Dietary sodium and arterial pressure: problems of studies within a single population

Impact of a community-based lifestyle intervention program on blood pressure and salt intake of normotensive adult population in a developing country

BACKGROUND

Data on the effect of lifestyle intervention programs on salt intake and blood pressure in developing countries are scarce. This study aimed to assess the impact of a healthy lifestyle community-based trial on salt intake and blood pressure among a representative sample of normotensive Iranian adults.

MATERIALS AND METHODS

We compared the data for salt intake, urinary sodium levels and blood pressure from three cross-sectional surveys in time points of 1999, 2001-2002 (beginning of the community interventions), and 2007 (after the community trial) for normotensive adult population of Isfahan, Iran in the framework of Isfahan Healthy Heart Program. Using multi-stage cluster sampling method, one of the family members at each household was randomly selected with Iranian adult population as a target. Dietary salt intake was estimated based on 24 hour urinary sodium levels. Systolic and diastolic blood pressures were measured according to standard methods.

RESULTS

Dietary sodium intake and urinary sodium levels as well as systolic and diastolic blood pressure were significantly decreased during the 9-year study period. Unlike systolic and diastolic blood pressures that had a consistent decrement between 1999 and 2007, dietary sodium intake and urinary sodium levels were slightly raised from 1999 to 2000-2001 and then reduced between 2001-2 and 2007 evaluations. The same findings were reached when data were analyzed separately by gender or weight status.

CONCLUSIONS

A lifestyle community trial was effective in controlling the escalating trend of blood pressure and salt intake in Iranian population. It can be considered as a model to be adopted in other developing countries.

Blood pressure and interactions between the angiotensin polymorphism AGT M235T and sodium intake: a cross-sectional population study

BACKGROUND

Intervention studies have indicated an interaction between the blood pressure response to a low-sodium or a low-fat and high-fruit and -vegetable diet and the angiotensinogen gene (AGT) polymorphisms G-6A and M235T.

OBJECTIVE

We investigated whether this interaction is also present in a large free-living population.

DESIGN

Urinary sodium, potassium as biomarkers of intake, and blood pressure were measured in 11 384 men and women aged 45-79 y participating in the Norfolk arm of the European Prospective Investigation of Nutrition and Cancer (EPIC). The M235T polymorphism was assessed by pyrosequencing.

RESULTS

Highly significant associations between sodium and blood pressure were shown for all genotypes (P < 0.001), but the regression coefficient for systolic blood pressure associated with each unit of sodium for each of the MT and TT genotypes was approximately double that for the MM homozygotes (P < 0.001 for heterogeneity between genotypes). Differences were evident at high exposures to sodium but not at low exposures. There were no significant associations between blood pressure and dietary or urinary potassium.

CONCLUSIONS

This large cross-sectional study supports public health recommendations to reduce salt consumption in the population as a whole, and it confirms intervention trial data showing the greatest response to intervention in persons with the AA and TT genotype in the AGT G-6A and M235T polymorphisms. Genotype effects in populations at low exposure to sodium are not likely to be seen.

Blood pressure and urinary sodium in men and women: the Norfolk Cohort of the European Prospective Investigation into Cancer (EPIC-Norfolk)

BACKGROUND

Abundant evidence indicates that a high sodium intake is causally related to high blood pressure, but debate over recommendations to reduce dietary sodium in the general population continues. A key issue is whether differences in usual sodium intake within the range feasible in free-living populations have clinical or public health relevance.

OBJECTIVE

We examined the relation between blood pressure and urinary sodium as a marker of dietary intake.

DESIGN

This was a study of 23104 community-living adults aged 45-79 y.

RESULTS

Mean systolic and diastolic blood pressure increased as the ratio of urinary sodium to creatinine increased (as estimated from a casual urine sample), with differences of 7.2 mm Hg for systolic blood pressure and 3.0 mm Hg for diastolic blood pressure (P < 0.0001) between the top and bottom quintiles. This trend was independent of age, body mass index, urinary potassium:creatinine, and smoking and was consistent by sex and history of hypertension. The prevalence of those with systolic blood pressure >/= 160 mm Hg halved from 12% in the top quintile to 6% in the bottom quintile; the odds ratio for having systolic blood pressure >/= 160 mm Hg was 2.48 (95% CI: 1.90, 3.22) for men and 2.67 (95% CI: 2.08, 3.43) for women in the top compared with the bottom quintile of urinary sodium. Estimated mean sodium intakes in the lowest and highest quintiles were approximately 80 and 220 mmol/d, respectively.

