Declaring independence: why we should be cautious

Sleep Disordered Breathing, Obesity and Atrial Fibrillation: A Mendelian Randomisation Study

It remains unclear whether the association between obstructive sleep apnoea (OSA), a form of sleep-disordered breathing (SDB), and atrial fibrillation (AF) is causal or mediated by shared co-morbidities such as obesity. Existing observational studies are conflicting and limited by confounding and reverse causality. We performed Mendelian randomisation (MR) to investigate the causal relationships between SDB, body mass index (BMI) and AF. Single-nucleotide polymorphisms associated with SDB (n = 29) and BMI (n = 453) were selected as instrumental variables to investigate the effects of SDB and BMI on AF, using genetic association data on 55,114 AF cases and 482,295 controls. Primary analysis was conducted using inverse-variance weighted MR. Higher genetically predicted SDB and BMI were associated with increased risk of AF (OR per log OR increase in snoring liability 2.09 (95% CI 1.10-3.98), p = 0.03; OR per 1-SD increase in BMI 1.33 (95% CI 1.24-1.42), p < 0.001). The association between SDB and AF was not observed in sensitivity analyses, whilst associations between BMI and AF remained consistent. Similarly, in multivariable MR, SDB was not associated with AF after adjusting for BMI (OR 0.68 (95% CI 0.42-1.10), p = 0.12). Higher BMI remained associated with increased risk of AF after adjusting for OSA (OR 1.40 (95% CI 1.30-1.51), p < 0.001). Elevated BMI appears causal for AF, independent of SDB. Our data suggest that the association between SDB, in general, and AF is attributable to mediation or confounding from obesity, though we cannot exclude that more severe SDB phenotypes (i.e., OSA) are causal for AF.

Correlation without a cause: an epidemiological odyssey

In the 1980s debate intensified over whether there was a protective effect of high-density lipoprotein cholesterol (HDL-C) or an adverse effect of triglycerides on coronary heart disease (CHD) risk. In a 1991 paper reprinted in the IJE we suggested that the high degree of correlation between the two, together with plausible levels of measurement error, made it unlikely that conventional epidemiological approaches could contribute to causal understanding. The consensus that HDL-C was protective, popularly reified in the notion of ‘good cholesterol’, strengthened over subsequent years. Reviewing the biostatistical and epidemiological literature from before and after 1991 we suggest that within the observational epidemiology pantheon only Mendelian randomization studies—that began to appear at the same time as the initial negative randomized controlled trials—made a meaningful contribution. It is sobering to realize that many issues that appear suitable targets for epidemiological investigation are simply refractory to conventional approaches. The discipline should surely revisit this and other high-profile cases of consequential epidemiological failure—such as that with respect to vitamin E supplementation and CHD risk—rather than pass them over in silence.

Cannabis use and psychosis: the origins and implications of an association

Evidence for the effectiveness of treatment or secondary prevention of psychotic illness such as schizophrenia is often disappointing. This situation reflects our limited understanding of the aetiology of psychosis. There is good evidence that both genetic and environmental factors are implicated but the precise identity of these is unclear. Cannabis use is one candidate as a possible, modifiable environmental influence on both incidence and prognosis of psychosis. Evidence supporting this candidature is exclusively observational, and its strength has perhaps been overestimated and problems related to its interpretation underestimated by some. Nevertheless the possibility that cannabis does cause psychosis remains. Because of this, and because there are other good public health reasons to prevent cannabis use, interventions targeting use need to be evaluated. This evaluation, along with other imaginative approaches to future research, is needed to further our understanding of the determinants of mental illness and how we can most effectively improve the population's mental health.

Explanation and Elaboration Document for the STROBE-Vet Statement: Strengthening the Reporting of Observational Studies in Epidemiology-Veterinary Extension

The STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) statement was first published in 2007 and again in 2014. The purpose of the original STROBE was to provide guidance for authors, reviewers, and editors to improve the comprehensiveness of reporting; however, STROBE has a unique focus on observational studies. Although much of the guidance provided by the original STROBE document is directly applicable, it was deemed useful to map those statements to veterinary concepts, provide veterinary examples, and highlight unique aspects of reporting in veterinary observational studies. Here, we present the examples and explanations for the checklist items included in the STROBE-Vet statement. Thus, this is a companion document to the STROBE-Vet statement methods and process document (JVIM_14575 "Methods and Processes of Developing the Strengthening the Reporting of Observational Studies in Epidemiology-Veterinary (STROBE-Vet) Statement" undergoing proofing), which describes the checklist and how it was developed.

