIONS

Unhealthy lifestyle behaviors, such as salty-snack eating and television/video-game viewing were strongly associated with the presence of asthma symptoms. Future interventions and public health messages should be focused on changing these behaviors from the early stages of life.

Salt intake, asthma, and exercise-induced bronchoconstriction: a review

Despite the heterogeneous treatment options for patients with asthma, there remains a substantial burden of unaddressed disease, even with optimal treatment. Epidemiological studies indicate that patients frequently resort to complimentary and alternative therapies when being treated for asthma and other chronic health conditions. Changes in diet associated with the development of a more affluent lifestyle is one of the environmental factors considered to contribute to the increased prevalence of asthma in the past few decades. Dietary sodium in particular has been considered to be a dietary constituent implicated in this phenomenon. This article reviews the studies conducted that have questioned whether reducing dietary salt intake potentially improves pulmonary function and airway hyper-responsiveness in asthmatics, as well as studies evaluating dietary salt intake on the severity of exercise-induced bronchoconstriction (EIB). The data presented supporting dietary salt restriction for reducing airway hyper-responsiveness in asthmatics is encouraging, though not clinically convincing. Studies conducted previously have been limited for a variety of reasons, including limitations related to the experiment and populations studied. However, in studies that evaluated the severity of EIB in asthmatic individuals and involved altered dietary salt intake, data have been more convincing. A low-sodium diet maintained for 1 to 2 weeks decreases bronchoconstriction in response to exercise in individuals with asthma. There are no data regarding the longer-term effects of a low-sodium diet on either the prevalence or severity of asthma or on EIB. As a low-sodium diet has other beneficial health effects, it can be considered a therapeutic option for adults with asthma, although it should be considered as an adjunctive intervention to supplement optimal pharmacotherapy, and not as an alternative.

A population-based epidemiologic study of Helicobacter pylori infection and its association with systemic inflammation

BACKGROUND

Infection with Helicobacter pylori is associated with a variety of non-gastrointestinal sequelae. These may be mediated by an increase in systemic inflammation. We assessed if serologic evidence of infection with H. pylori is associated with increased serum C-reactive protein (CRP) levels.

METHODS

The study design consisted of a randomly selected, cross-sectional population-based study of 2633 individuals phenotyped in 1991, of whom 2361 participants provided serum samples to permit measurement of H. pylori's serologic status and CRP levels.

RESULTS

Male gender (odds ratio (OR): 1.65; 95% confidence interval (CI): 1.23-2.21), age (OR per year: 1.05; 95% CI: 1.04-1.06), height (OR per meter: 0.05; 95% CI: 0.01-0.24), current smoking habit (compared with never smokers, OR: 1.46; 95% CI: 1.13-1.88), and less affluent socioeconomic status were associated with increased odds of being seropositive for H. pylori. Helicobacter pylori infection was associated with increased risk of having an elevated serum CRP (above 3 mg/L) after adjustment for gender, age, height, smoking status, and socioeconomic status (OR: 1.32; 95% CI: 1.05-1.67). Similar associations were seen using a threshold for elevated serum CRP of greater than 1 mg/L.

CONCLUSIONS

Our data suggest that infection with H. pylori is associated with increased systemic inflammation. This suggests one potential mechanism to explain the extra-gastrointestinal conditions associated with H. pylori infection.

Is higher sodium intake associated with elevated systemic inflammation? A population-based study

BACKGROUND

Observational epidemiologic studies have suggested that a low-sodium diet is associated with reduced mortality.

OBJECTIVE

The objective was to test the hypothesis that a higher dietary intake of sodium is associated with increased systemic inflammation--a potential risk factor for both cardiovascular disease and cancer.

DESIGN

The study design consisted of a randomly selected, cross-sectional, population-based study of 2633 individuals surveyed in 1991, of whom 1597 participants provided paired urinary and blood samples permitting measurement of 24-h urinary sodium excretion and serum C-reactive protein (CRP) concentrations.

RESULTS

The mean (+/-SD) 24-h sodium intake for the population was 177 +/- 69 mmol. In the basic model adjusted for age, sex, and smoking, higher levels of 24-h sodium excretion were directly associated with serum CRP, with an increase in serum CRP of 1.20 mg/L per 100-mmol increment in sodium excretion (95% CI: 1.11, 1.30). However, this association was reduced after adjustment for body mass index, with an increase in serum CRP of 1.06 mg/L per 100-mmol increment in sodium excretion (95% CI: -1.02, 1.15).

CONCLUSIONS

We observed a linear association between an objective measure of sodium intake and serum CRP that may be influenced by confounding by body mass index. The magnitude of these associations suggests that dietary sodium consumption is unlikely to be an important modifiable risk factor for increased systemic inflammation.

A multivariate analysis of serum nutrient levels and lung function

Background

There is mounting evidence that estimates of intakes of a range of dietary nutrients are related to both lung function level and rate of decline, but far less evidence on the relation between lung function and objective measures of serum levels of individual nutrients. The aim of this study was to conduct a comprehensive examination of the independent associations of a wide range of serum markers of nutritional status with lung function, measured as the one-second forced expiratory volume (FEV₁).