CONCLUSIONS

Within the usual range found in a free-living population, differences in urinary sodium, an indicator of dietary sodium intake, are associated with blood pressure differences of clinical and public health relevance. Our findings reinforce recommendations to lower average sodium intakes in the general population.

By how much does dietary salt reduction lower blood pressure? II--Analysis of observational data within populations

OBJECTIVE

To determine whether the estimates of the size of the association between blood pressure and sodium intake derived from studies of individuals within populations can be quantitatively reconciled with our estimates derived from comparisons of the average blood pressure and sodium intake between different populations.

DESIGN

Examination of data from 14 published studies that correlated blood pressure recordings in individuals against measurements of their 24 hour sodium intake (within population studies).

MAIN OUTCOME MEASURE

Comparison of observed differences in blood pressure per 100 mmol/24 h difference in sodium intake in each within population study with predicted differences calculated from the between population data, after allowing for the underestimation of the true association of blood pressure with sodium intake caused by the large day to day variation in 24 hour sodium intake within individuals.

RESULTS

The underestimation bias inherent in the within populations studies reduced the regression slope of blood pressure on single measures of 24 hour sodium intake to between a half and a quarter of the true value (for example, in one study from 6.0 to 2.4 mm Hg/100 mmol/24 h). Estimates from between population comparisons of the regression slope of blood pressure on sodium intake, after adjustment to take this underestimation bias into account, were similar to the values actually observed in the within population studies.

CONCLUSION

The within population studies confirm our estimates from between population comparisons of the magnitude of the association between blood pressure and sodium intake.

[Blood pressure and relative body weight, alcohol consumption and electrolyte excretion in the FRG and the GDR: the Intersalt Study. The Intersalt Study Group form the FRG and the GDR]

The relationships between body mass index (BMI) and age, alcohol consumption, 24-hr urinary electrolyte excretion, and BP were studied in 588 subjects from three German centers participating in Intersalt, a highly standardized, previously reported protocol. Men and women aged 20-59 were sampled in Bernried, FRG; Cottbus, GDR; and Heidelberg, FRG. The subjects from the three centers did not differ in BMI, level of education, physical activity, cigarette- or alcohol-consumption patterns, or urinary Cl excretion. Mean Na excretion was 167, 147, and 172 mmol/24 hr in Bernried, Cottbus, and Heidelberg, while mean K excretion was 72, 55, and 73 mmol/24 hr, respectively. The excretion of these electrolytes was significantly lower in Cottbus than in Bernried or Heidelberg. BMI increased progressively in men with age; in women BMI plateaued until the 5th decade, after which it increased to equal that of men. In individual centers, the excretion of electrolytes was correlated with BMI. Sodium and chloride excretion were highly correlated. The data from each individual center were fitted to a multiple regression model. Age, BMI, sex, and alcohol consumption entered the model.

Diet, alcohol and hypertension

Obesity, diet and alcohol consumption constitute major environmental determinations of blood pressure elevation. The long term setting of blood pressure in response to these factors will be determined by genetic susceptibility, and interactions with effects of physical fitness and smoking. Dietary changes which independently influence both atherosclerosis and hypertension are likely to be of greatest value in helping to control morbidity and mortality from hypertensive cardiovascular disease. Recommendations should focus on diets low in total and saturated fat intake and high in fruit and vegetables, containing potassium and fibre, coupled with weight control, alcohol moderation to less than two drinks per day in drinkers and regular physical exercise. Sodium restriction will help lower blood pressure in older hypertensives in particular. The role of dietary calcium or fish oils in blood pressure regulation is still uncertain. Dietary and related recommendations on smoking and exercise should be 'first line' treatment in mild hypertensives, and complimentary to therapy in all patients requiring drugs.

Urinary electrolyte excretion, alcohol consumption, and blood pressure in the Scottish heart health study

As part of a study of risk factors for coronary heart disease 24 hour urine collections were obtained from 7354 men and women aged 40-59 selected at random from 22 districts throughout Scotland (Scottish heart health study). The mean of two standardised measurements of blood pressure was related to the reported consumption of alcohol and measurements of height, weight, pulse rate, and electrolyte excretion. Several significant correlations were found with both systolic and diastolic pressure, but only the coefficients for age, body mass index, and pulse rate were greater than 0.1. Alcohol consumption showed a weak positive correlation with blood pressure in men. Sodium excretion showed a weak positive correlation with blood pressure in both sexes, and potassium excretion showed weak negative correlations. In multiple regression analysis age, pulse rate, body mass index, alcohol consumption, and potassium excretion had significant independent effects but sodium excretion did not. Although measuring blood pressure twice on one occasion and 24 hour urinary sodium excretion only once may have weakened any potential correlation, the most likely explantation of these results is that the relation between sodium and blood pressure in the population is weak and that potassium and alcohol are of greater importance.