Some Observations on Health and Socio economic Status

Health and socio-economic status are powerfully linked. This association cannot be attributed to social-selection effects, and the unequal distribution of behavioural risk factors, such as smoking, explains only a part of the variance. Differential exposure to physical hazards plays a role, but the persistence of health differentials into the better-off social strata and the significance of relative as well as absolute living standards suggest psychosocial factors also. We outline a conceptual model that regards the clustering of adverse physical and psychosocial factors over the life course as critical. Identifying the salient physical and psychosocial factors is a formidable research mission. In pursuing this mission we should not lose sight of the key fact that socio economic health differentials are intimately bound up with material differentials, and that remediation demands strategies that counter socio-economic disparity.

The health effects of the global financial crisis: can we reconcile the differing views? A network analysis of literature across disciplines

Why are researchers studying the health effects of economic change reaching markedly varying conclusions? To understand these differences, we first systematically searched Web of Science for the literature on recessions and health yielding 461 articles and 14,401 cited documents. We then undertook a network analysis of co-citation pattern by disciplines, journals and backgrounds of the authors, followed by a chronological review of the literature, to trace the evolution of ideas. We then examined the extent to which earlier literature predicted what has happened in the 2007-2012 crisis. Our analysis finds the literature is dominated by disciplinary silos, with economics studies predominantly citing each other and relative isolation of psychiatry and substance abuse journals. Different philosophical approaches to assessing causality appear to contribute to varying interpretations, a tendency that is unlikely to be resolved without a shift in research norms. We conclude by calling for more inter-disciplinary research that combines empirical findings with a search for plausible mechanisms. This approach would evaluate not only the effects of economic shocks but also the mechanisms that offer protection against them.

Strengthening the Reporting of Observational Studies in Epidemiology (STROBE): explanation and elaboration

Much medical research is observational. The reporting of observational studies is often of insufficient quality. Poor reporting hampers the assessment of the strengths and weaknesses of a study and the generalisability of its results. Taking into account empirical evidence and theoretical considerations, a group of methodologists, researchers, and editors developed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) recommendations to improve the quality of reporting of observational studies. The STROBE Statement consists of a checklist of 22 items, which relate to the title, abstract, introduction, methods, results and discussion sections of articles. Eighteen items are common to cohort studies, case-control studies and cross-sectional studies and four are specific to each of the three study designs. The STROBE Statement provides guidance to authors about how to improve the reporting of observational studies and facilitates critical appraisal and interpretation of studies by reviewers, journal editors and readers. This explanatory and elaboration document is intended to enhance the use, understanding, and dissemination of the STROBE Statement. The meaning and rationale for each checklist item are presented. For each item, one or several published examples and, where possible, references to relevant empirical studies and methodological literature are provided. Examples of useful flow diagrams are also included. The STROBE Statement, this document, and the associated Web site (http://www.strobe-statement.org/) should be helpful resources to improve reporting of observational research.

[Strengthening the reporting of observational studies in epidemiology (STROBE): explanation and elaboration]

Much medical research is observational. The reporting of observational studies is often of insufficient quality. Poor reporting hampers the assessment of the strengths and weaknesses of a study and the generalisability of its results. Taking into account empirical evidence and theoretical considerations, a group of methodologists, researchers, and editors developed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) recommendations to improve the quality of reporting of observational studies. The STROBE Statement consists of a checklist of 22 items, which relate to the title, abstract, introduction, methods, results and discussion sections of articles. Eighteen items are common to cohort studies, case-control studies and cross-sectional studies and four are specific to each of the three study designs. The STROBE Statement provides guidance to authors about how to improve the reporting of observational studies and facilitates critical appraisal and interpretation of studies by reviewers, journal editors and readers. This explanatory and elaboration document is intended to enhance the use, understanding, and dissemination of the STROBE Statement. The meaning and rationale for each checklist item are presented. For each item, one or several published examples and, where possible, references to relevant empirical studies and methodological literature are provided. Examples of useful flow diagrams are also included. The STROBE Statement, this document, and the associated Web site (http://www.strobe-statement.org/) should be helpful resources to improve reporting of observational research.