Methods

Using data from the Third National Health and Nutrition Examination Survey, a US population-based cross-sectional study, we investigated the relation between 21 serum markers of potentially relevant nutrients and FEV₁, with adjustment for potential confounding factors. Systematic approaches were used to guide the analysis.

Results

In a mutually adjusted model, higher serum levels of antioxidant vitamins (vitamin A, beta-cryptoxanthin, vitamin C, vitamin E), selenium, normalized calcium, chloride, and iron were independently associated with higher levels of FEV₁. Higher concentrations of potassium and sodium were associated with lower FEV₁.

Conclusion

Maintaining higher serum concentrations of dietary antioxidant vitamins and selenium is potentially beneficial to lung health. In addition other novel associations found in this study merit further investigation.

Wheeze and asthma in children: associations with body mass index, sports, television viewing, and diet

BACKGROUND

Obesity, physical activity, and dietary habits are distinct but strongly interrelated lifestyle factors that may be relevant to the prevalence of wheeze and asthma in children. Our goal was to analyze the relationship of body mass index (BMI), regular sports participation, TV viewing, and diet with current wheezing and asthma.

METHODS

We investigated 20,016 children, aged 6-7 years, who were enrolled in a population-based study. Parents completed standardized questionnaires. Logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs), while adjusting for several confounders and simultaneously considering BMI, regular sports activity, TV viewing and selected dietary items.

RESULTS

A total of 1575 children (7.9%) reported current wheezing and 1343 (6.7%) reported current asthma. In a multivariate model, an elevated BMI was associated with wheeze and current asthma: children from the highest quintile (compared with the lowest quintile) had an increased risk of wheeze (OR = 1.47; CI = 1.20-1.82) or current asthma (1.61; 1.28-2.01). Wheeze or asthma was not associated with regular sports activity. Subjects who spent 5 or more hours per day watching television were more likely to experience wheeze (1.53; 1.08-2.17) or current asthma (1.51; 1.04-2.2) compared with those who viewed TV less than 1 hour a day. Adding salt to food was strongly and independently associated with current wheeze (2.58; 1.41-4.71) and current asthma (2.68; 1.41-5.09).

CONCLUSIONS

Our data support the hypothesis that high body weight, spending a lot of time watching television, and a salty diet each independently increase the risk of asthma symptoms in children.

Does a low sodium diet improve asthma control? A randomized controlled trial

RATIONALE

Observational studies and initial randomized trials have indicated that a low sodium diet may improve asthma control.

OBJECTIVES

We tested the hypothesis that a low sodium diet would improve asthma control over a 6-week period.

METHODS

Participants with a physician diagnosis of asthma and measurable bronchial reactivity to methacholine entered a randomized double-blind placebo-controlled trial. All adopted a low sodium diet and were randomized to receive either 80 mmol/day of oral sodium supplements (normal sodium intake) or matched placebo (low sodium intake) for 6 weeks. The primary outcome was change in bronchial reactivity to methacholine; secondary outcomes were change in lung function, morning and evening peak expiratory flow, asthma symptoms score, daily bronchodilator use, Juniper Standardized Asthma Quality of Life Questionnaire score, and atopy.

MEASUREMENTS AND MAIN RESULTS

A total of 220 individuals entered the study, of whom 199 completed the protocol. In the low sodium-intake group, mean daily urinary sodium excretion decreased by 20 mmol (SD, 64 mmol) and in the normal-sodium-intake group increased by 28 mmol (SD, 74 mmol). There were no differences between the two groups in the primary or secondary outcome measures; the mean difference in bronchial reactivity between the low- and normal-intake groups was -0.03 doubling doses of methacholine (95% confidence interval, -0.60 to 0.53).

CONCLUSIONS

The use of a low sodium diet as an adjunctive therapy to normal treatment has no additional therapeutic benefit in adults with asthma and bronchial reactivity to methacholine.

A prospective study of weight change and systemic inflammation over 9 y

BACKGROUND

An increase in weight is a risk factor for cardiovascular disease and cancer. This increased risk may be mediated by inflammation, but no long-term data are available on the effect of weight gain on systemic inflammation.

OBJECTIVE

We tested the hypothesis that weight gain is associated with an increase in systemic inflammation during a 9-y period.

DESIGN

In 1991 data on body weight and a blood sample were collected from a random sample of 2425 randomly selected adults from a community-based cohort in Nottingham, United Kingdom. In 2000, these measures were repeated in 1301 of these participants. The main outcome measure was change in systemic inflammation as measured by serum C-reactive protein (CRP) from the 1222 participants who provided paired samples.

RESULTS

The mean change in weight from 1991 to 2000 was 2.9 kg (95% CI: 2.6, 3.2 kg). The geometric mean of CRP in 1991 was 1.22 mg/L (95% CI: 0.03, 125.0 mg/L), and it increased to 1.76 mg/L (95% CI: 0.09, 62.0 mg/L) in 2000 (P<0.001). A linear association was observed between increase in weight and serum CRP, with a 1-kg increment in weight being associated with an additional increase in CRP of 0.09 mg/L (95% CI: 0.02, 0.16 mg/L) during this time period.

CONCLUSION

During a 9-y period, an increase in weight is associated with an increase in systemic inflammation. This provides a mechanism that may explain some of the previously reported association of weight gain with an increased risk of both cancer and cardiovascular disease.