The association between blood pressure, age, and dietary sodium and potassium: a population study

We examined the relationship between blood pressure and dietary sodium and potassium intake estimated from 24 hr diet recall in a population of 584 men and 718 women 30 to 79 years old in Southern California. In men, but not women, age-adjusted systolic and diastolic blood pressure correlated significantly with dietary sodium intake. In both men and women, age-adjusted diastolic blood pressure significantly inversely correlated with dietary potassium intake. Age-adjusted systolic and diastolic blood pressure correlated significantly with the dietary sodium/potassium ratio in each sex; correlations were better for the ratio than for either sodium or potassium alone. The relationship was apparent over the whole range of blood pressure and dietary intake. A marked age gradient was apparent in men, the regression slope for blood pressure vs sodium/potassium ratio increasing with increasing age, suggesting increasing sensitivity to dietary sodium/potassium ratio with age. Adjusting for intake of other dietary variables, including calories, protein, carbohydrate, saturated fat, alcohol, calcium, and fiber, did not alter the relationships; adjusting for body mass index reduced the strength of the association in women but not in men. These results support the hypothesis that dietary sodium and potassium are related to blood pressure within a population.

Blood pressure and salt intake in Malawi: an urban rural study

A significant difference between the blood pressures of rural and urban Malawians was found in both sexes, was present at the age of 15 years, and was associated with obesity but not with smoking, alcohol consumption, occupation or housing. Pulse rate was significantly lower in the urban group. These differences were accompanied by low potassium and sodium intake although the sodium intake in the urban group was double that in the rural group. No direct relation between blood pressure and urinary electrolytes was found.

Dietary sodium and arterial blood pressure: evidence against genetic susceptibility

Thirty five subjects with both parents in the top third of their age specific blood pressure distributions and 31 subjects with both parents in the bottom third of their blood pressure distributions restricted their intake of sodium for eight weeks while taking part in a double blind, randomised crossover trial of supplements of sodium and placebo. A comparison of two periods of four weeks at different intakes of sodium showed no differences in blood pressure in either the groups as a whole or the subgroups who complied best with the diet and tablets. In the compliant subgroups mean urinary sodium excretions were above 120 mmol(mEq) and below 50 mmol/day. The study provides evidence against the hypothesis that people with a family history of high blood pressure are more susceptible in their blood pressure response to dietary sodium.

Monovalent and divalent cations in hypertension

The relationship of four cations (sodium, potassium, calcium, magnesium) to hypertension is reviewed. It seems reasonable to advise some reduction in sodium intake, and an initial goal of a mean intake of 120 mmole per day for men and 90-100 mmole per day for women is suggested. Some increase in potassium intake may well be justifiable but is better achieved through potassium-containing foods than by any artificial supplements. The data for calcium and magnesium are not sufficiently strong to warrant any recommendation for a change in intake at present.

Group comparisons of blood pressure and indices of obesity and salt intake in Pacific populations

Group comparisons of blood pressure and indices of obesity and salt intake from epidemiological studies conducted in the Pacific early in the 1980s are presented. The comparisons were of a rural-urban type, and showed a trend for a lower prevalence of hypertension and for lower mean blood pressures in adults from rural areas, compared with urban populations. Rural residents were also slimmer and appeared to have a lower salt intake than did urban dwellers. These findings give possible support to the role of adiposity and salt intake in the genesis of essential hypertension.

Childhood antecedents of adult degenerative disease

The pathogenetic links between diet and diseases such as hypertension and atherosclerosis remain the subject of much controversy. This article reviews the evidence about the relationship between diet and these two widespread adult conditions, proposes an approach for their early recognition, examines the rationale and safety of dietary changes, and formulates specific recommendations.

Hypothesis: discrepancy between intra- and interpopulation studies of the relationship between dietary salt and blood pressure: fact or fiction?

It is a paradox that intra-population studies fail to show significant correlation between sodium excretion and blood pressure while a clear relationship exists in cross-cultural studies. Since daily variation of sodium excretion is high, the discrepancy between the two observations could be due to non-comparable data on sodium excretion. This is a discussion of the hypothesis that the finding of a significant correlation or not between sodium excretion and blood pressure depends on the statistical analysis of the data.