Strengthening the Reporting of Observational Studies in Epidemiology (STROBE): explanation and elaboration

Much medical research is observational. The reporting of observational studies is often of insufficient quality. Poor reporting hampers the assessment of the strengths and weaknesses of a study and the generalisability of its results. Taking into account empirical evidence and theoretical considerations, a group of methodologists, researchers, and editors developed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) recommendations to improve the quality of reporting of observational studies. The STROBE Statement consists of a checklist of 22 items, which relate to the title, abstract, introduction, methods, results and discussion sections of articles. Eighteen items are common to cohort studies, case-control studies and cross-sectional studies and four are specific to each of the three study designs. The STROBE Statement provides guidance to authors about how to improve the reporting of observational studies and facilitates critical appraisal and interpretation of studies by reviewers, journal editors and readers. This explanatory and elaboration document is intended to enhance the use, understanding, and dissemination of the STROBE Statement. The meaning and rationale for each checklist item are presented. For each item, one or several published examples and, where possible, references to relevant empirical studies and methodological literature are provided. Examples of useful flow diagrams are also included. The STROBE Statement, this document, and the associated Web site (http://www.strobe-statement.org/) should be helpful resources to improve reporting of observational research.

Worksite interventions to increase stair climbing; reasons for caution

Point-of-choice prompts to use the stairs rather than the escalator consistently increase physical activity at public access staircases such as those in shopping malls. More recently, exercise promoters have targeted stair climbing in the worksite. A review of interventions in worksites reveals little hard evidence of successful increases in stair climbing, though the increases in stair usage are encouraging. The contrast between the worksites and public access staircases, however, is not simply one of location. In a worksite, the choice is between the stairs and an elevator rather than an escalator. We reason that the availability of the elevator or the stairwell may be the major immediate determinant of stair climbing in worksites and dilute any possible effects of an intervention.

Shifting dollars, saving lives: What might happen to mortality rates, and socio-economic inequalities in mortality rates, if income was redistributed?

Personal or household income predicts mortality risk, with each additional dollar of income conferring a slightly smaller decrease in the mortality risk. Regardless of whether levels of income inequality in a society impact on mortality rates over and above this individual-level association (i.e., the 'income inequality hypothesis'), the current consensus is that narrowing income distributions will probably improve overall health status and reduce socio-economic inequalities in health. Our objective was to quantify this impact in a national population using 1.3 million 25-59-year-old respondents to the New Zealand 1996 census followed-up for mortality over 3 years. We modelled 10-40% shifts of everyone's income to the mean income (equivalent to 10-40% reductions in the Gini coefficient). The strength of the income-mortality association was modelled using rate ratios from Poisson regression of mortality on the logarithm of equivalised household income, adjusted for confounders of age, marital status, education, car access, and neighbourhood socio-economic deprivation. Overall mortality reduced by 4-13% following 10-40% shifts in everyone's income, respectively. Inequalities in mortality reduced by 12-38% following 10-40% shifts in everyone's income. Sensitivity analyses suggested that halving the strength of the income-mortality association (i.e., assuming our multivariable estimate still overestimated the causal income-mortality association) would result in 2-6% reductions in overall mortality and 6-19% reductions in inequalities in mortality in this New Zealand setting. Many commentators have noted the non-linear association of income with mortality predicts that narrowing the income distribution will both reduce overall mortality rates and reduce inequalities in mortality. Quantifying such reductions can only be done with considerable uncertainty. Nevertheless, we tentatively suggest that the gains in overall mortality will be modest (although still potentially worthwhile from a policy perspective) and the reductions in inequalities in mortality will be more substantial.

Psychosocial factors and public health: a suitable case for treatment?