Sodium and asthma: something borrowed, something new?

Some early studies have called attention to the potential contribution of sodium (both dietary and serum levels) in airway-related disease, although the picture was not entirely clear. Two recent developments may now allow a more careful consideration of this: first, the greatly improved understanding of the role of salt in hypertension (particularly the identification of subgroups of salt-sensitive individuals within the general population), and second, the recent discovery of the role of the Na(+)/Ca(2+) exchanger in smooth muscle function. Here, we first review those two developments and then apply them to airway smooth muscle and asthma.

Early life events in asthma--diet

It has been hypothesized that the recent increase in the prevalence of asthma may, in part, be a consequence of changing diet. There is now increasing interest in the possibility that childhood asthma may be influenced by maternal diet during pregnancy and/or diet during early childhood. A number of observational studies and a childhood fish oil supplementation study provide little support for the notion that early childhood intake of polyunsaturated fatty acids (PUFAs) influence the development of childhood asthma. Recent work however, suggests that supplementation of maternal diet with fish oil is associated with altered neonatal immune responses to allergens. Further work is required to establish whether this immunological observation is translated into clinical outcomes. Two birth cohorts have now reported reduced maternal intake of vitamin E, zinc and vitamin D during pregnancy to be associated with increased asthma and wheezing outcomes in children up to the age of 5 years. Early life diet could modulate the likelihood of childhood asthma by affecting fetal airway development and/or influencing the initial early life interactions between allergens and the immune system. In animal models, vitamin E, zinc and vitamin D have been shown to modify fetal lung development and vitamin E, zinc, vitamin D and PUFA can modulate T-cell responses. Further research, particularly, early life intervention studies need to be carried out to establish whether early life dietary intervention can be used as a public health measure to reduce the prevalence of childhood asthma.

Systemic inflammation and decline in lung function in a general population: a prospective study

BACKGROUND

An increase in levels of C-reactive protein (CRP), a marker of systemic inflammation, is associated with reduced forced expiratory volume in 1 s (FEV1), supporting the hypothesis that the pathophysiology of chronic obstructive pulmonary disease has a systemic inflammatory component. However, few large studies have assessed the relationship between systemic inflammation as measured by CRP and decline in lung function prospectively in a randomly selected population.

METHODS

In 1991, data were collected on FEV1 and forced vital capacity (FVC) and a blood sample was taken from 2442 randomly selected adults in a community-based cohort. In 2000 these measures were repeated in 1301 individuals. The level of serum CRP was analysed in these samples from 1991 and 2000.

RESULTS

In cross-sectional analyses of data from 1991 and 2000, serum CRP levels were inversely related to FEV1 and FVC. After adjustment for smoking and other confounders, the difference in FEV1 was reduced by -9 ml (95% CI -13 to -5) and -7 ml (95% CI -13 to -2) for each mg/l increment in serum CRP in 1991 and 2000, respectively. There was no significant association between baseline serum CRP levels and decline in FEV1 and FVC over 9 years.

CONCLUSIONS

Although serum CRP levels are inversely associated with lung function in cross-sectional studies, there was no effect of a marker of systemic inflammation on decline in lung function over 9 years.

Dietary sodium intake and asthma: an epidemiological and clinical review

The changes in diet associated with the development of a more affluent lifestyle have been considered one of the environmental factors that may have contributed to the rise in the prevalence of asthma over the past few decades, and dietary sodium has been considered to be a dietary constituent which may be implicated in this phenomenon. The data presented in this review demonstrate that adoption of a low sodium diet for a period of 2-5 weeks may improve lung function and decrease bronchial reactivity in adults with asthma, while sodium loading appears to have a detrimental effect. Similarly, a low sodium diet maintained for 1-2 weeks decreases bronchoconstriction in response to exercise in individuals with asthma. There is no data as to the longer-term effect of a low sodium diet on either the prevalence or severity of asthma or on exercise-induced bronchoconstriction. As a low sodium diet has other beneficial health effects, it can be considered as a therapeutic option for adults with asthma, although it should be considered as an adjunctive intervention to supplement optimal pharmacological management of asthma and not as an alternative. If the relationship between higher sodium intake and increased prevalence and severity of asthma is causal, then there are potential population benefits for asthma as well as cardiovascular disease to be derived from public health measures to reduce sodium consumption.

The increase in the prevalence of asthma and allergy: food for thought

Since about 1960, the prevalence of asthma and allergic disease has increased sufficiently to become a major public-health concern. Concurrently, there have been marked changes in our diet, and it has been proposed that these changes have contributed to the increase in the prevalence of asthma and allergy. In this article, these hypotheses about diet are described, together with the postulated mechanisms and the evidence for and against, leading to the most recent evidence indicating that maternal diet during pregnancy might be particularly important in the development of childhood asthma.

Risk factors and characteristics associated with severe and difficult to treat asthma phenotype: an analysis of the ENFUMOSA group of patients based on the ECRHS questionnaire

BACKGROUND

Severe and difficult to treat asthma impairs health status and accounts for about half of asthma expenditure. In 1994, a European Network For Understanding Mechanisms of Severe Asthma (ENFUMOSA) was formed. A large group of patients from nine European countries has been selected.