Adverse psychosocial exposure or "misery" is associated with physical disease. This association may not be causal. Rather it may reflect issues of reverse causation, reporting bias, and confounding by aspects of the material environment typically associated with misery. A non-causal relation will not form the basis of effective public health interventions. This may be why psychosocial interventions have, so far, showed little effect on objective physical health outcomes. This paper reviews evidence for the "psychosocial hypothesis" and suggests strategies for clarifying these issues. It concludes that, although misery is clearly a bad thing as it erodes people's quality of life, there is little evidence that psychosocial factors cause physical disease. In the absence of better evidence, suggestions that psychosocial interventions are needed to improve population physical health, in both absolute and relative terms, seem premature.

Mortality and quality of life 12 months after myocardial infarction: effects of depression and anxiety

OBJECTIVE

The purpose of this study was to determine the impact of symptoms of depression and anxiety on mortality and quality of life in patients hospitalized for acute myocardial infarction (MI).

METHODS

The Beck Depression Inventory and the State-Trait Anxiety Inventory were completed by 288 patients hospitalized for MI. Twelve-month survival status was ascertained, and quality of life among survivors was assessed at 12 months using the Dartmouth COOP charts.

RESULTS

Thirty-one (10.8%) patients died, 27 of cardiac causes, during the 12-month follow-up. Symptoms of depression and anxiety predicted neither cardiac nor all-cause mortality. Severity of infarction and evidence of heart failure predicted both cardiac and all-cause mortality. The same findings emerged from supplementary analyses of data from patients who died after discharge from the hospital. Symptoms of depression and anxiety, measured at entry, predicted 12-month quality of life among survivors, as did gender, partner status, employment status, living alone, previous frequency of exercise, and indices of disease severity (Killip class and Peel Index). In a multiple regression model in which all of these variables were entered, initial depression scores provided the best independent prediction of quality of life, although living alone, severity of infarction, and state anxiety also entered the model.

CONCLUSIONS

Symptoms of depression and anxiety did not predict either cardiac or all-cause mortality after MI, but they did predict quality of life among those who lived to 12 months.

Effects of depression and anxiety on mortality and quality-of-life 4 months after myocardial infarction

OBJECTIVE

The purpose of this study was to determine the impact of depression and anxiety on mortality and quality-of-life in patients hospitalized for an acute myocardial infarction (MI).

METHODS

Questionnaire measures of depression and anxiety were completed during hospitalization by 288 MI patients. The main outcomes were mortality and quality-of-life, assessed by the Dartmouth COOP charts, at 4 months.

RESULTS

A total of 25 patients died, 22 from cardiac causes, during the 4-month follow-up. Symptoms of depression and anxiety did not predict either cardiac or all-cause mortality. Severity of infarction, extent of heart failure, and a longer stay in hospital predicted mortality. Symptoms of depression and anxiety predicted 4-month quality-of-life among survivors, as did gender, partner status, occupational status, living alone, previous exercise behaviour, length of hospital admission, and Peel Index scores. In a multiple regression model, depression emerged as the strongest predictor of quality-of-life. State anxiety, severity of infarction, and partner status also entered the model.

CONCLUSION

Neither depression nor anxiety predicted mortality 4 months after MI. Both depression and anxiety predicted quality-of-life at 4 months among survivors.

Hyperleptinemia as a component of a metabolic syndrome of cardiovascular risk

In humans, production of the adipocyte-derived peptide leptin has been linked to adiposity, insulin, and insulin sensitivity. We therefore considered that alterations in plasma leptin concentrations could constitute an additional component of a metabolic syndrome of cardiovascular risk. To explore this hypothesis, we employed factor analysis, a multivariate statistical technique that allows reduction of large numbers of highly intercorrelated variables to composite, biologically meaningful factors. Seventy-four men [age, 48.4+/-1.3 years (mean+/-SEM); body mass index (BMI), 25.6+/-0.3 kg/m2] who were free of coronary heart disease and diabetes underwent anthropometric measurements (subscapular-to-triceps [S:T] and subscapular-to-biceps [S:B] skinfold thickness ratios, measurement of fasting plasma leptin, and an intravenous glucose tolerance test (IVGTT) for assessment of insulin sensitivity. Plasma leptin concentrations were correlated with BMI (r=0.57, P<0.001), S:T (r=0.34, P=0.003), S:B (r=0.37, P<0.001), systolic and diastolic blood pressures (both r=0.24, P=0.044), fasting triglycerides (r=0.31, P=0.007), serum uric acid (r=0.35, P=0.003), fasting glucose (r=0.32, P=0.003) and insulin (r=0.33, P=0.004), and IVGTT insulin (r=0.63, P<0.001). A negative correlation was observed between leptin and insulin sensitivity (r=-0.32, P=0.006). No significant correlations emerged between plasma leptin concentrations and age, high density lipoprotein cholesterol, or IVGTT glucose. In multivariate regression analyses, BMI (standardized coefficient [SC]=0.40, P=0.001), fasting insulin (SC=0.23, P=0.036), and IVGTT insulin (SC=0.51, P<0.001) emerged as independent predictors of plasma leptin concentrations (R2=0.56, P<0.001). After adjustment for BMI, only IVGTT insulin emerged as a significant predictor of plasma leptin concentrations (SC=0.56, P<0.001, R2=0.45, P<0.001). Factor analysis of plasma leptin concentrations and the variables that are considered relevant to the insulin resistance syndrome revealed a clustering of plasma leptin concentrations with a factor dominated by insulin resistance and high IVGTT insulin, separate from a high IVGTT glucose/central obesity factor and a high triglyceride/low high density lipoprotein cholesterol factor. Together, these factors accounted for 55.9% of the total variance in the dataset. In conclusion, interindividual variations in plasma leptin concentrations are strongly related to the principal components of the insulin resistance syndrome. Further studies are needed to determine whether the insulin-leptin axis plays a coordinating role in this syndrome and whether plasma leptin concentrations could provide an additional measure of cardiovascular risk.

Factors of the metabolic syndrome: baseline interrelationships in the first follow-up cohort of the HDDRISC Study (HDDRISC-1). Heart Disease and Diabetes Risk Indicators in a Screened Cohort

Syndromes of risk factor disturbance may contribute to the development of coronary heart disease and non-insulin-dependent diabetes mellitus, but their definition and quantification remain problematic. Using factor analysis, constellations of risk factor variables that could indicate distinct syndromes of metabolic disturbance were explored in the baseline data of the first follow-up cohort of 742 men from the Heart Disease and Diabetes Risk Indicators in a Screened Cohort (HDDRISC) study. The primary analysis considered 16 intercorrelated variables measured in more than 90% of cohort participants. A missing-values estimation routine was used to ensure inclusion of all participants in the analysis. Subanalyses were undertaken, including a repeat of the primary analysis on the 522 individuals who had received measurement of HDL cholesterol, an oblique rather than orthogonal factor rotation procedure performed on primary and HDL subset analyses, a repeat of these two primary and HDL subset analyses using only those participants with complete measurements, and a repeat of these six analyses including only the seven variables conventionally associated with the metabolic syndrome. The principal factor that emerged in all analyses undertaken comprised oral glucose tolerance test insulin and glucose response, serum uric acid, and body mass index. Fasting serum triglyceride concentration was included in this factor in 11 of the 12 analyses undertaken, fasting plasma insulin in 8, fasting plasma glucose in 5, and mean arterial pressure in 3. HDL cholesterol factored in isolation from insulin in all analyses undertaken. These findings provide strong support for a core metabolic cluster, which is unlikely to include blood pressure and does not include HDL. The factor scores relating to this cluster will provide a means of assessing its quantitative importance in prospective analysis of the development of CHD and diabetes in this cohort.

Does insulin resistance unite the separate components of the insulin resistance syndrome? Evidence from the Miami Community Health Study