OBJECTIVE

To examine the risk factors and symptoms associated with a phenotype of severe/difficult to treat asthma.

METHODS

The present report presents data assessed through the use of the European Community Respiratory Health Survey (ECRHS) Questionnaire in 148 mild-moderate controlled and 155 severe asthmatics from the ENFUMOSA group.

RESULTS

There is a negative association of severe asthma with reported allergy and with a family history of allergy (Odds ratio (OR)=0.45). Sharing a bedroom before the age of five is associated with a higher risk of severe asthma (OR=1.5) while childhood infections, play school attendance and exposure to allergens or animals are not. A larger proportion of severe asthma patients report symptoms at work (OR=2.7) or have to change jobs (OR=4.3) and fewer severe than mild patients are currently employed (OR=0.39). Smoking and exposure to smoke is similar in mild and severe asthma. Dietary habits do not differ between the groups, but severe asthmatics report eating less savoury snacks and there is a trend for lower intake of sweets.

CONCLUSIONS

Analysis of the ECRHS questionnaire in the ENFUMOSA study shows that severe asthma patients experience more symptoms and their health status is impaired by their inability to work and perhaps eat freely. Personal and maternal history of allergy is associated with mild but not severe asthma. Other than sharing a bedroom before the age of 5 years, no childhood exposure risk factors associated with severe asthma could be identified from this analysis.

Lung function and blood levels of copper, selenium, vitamin C and vitamin E in the general population

BACKGROUND

Increased dietary intake of antioxidants has been associated with higher lung function, but few studies have used biological markers of antioxidant intake.

OBJECTIVE

This study aimed to determine if antioxidant status, as measured by blood levels, influences lung function.

DESIGN

Using a random subsample of 479 participants, aged 18-65 y old, from a larger cross-sectional observational study, the association of forced expiratory volume in 1 s (FEV1) with plasma copper, vitamin C, vitamin E and serum selenium was assessed.

RESULTS

An s.d. increase in blood copper level was associated with a difference in FEV1 of -48 ml (95% confidence intervals: -95, -2 ml, P = 0.04), vitamin C +49 ml (+4, +94, P = 0.03), vitamin E -15 ml (-62, +32, P = 0.53) and selenium +52 ml (+7, +96, P = 0.02). The sizes of association were not appreciably altered in a mutually adjusted model.

CONCLUSIONS

Higher levels of serum vitamin C and selenium appear to be associated with higher FEV1. The association between higher serum copper and lower FEV1 requires further study in view of the ubiquitous exposure to this mineral.

Sodium intake and bronchial hyperresponsiveness in adults

BACKGROUND

Several investigations suggested a relationship between sodium intake and asthma and bronchial hyperresponsiveness (BHR), respectively. However, clinical and epidemiological studies did not show consistent finding.

OBJECTIVE

We analysed the association between dietary sodium intake and BHR to methacholine among 613 adults aged 20-65 years as one of the two German centres of the European Community Respiratory Health Survey (ECRHS).

METHODS

Dietary sodium intake was estimated from a 3-day weighed record of food intake. We applied multiple logistic regression models contrasting the three higher quartiles of sodium intake versus the lowest to assess the risk of BHR and mild BHR estimated by PD20 and PD10, respectively, controlling for potential confounders and stratified for sex. In addition, we analysed PD20 (dose of methacholine causing a fall of 20% in forced expiratory volume in 1s) as continuous variable expressed as transformed dose-response slope (tDRS) in the linear model.

RESULTS

Women were as expected more likely to be bronchial hyperresponsive (PD20: 26.1%; PD10: 52.2%) than men (PD20:15.8%; PD10: 34.8%) and had a lower mean daily sodium intake (2.36 g) compared with men (3.15 g). Logistic regression did not show any significant relationship between sodium intake and BHR in terms of PD20 after adjustment for age group, education, smoking status, body mass index and height in men or women. However, mild BHR assessed as PD10 was statistically significant positively related to the third (OR: 2.35; CI: 1.11-5.00) and highest quartile of sodium intake (OR: 2.28; CI: 1.06-4.88) in women, but not in men for third quartile (OR: 1.29; CI: 0.68-2.44) and for fourth quartile (OR: 1.07; CI: 0.56-2.07), respectively.

CONCLUSION

Sodium intake by several food items does not alter BHR assessed as PD20 to methacholine but may increase mild BHR assessed as PD10. We conclude that, in addition, PD10 has to be considered when the effect of sodium intake on BHR is studied.