A number of coronary heart disease risk factors have been identified that often cluster together to increase the risk of macrovascular disease. This cluster is referred to as the insulin resistance syndrome, and the risk factors commonly include dyslipidemia, elevated blood pressure, an android pattern of body fat distribution, and glucose intolerance. Whether hyperinsulinemia or insulin resistance per se provides a common pathway for these metabolic abnormalities is unclear. The authors studied 50 nondiabetic persons who had completed a euglycemic hyperinsulinemic clamp protocol in addition to a 75-g oral glucose tolerance test and other measures of the coronary risk profile. Using principal-component analysis, we reduced nine coronary risk factors to two uncorrelated factors that explained 54.5% of the variance. Factor 1 consisted of positive loadings for uric acid, systolic and diastolic blood pressure, triglyceride concentration, and waist girth and negative loadings for HDL cholesterol and the rate of insulin-mediated glucose disposal (M, in milligrams per kilogram of body weight per minute). M also loaded on factor 2, along with fasting insulin and glucose concentrations, diastolic blood pressure, and waist girth. The observation that M loaded on both factors suggests that a resistance to insulin action may provide the mechanism uniting the features of the insulin resistance syndrome. Hyperinsulinemia with concomitant insulin resistance may be necessary to produce this metabolic derangement, as well as the increased risk of macrovascular complications.

Interpreting epidemiologic studies of diet-disease relationships

The purpose of this paper is to examine key issues in the interpretation of nutritional epidemiologic study results when the focus is on major chronic degenerative diseases of multifactorial etiology. The estimation of disease risk associated with a particular dietary factor is influenced by the presence of other risk factors within the study population, complicating the interpretation of relative risk and odds ratio estimates in this context. Identifying the precise role(s) that dietary factors play in the onset or progression of chronic diseases is further complicated by the intercorrelation of dietary components and by the correlation of dietary patterns with other behavioral and environmental factors which may also impart or exacerbate risk of disease. Issues of study design and measurement make it difficult to identify relationships in nutritional epidemiology, but also thwart the rejection of hypotheses regarding diet-disease relationships when studies fail to yield significant associations. In drawing causal inferences from epidemiologic findings, it is important to examine evidence from a variety of sources and to look for congruence between epidemiologic, clinical and laboratory research findings.

Socioeconomic differentials in mortality risk among men screened for the Multiple Risk Factor Intervention Trial: I. White men

OBJECTIVES

This study examined socioeconomic differentials in risk of death from a number of specific causes in a large cohort of White men in the United States.

METHODS

For 300 685 White men screened for the Multiple Risk Factor Intervention Trial between 1973 and 1975, data were collected on median income of White households in the zip code of residence, age, cigarette smoking, blood pressure, serum cholesterol, previous myocardial infarction, and drug treatment for diabetes. The 31 737 deaths that occurred over the 16-year follow-up period were grouped into specific causes and related to median White family income.

RESULTS

There was an inverse association between age- adjusted all-cause mortality and median family income. There was no attenuation of this association over the follow-up period, and the association was similar for the 22 clinical centers carrying out the screening. The gradient was seen for many-but not all-of the specific causes of death. Other risk factors accounted for some of the association between income and coronary heart disease and smoking-related cancers.

CONCLUSIONS

Socioeconomic position, as measured by median family income of area of residence, is an important determinant of mortality risk in White men.

The design of prospective epidemiological studies: more subjects or better measurements?

Prospective epidemiological studies which seek to relate potential risk factors to the risk of disease are subject to appreciable biases which are often unrecognized. The inability to precisely measure subjects' true values of the risk factors under consideration tends to result in bias towards unity in the univariate relative risks associated with them--the more imprecisely a risk factor is measured, the greater the bias. When correlated risk factors are measured with different degrees of imprecision the adjusted relative risk associated with them can be biased towards or away from unity. When designing a new prospective study cost considerations usually limit the total number of subject-evaluations that are available. The usual design approach is to maximize the study size and evaluate each subject on one occasion only. An alternative approach involves recruitment of a smaller number of subjects so that each can be evaluated on more than one occasion, thus resulting in a more precise measure of subjects' risk factor values and hence less bias in the relative risk estimates. In this paper we use a simulation approach to show that under conditions that prevail for most major prospective epidemiological studies the latter approach is actually more likely to produce accurate relative risk estimates. This emphasizes the importance of bias due to exposure measurement imprecision and suggests that attempts to anticipate and control it be given at least as high a priority as that given to sample size assessment in the design of epidemiological studies.

Relation of passive smoking as assessed by salivary cotinine concentration and questionnaire to spirometric indices in children

BACKGROUND

Previous studies of the effects of passive exposure to smoke on spirometric indices in children have largely relied on questionnaire measures of exposure. This may have resulted in underestimation of the true effect of passive smoking. Biochemical measures offer the opportunity to estimate recent exposure directly.

METHODS

The relation between spirometric indices and passive exposure to tobacco smoke was examined in a large population sample of 5-7 year old children from 10 towns in England and Wales. The effects of passive exposure to smoke on lung function were assessed by means of both salivary cotinine concentration and questionnaire measurements of exposure. Analyses of the relation between spirometric values and cotinine concentrations were based on 2511 children and of the relation between spirometric values and questionnaire measures on 2000 children.

RESULTS

Cotinine concentration was negatively associated with all spirometric indices after adjustment for confounding variables, which included age, sex, body size, and social class. The strongest association was with mid expiratory flow rate (FEF50), the fall between the bottom and top fifths of the cotinine distribution being 6%, equivalent to a reduction of 14.3 (95% confidence limits (CL) 8.6, 20.0) ml/s per ng/ml cotinine. Salivary cotinine concentrations were strongly related to exposure to cigarette smoke at home but 88% of children who were from non-smoking households and not looked after by a smoker had detectable cotinine concentrations, 5% being in the top two fifths of the cotinine distribution. A composite questionnaire score based on the number of regular sources of exposure was as strongly related to mid and end expiratory flow rates as the single cotinine measure. The fall in FEF50 per smoker to whom the child was exposed was 51.0 (26.5, 75.5) ml/s. The relationships between the questionnaire score and forced vital capacity (FVC) or forced expiratory volume in one second (FEV1) were not statistically significant.

CONCLUSIONS

These effects of passive smoking on respiratory function are consistent with the results of previous studies and, although small in absolute magnitude, may be important if the effects of exposure are cumulative. In children aged 5-7 years the use of a single salivary cotinine concentration as a marker of passive exposure to smoke resulted in clear relationships between exposure and FVC and FEV1, whereas the associations were much weaker and not significant when based on the questionnaire score. The associations between exposure and mid or end expiratory flow rates were of similar magnitude for cotinine concentration and the questionnaire score. The use of salivary cotinine concentration in longitudinal studies may help to determine the extent to which these effects are cumulative or reversible.

Plasma triglyceride and high density lipoprotein cholesterol as predictors of ischaemic heart disease in British men. The Caerphilly and Speedwell Collaborative Heart Disease Studies

OBJECTIVE

To assess the roles of plasma triglyceride and high density lipoprotein (HDL) cholesterol concentrations in predicting ischaemic heart disease.

DESIGN

Two prospective cohort studies with common core protocols.

SETTING AND PARTICIPANTS

Both cohorts are 100% samples of middle aged men. In Caerphilly the 2512 men were living within a defined area. In Speedwell the 2348 men were registered with local general practitioners.

MAIN OUTCOME MEASURES

Fasting blood samples were taken at initial examination and plasma lipid concentrations were measured. Major ischaemic heart disease events were assessed from hospital notes, death certificates, and electrocardiograms.

RESULTS

At first follow up, after an average of 5.1 years in Caerphilly and 3.2 years in Speedwell, 251 major ischaemic heart disease events had occurred. Men with triglyceride concentrations in the top 20% of the distribution had a relative odds value for ischaemic heart disease of 2.3 (95% confidence interval (95% CI) 1.3 to 4.1) compared with men in the bottom 20%, after adjusting for both plasma total and HDL cholesterol, and non-lipid risk factors. Men in the lowest 20% of the distribution of HDL cholesterol concentration had a relative odds value of 1.7 (95% CI 1.0 to 2.8) compared with the top 20%, after adjustment was made for total cholesterol and triglyceride concentrations, and non-lipid risk factors. These relations were not caused by beta blockers, which were being taken by 5% of the men.

CONCLUSIONS

Plasma triglyceride concentration predicts major ischaemic events after allowance is made for total and HDL cholesterol concentrations and other risk factors. In these populations, triglyceride is a more important predictor than total cholesterol concentration.