Dietary salt intake as a potential modifier of airway responsiveness in bronchial asthma

While pharmacologic treatment of chronic asthma is usually highly effective, medications often have significant side-effects or exhibit tachyphylaxis. Alternative and/or complementary treatments that reduce dependence on pharmacologic medications are of interest in reducing the severity of asthma. This review analyzes the literature that has evaluated dietary salt intake as a potential modifier of the severity of asthma and airway responsiveness. High dietary intakes of salt, greater than 9 g/d, are common in Western civilizations, as is asthma. The question is whether reducing dietary salt intake potentially would improve pulmonary function and airway responsiveness in individuals with asthma. This review details the existing studies in this regard and includes the studies that have evaluated dietary salt on the severity of exercise-induced asthma (exercise-induced bronchoconstriction [E1B]). From a critical analysis of the existing literature, the data that support a role for dietary salt reduction for reducing severity of asthma and airway responsiveness in individuals with asthma is considered encouraging but not clinically convincing. The existing studies have suffered from a variety of experimental and population limitations. In contrast, the data from studies that have altered dietary salt and evaluated severity of EIB in nonatopic individuals is much more convincing. In each study so far, lowering dietary salt has reduced the severity of EIB to subclinical levels. Correspondingly, the supplementing of diets to higher than normal salt intake increased EIB significantly. This review concludes that the data are sufficient to warrant a clinical trial that is properly controlled and randomized to further investigate the influence of dietary salt intake on pulmonary function, airway responsiveness, symptoms, quality of life, and medication requirements in asthma and EIB.

Dietary salt reduction or exclusion for allergic asthma

BACKGROUND

There is a wide geographical variation in asthma prevalence and one explanation may be in dietary salt consumption.

OBJECTIVES

To assess the effect of dietary sodium reduction in patients with asthma.

SEARCH STRATEGY

A search was conducted using the Cochrane Airways Group asthma register. Bibliographies of included randomised controlled trials (RCTs) were searched for additional studies. Authors of identified RCTs were contacted for other studies. The most recent search was carried out in February 2004.

SELECTION CRITERIA

All studies were to be RCTs that involved dietary salt reduction or increased salt intake in patients with asthma. Studies of other allergic conditions such as hay fever, allergic rhinitis and eczema were considered patients with asthma were separately identified.

DATA COLLECTION AND ANALYSIS

Study quality was assessed and data extracted by two reviewers. All data analysis was conducted using the Cochrane Collaboration software (RevMan).

MAIN RESULTS

Six RCTs were included in this review. All studies were small size and of short duration. Data from only four studies could be pooled. Low sodium diet was associated with a significantly lower urine sodium excretion than normal or high salt diets. There were no significant differences in any asthma outcome between low salt and normal or high salt diets, however the confidence intervals were wide. FEV(1) with low salt compared to normal diet showed a WMD 0.09 L with a 95% confidence interval (95%CI) -0.19 L to 0.38 L, and compared to a high salt diet WMD 0.18 L; 95%CI -0.11 L to 0.48 L. Daily PEFR was also non-significantly higher with low salt diet compared to normal (WMD 19.52 L/min; 95% CI -21.22 to 60.25) and high salt diet (WMD 7.57 L/min; 95% CI -37.52 to 52.67). Reliever bronchodilator with the low salt diet when compared to both the normal and high salt diets showed WMD -0.07 puffs/day; 95%CI -0.94 to 0.81 & WMD -0.65 puffs/day; 95%CI -1.75 to 0.45, respectively.

REVIEWERS' CONCLUSIONS

Based on currently available evidence it is not possible to conclude whether dietary salt reduction has any place in the treatment or management of asthma. The results of this review do indicate an improvement in pulmonary function with low salt diet, however further large scales trials are required before any firm conclusions can be reach.

Food and nutrient intakes and asthma risk in young adults

BACKGROUND

Some aspects of diet are relatively newly recognized potential risk factors for asthma, but the evidence to date is conflicting.

OBJECTIVE

The goal was to determine whether the food and nutrient intakes of adults with asthma differ from those of adults without asthma.

DESIGN

This was a community-based, cross-sectional study of 1601 young adults ( +/- SD age: 34.6 +/- 7.1 y) who were initially recruited by random selection from the federal electoral rolls in Melbourne in 1999. Subjects completed a detailed respiratory questionnaire, a validated semiquantitative food-frequency questionnaire, skin-prick testing, and lung function tests, including a methacholine challenge test for bronchial hyperreactivity (BHR). A total of 25 nutrients and 47 food groups were analyzed by using multiple logistic regression with alternate definitions of asthma and atopy as the outcomes.

RESULTS

Whole milk appeared to protect against current asthma (odds ratio: 0.66; 95% CI: 0.46, 0.97), doctor-diagnosed asthma (0.73; 0.54, 0.99), BHR (0.68; 0.48, 0.92), and atopy (0.71; 0.54, 0.94). Conversely, soy beverage was associated with an increased risk of current asthma (2.05; 1.19, 3.53), doctor-diagnosed asthma (1.69; 1.04, 2.77), and BHR (1.65; 1.00, 2.71). Apples and pears appeared to protect against current asthma (0.83; 0.71, 0.98), asthma (0.88; 0.78, 1.00), and BHR (0.88; 0.77, 1.00).

CONCLUSIONS

The consumption of dairy products, soy beverages, and apples and pears, but not of nutrients per se, was associated with a range of asthma definitions. Dietary modification after diagnosis is one possible explanation for this finding. Intervention studies using whole foods are required to ascertain whether such modifications of food intake could be beneficial in the prevention or amelioration of asthma.

Obesity and asthma: evidence for and against a causal relation

The evidence for an association between asthma and obesity in adults, and in children and adolescents, is reviewed. Few studies in adults measured height and weight, whereas the majority in children did. Evidence for the association is strong, but that for a gender interaction is weak. There is sufficient evidence to rule out asthma preceding obesity as an explanation, and that increased perception of symptoms in the obese, or a purely mechanical effect, is responsible. However, direct causality is unlikely, because in children the association is of recent origin, and trends in obesity do not explain the rising prevalence of asthma. Atopy was not associated with obesity in a large adult study. Potential explanations that require further investigation are that gastroesophageal reflux as a result of obesity causes asthma, that physical inactivity may promote both obesity and asthma, and that the diets of obese subjects may potentiate asthma.

Comparing and combining studies of bronchial responsiveness

BACKGROUND

There is no standardised protocol for the measurement of bronchial responsiveness. Results from different studies are difficult to compare and combine.

METHODS

Analyses are divided between those of a continuous outcome, which can be directly standardised as effect size, and those based on a binary outcome. A published method is used to convert an odds ratio to equivalent effect size.

RESULTS

The use of effect size allows comparison between studies using a continuous outcome but different protocols, provided the relevant standard deviation is reported. Effect size from a continuous outcome and that derived from an odds ratio from an equivalent analysis gave similar results.

CONCLUSIONS

Systematic reviews which include both continuous effect estimates and odds ratios can include both in one meta-analysis, provided relevant standard deviations are published for the former. Authors are encouraged to report these in all fields in which measurement protocols vary.

Prospective study of diet and decline in lung function in a general population

We have investigated the relationship between decline in lung function and dietary intakes of magnesium, vitamin C, and other antioxidant vitamins in a general population cohort in Nottingham, United Kingdom. In 1991, we measured dietary intake by semiquantitative food frequency questionnaire, forced expiratory volume in 1 second (FEV1), and respiratory symptoms in a cross-sectional survey of 2,633 adults aged 18-70. Nine years later we repeated these measures in 1,346 of these individuals. In cross-sectional analyses, after adjustment for smoking and other confounders, higher intakes of vitamin C and magnesium, but not vitamins A or E, were associated with higher levels of FEV1 in both 1991 and 2000. In longitudinal analysis with adjustment for confounders, decline in FEV1 between 1991 and 2000 was lower amongst those with higher average vitamin C intake by 50.8 ml (95% confidence interval, 3.8-97.9) per 100 mg of vitamin C per day, but was unrelated to magnesium intake. There was no relationship between decline in FEV1 and intake of vitamins A or E. This study suggests that a high dietary intake of vitamin C, or of foods rich in this vitamin, may reduce the rate of loss of lung function in adults and thereby help to prevent chronic obstructive pulmonary disease.

Harmful effects of dietary salt in addition to hypertension

In addition to raising the blood pressure dietary salt is responsible for several other harmful effects. The most important are a number which, though independent of the arterial pressure, also harm the cardiovascular system. A high salt intake increases the mass of the left ventricle, thickens and stiffens conduit arteries and thickens and narrows resistance arteries, including the coronary and renal arteries. It also increases the number of strokes, the severity of cardiac failure and the tendency for platelets to aggregate. In renal disease, a high salt intake accelerates the rate of renal functional deterioration. Apart from its effect on the cardiovascular system dietary salt has an effect on calcium and bone metabolism, which underlies the finding that in post-menopausal women salt intake controls bone density of the upper femur and pelvis. Dietary salt controls the incidence of carcinoma of the stomach and there is some evidence which suggests that salt is associated with the severity of asthma in male asthmatic subjects.

Dietary salt reduction or exclusion for allergic asthma

BACKGROUND

There is a wide geographical variation in asthma prevalence and one explanation may be in dietary salt consumption.

OBJECTIVES

To assess the effect of dietary sodium reduction in patients with asthma.

SEARCH STRATEGY

A search was conducted using the Cochrane Airways Group asthma register. Bibliographies of included randomised controlled trials (RCTs) were searched for additional studies. Authors of identified RCTs were contacted for other studies.

SELECTION CRITERIA

All studies were to be RCTs that involved dietary salt reduction or increased salt intake in patients with asthma. Studies of other allergic conditions such as hay fever, allergic rhinitis and eczema were considered patients with asthma were separately identified.

DATA COLLECTION AND ANALYSIS

Study quality was assessed and data extracted by two reviewers. All data analysis was conducted using the Cochrane Collaboration software (RevMan 4.1.1).

MAIN RESULTS

Fifty-six abstracts were identified and 15 studies were reviewed in full text. Five fulfilled the inclusion criteria. Nine were excluded. One was published in duplicate. Complete agreement was reached between the reviewers on inclusion or exclusion of all studies. All studies were small and of short duration. Data from only three could be pooled. Low sodium diet was associated with a significantly lower urine sodium excretion than normal or high salt diets. There were no significant differences in any asthma outcome between low salt and normal or high salt diets, but FEV1 was slightly higher with low salt compared to normal, WMD 0.09 L (95% confidence interval (CI) -0.26, 0.44 L, n=88), as was daily PEFR, WMD 11 l/min (95% CI -81, 103 l/min, n=78). With low compared to high salt, FEV1 was slightly higher WMD 0.22 L (95% CI -0.14, 0.59 L, n=88), as was daily PEFR, WMD 14 l/min (95% CI -41, 68 l/min, n=78). Bronchodilator use was slightly lower, WMD 0.7 puffs/day (95% CI -1.8, 0.5 puffs/day, n=62).