Confounding in epidemiological studies: why "independent" effects may not be all they seem

Images

Bias in relative odds estimation owing to imprecise measurement of correlated exposures

A series of graphs is presented that show the estimated degree of bias in logistic coefficient estimates for two correlated continuous exposures measured with imprecision. These graphs indicate that even when the correlation coefficient between the exposure of interest and a correlated exposure is as low as 0.2, imprecision in the measurement of the latter exposure can result in at least as serious bias in the logistic coefficient estimate for the exposure of interest as measurement imprecision in the exposure of interest itself. The implications for the design and interpretation of epidemiological studies are discussed.

Risk of severe hypoglycaemia in insulin treated diabetic patients transferred to human insulin: a case control study

OBJECTIVE

To examine whether transfer from animal insulin to human insulin is associated with an increased risk of severe hypoglycaemia.

DESIGN

Matched case-control study of insulin treated diabetic patients admitted to hospital because of hypoglycaemia during 1984-7, the period when human insulin was introduced into treatment.

SETTING

Case admissions and control admissions were obtained from eight public hospitals within the Swiss canton of Berne and a second control group comprised members of the Bernese section of the Swiss Diabetes Association.

SUBJECTS

94 patients with insulin treated diabetes with a total of 112 admissions for hypoglycaemia during 1984-7 (case admissions), 182 patients with insulin treated diabetes seen in the same hospitals for reasons other than hypoglycaemia with a total of 225 admissions (control admissions), and 86 insulin treated diabetic patients who were members of the Bernese section of the Swiss Diabetes Association.

MAIN OUTCOME MEASURES

Type of insulin used at time of admission, glycaemic control as measured by amount of glycated haemoglobin or glucose concentration; severity of hypoglycaemia.

RESULTS

Treatment with human insulin at admission was more common in cases than controls (52/112 (46%) admissions v 77/225 (34%); p = 0.003). 116 out of 129 (90%) of admissions taking human insulin had been transferred from animal insulin, mainly because of non-availability of porcine insulins. The ratio of rate of hypoglycaemia in those taking human insulin to the rate in those taking animal insulin was 2.4 (95% confidence interval 1.3 to 4.4). Other risk factors for hypoglycaemia were a history of hypoglycaemic coma (rate ratio of history to no history 3.8, 2.3 to 6.4) and good glycaemic control (rate ratio of good to poor control 3.9, 1.4 to 7.5). With multivariate analysis the increase in rate ratio associated with use of human insulin rose to 3.0 (1.4 to 6.4). Comparison with the diabetes association controls also showed an increased risk associated with use of human insulin (2.2; 1.1 to 4.8).

CONCLUSIONS

Transfer of treatment from animal insulin to human insulin was associated with an increased risk of severe hypoglycaemia. Caution should be exercised when transferring diabetic patients to human insulin. Further studies are required to elucidate why this effect occurs.

Risk factors for stroke in middle aged British men

OBJECTIVE

To determine the risk factors for stroke in a cohort representative of middle aged British men.

DESIGN

Prospective study of a cohort of men followed up for eight years.

SETTING

General practices in 24 towns in England, Wales, and Scotland (the British regional heart study).

SUBJECTS

7735 men aged 40-59 at screening, selected at random from one general practice in each town.

MAIN OUTCOME MEASURE

Fatal and non-fatal strokes.

RESULTS

110 of the men had at least one stroke; there were four times as many non-fatal as fatal strokes. The relative risk of stroke was 12.1 in men who had high blood pressure (systolic blood pressure greater than or equal to 160 mm Hg) and were current smokers compared with normotensive, non-smoking men. Diastolic blood pressure yielded no additional information, and former cigarette smokers had the same risk as men who had never smoked. Heavy alcohol intake was associated with a relative risk of stroke of 3.8 in men without previously diagnosed cardiovascular disease. Men with pre-existing ischaemic heart disease had an increased risk of stroke, but only when left ventricular hypertrophy on electrocardiography was also present.

CONCLUSIONS

Systolic blood pressure, cigarette smoking, and left ventricular hypertrophy on electrocardiography in men with pre-existing ischaemic heart disease were found to be the major risk factors for stroke in middle aged British men. Heavy alcohol intake seemed to increase the risk of stroke in men without previously diagnosed cardiovascular disease. A large proportion of strokes should be preventable by controlling blood pressure and stopping smoking.