REVIEWER'S CONCLUSIONS

Based on currently available evidence it is not possible to conclude whether dietary salt reduction has any place in the treatment or management of asthma.

Dietary salt restriction improves pulmonary function in exercise-induced asthma

PURPOSE

Exercise-induced asthma (EIA) occurs in approximately 90% of persons with asthma. The mechanism has not been delineated. Epidemiological studies have suggested that dietary salt may play a role in airway responsiveness. Therefore, the purpose of this study was to determine the influence of both elevated and restricted salt diets on pulmonary function in subjects with EIA.

METHODS

Eight subjects with EIA and eight subjects without EIA (control) participated in a double-blind crossover study. Pulmonary function was determined pre- and post-exercise challenge before and after 2 wk on a normal salt, sodium chloride, diet (NSD), a low salt diet (LSD), and a high salt diet (HSD). A 1-wk washout occurred between diets.

RESULTS

Diet had no effect on preexercise pulmonary function values in either group and had no effect on postexercise pulmonary function values in control subjects. However, LSD improved and HSD worsened postexercise pulmonary function values in EIA subjects. Forced expiratory volume in 1 s (FEV1) decreased by at least 10% in EIA subjects with exercise. In EIA subjects, FEV1 decreased by 14+/-6% on LSD, 20+/-7% on NSD, and 24+/-6% on HSD at 15 min postexercise. Similar patterns were observed for forced vital capacity and peak expiratory flow rates. Although LSD did not normalize pulmonary function in EIA, it did improve it.

CONCLUSIONS

These data suggest that individuals with EIA might benefit from lower salt diets.

Diet and asthma

The role of food intolerance in asthma is well recognized, and where food avoidance measures are instituted considerable improvement in asthma symptoms and in reduction in drug therapy and hospital admissions can result. These benefits may have a greater impact in those patients with greater symptoms. However, the promise of such benefits should not result in an approach which ignores inhaled drug therapy, or in a dietary regime which is inappropriate in the face of mild symptoms. Whilst sub-optimal intake of dietary nutrients is also a recently recognized potential risk factor for asthma, available data are insufficient to implicate any as casual. A number of studies have sought to establish the role of the antioxidant vitamins, A, C and E and selenium, yet others of the elements sodium and magnesium. Sub-optimal nutrient intake may enhance asthmatic inflammation, consequently contributing to bronchial hyperreactivity. Prospective studies of supplementation therapy are needed to confirm this.

Asthma, oxidant stress, and diet

It has been suggested that the increased prevalence of atopy and asthma observed in many developed countries over the past 30 y is in part the result of a decrease in the incidence and severity of early childhood infections. The immunologic consequence of this phenomenon has been the expansion of T-lymphocyte populations away from the T-helper 1 (Th1) subset and in the direction of the Th2 subset. This leads to the creation of a cytokine-mediated propensity for the development of an intense inflammatory response in the airways, resulting in oxidative stress, airway tissue injury, and the development of atopy and asthmatic symptomatology. Over this same period, there has been a decreased intake of dietary substances that contribute to antioxidant defense, and this appears to have contributed to the rise of atopy and asthma. Studies evaluating the efficacy of these antioxidant substances in the prevention of asthma and as adjuvants in the treatment of asthma are reviewed, and suggestions are made for the direction of future studies.

Dietary influences on chronic obstructive lung disease and asthma: a review of the epidemiological evidence

The epidemiological evidence for a relationship between diet and indicators of asthma and chronic obstructive pulmonary disease (COPD) is evaluated. The review focuses on the intake of Na, n-3 fatty acids, and antioxidant vitamins as well as fruit and vegetables. Experimental studies suggest that a high-Na diet has a small adverse effect on airway reactivity in asthma patients. However, observational studies provide no clear evidence that high Na intake has adverse effects on airway reactivity or asthma symptoms in open populations. n-3 Polyunsaturated fatty acids, which are present in fish oils, are metabolized into less broncho-constricting and inflammatory mediators than n-6 polyunsaturated fatty acids. Studies in the general adult population suggest that a high fish intake has a beneficial effect on lung function, but the relationship with respiratory symptoms and clinically-manifest asthma or COPD is less evident. Also, experimental studies in asthma patients have not demonstrated an improvement in asthma severity after supplementations with fish oil. Several studies showed a beneficial association between fruit and vegetable intake and lung function, but the relationship with respiratory symptoms and the clinically-manifest disease was less convincing. A similar pattern was found for vitamin C in relation to indicators of asthma and COPD, but there are still conflicting results with respect to vitamin E and beta-carotene. In conclusion, the epidemiological evidence for a beneficial effect on indicators of asthma and COPD of eating fish, fruit and vegetables is increasing. However, the effectiveness of dietary supplementation in open-population samples is often not demonstrated. Several unresolved questions are raised, which should be addressed in future studies on the relationship between diet and respiratory disease.

Reversing the trend: reducing the prevalence of asthma

BACKGROUND

We urgently need to take steps towards reducing the prevalence of asthma in countries where the prevalence has become unacceptably high in recent years. Because we do not have any good information about causes of the increased prevalence, we cannot act directly to reverse the trend. Therefore we need to take an indirect approach and use known information of etiologic factors to try to reduce asthma in the next generation, while acknowledging that we may be dealing with different factors from those responsible for the increased prevalence. Any successful strategies will also help to ensure that developing countries do not attain such high rates of asthma in their children in coming years.

OBJECTIVE

This article summarizes the roles of the risk factors that identify "high-risk" children, that provide insights into mechanisms, or that have potential for primary prevention. The factors with the most potential for primary prevention are allergen exposure, parental smoking, breast-feeding, and dietary fatty acids.

CONCLUSION

In other health models, information about risk factors has been used in successful public health interventions. It is disappointing that the important risk factors for asthma are well documented but that there have been few trials of primary prevention and no changes in public health policies.

Asthma: a longitudinal perspective

In the future, the important longitudinal studies will be those that divide the broad spectrum of asthma into phenotypic groups in order to provide more precise information about mechanisms and outcomes. Asthmatics who present in a clinical setting often have a combination of two or three different phenotypes, and may have more severe illness as a result, but subjects who are tested in epidemiological studies are frequently characterized by only one phenotype. Although the definition of asthma as a combination of symptoms plus either AHR or reduced lung function has greater predictive isolation for assessing the prognosis of respiratory illness that occurs in childhood (27,40,55,66-68), studies that are able to classify subjects according to their different phenotypes are more likely to lead to a better understanding of the factors that influence prognosis. More importantly, by using this approach, future research studies should be better able to separate the mechanisms whereby therapeutic or environmental interventions influence the prognosis of each phenotype. The potential benefits will be a better understanding and evaluation of the factors that make an important contribution to respiratory health.

New insights into risk factors of asthma

The prevalences of bronchial asthma in the Asia-Pacific region range from 0 to 24% and appear to be increasing in some countries. The increased prevalence of asthma may be related to the urbanization of these countries or areas. Risk factors relating to the development of asthma are multiple and complex. These include: Predisposing or genetic factors: (atopy and bronchial hyperresponsiveness; BHR) that increase an individuals susceptibility. Longitudinal studies in children have shown that BHR precedes asthma in some individuals. A gene governing BHR is located near a major locus that regulates serum IgE levels on chromosome 5 q. An additional gene that determines the specificity of the immune response located in the human leukocyte antigen complex (HLA) may govern the specificity of the immune response to common aeroallengens in some individuals. Causal Factors: inhaled allergens are the most important causal factors of asthma, which include indoor allergens (domestic mites, animal, cockroach and fungus allergens), domestic mites being the most common potential allergen, and outdoor allergens (pollens from trees, grasses and weeds). Owing to the geographic location and different sensitivity to allergen between races, allergens vary from area to area. Certain drugs, food and food additives are also the cause of asthma attack. Contributing factors: smoking is an important trigger and a serious problem in most Asian-Pacific countries. Air pollution in particular sulfur dioxide (SO2), nitrogen dioxide (NO2) and respirating particles are common contributing factors. Very low concentration of SO2 (0.5 ppm) can cause bronhospasm in asthmatics. A combination of low concentration of SO2 and NO2 often encountered in heavy traffic further enhances the airway responsiveness to inhaled allergen. In addition, respiratory virus infection is closely associated to the development of asthma in childhood.

Effect of dietary sodium on airways responsiveness and its importance in the epidemiology of asthma: an evaluation in three areas of northern England

BACKGROUND

Although several investigations have shown a relationship between asthma (or its surrogate, airways responsiveness) and dietary or urinary sodium, others have not, and the matter remains controversial. This "salt effect" has been investigated during two recent epidemiological surveys of men in northern England. The first assessed the possible effect on airways responsiveness of occupational exposure to welding fumes, and the second characterised airways responsiveness in two geographically distinct residential areas. Thus, three separate study areas/populations were involved.

METHODS

Investigation 1 involved 1059 shipyard workers aged 16-27 years who were exposed variously to welding fumes, and Investigation 2 involved 587 men aged 20-44 years who lived in rural West Cumbria or in urban Newcastle upon Tyne. In Investigation 1, a 24 hour urine specimen was requested from each subject with quantifiable airways responsiveness (PD20 < or = 6400 micrograms methacholine) and from an equal number of subjects without measurable airways responsiveness from the same occupational subgroup. In Investigation 2, every subject was asked to provide a 24 hour urine specimen.

RESULTS

Of the men undergoing methacholine tests, satisfactory 24 hour urine specimens were obtained from 234 (22.1%) in Investigation 1 and 232 (39.5%) in Investigation 2. Analysis using multiple linear regression, multiple linear logistic regression, and multiple regression for censored data produced consistent results within each study population but conflicting results between them, such that there was no hint of a relationship between airways responsiveness and 24 hour urinary sodium excretion in the shipyard workers of Investigation 1 nor in the rural West Cumbrian population of Investigation 2, but an association was found in the urban Newcastle population of Investigation 2. All study populations were sufficiently large to demonstrate anticipated relationships between airways responsiveness and atopy, baseline FEV1, and (Newcastle only) age.

CONCLUSIONS

If airways responsiveness is related to dietary sodium the relationship is not likely to be strong.