Letters

Review of interventions to prevent heart disease

Study neglected to examine benefits of exercise

Editor–In their systematic review of interventions to reduce risk factors for coronary heart disease Shah Ebrahim and George Davey Smith examined changes in blood pressure, smoking, and blood cholesterol concentration.¹ Meta-analysis supports physical inactivity as a fourth risk factor.^{2 3} Although the relative risk of physical inactivity (1.9) is similar to that of hypertension, smoking, and hypercholesterolaemia,⁴ the population attributable risk is higher owing to its greater prevalence.⁵ This suggests that interventions to increase physical activity may be efficacious in reducing the incidence of coronary heart disease.

Exercise was an intervention in nine of the 14 studies reviewed, yet no analysis of change in exercise was reported. Although disparate measurement methods often make the comparison of physical activity difficult between studies, conclusions about the efficacy of the exercise interventions would inform policymakers and be topical in light of the Health Education Authority's current national campaign to promote physical activity.

David Batty, Researcher ^a

^a Exercise and Health Research Unit, University of Bristol, Bristol BS8 2LU

1. Ebrahim S, Davey Smith G. Systematic review of randomised controlled trials of multiple risk factor interventions for preventing coronary heart disease. BMJ 1997;314:1666-74. (7 June.) [Abstract/Free Full Text]
2. Powell KE, Thompson PD, Caspersen CJ, Kendrick JS. Physical activity and the incidence of coronary heart disease. Annu Rev Public Health 1987;8:253-87.
3. Berlin JA, Colditz GA. A meta-analysis of physical activity in the prevention of coronary heart disease. Am J Epidemiol 1990;132:612-28.
4. Pooling Project Research Group. Relationship of blood pressure, serum cholesterol, smoking habit, relative weight and ECG abnormalities to incidence of major chronic events: final report of the pooling project. J Chron Dis 1978;31:201-306.
5. Caspersen CJ, Christenson GM, Pollard RA. Status of the 1990 physical fitness and exercise objectives—evidence from NHIS 1985. Public Health Rep 1986;101:587-92. [Medline]

Authors' reply

Editor–We agree with David Batty that a lack of exercise is an important risk factor in cardiovascular disease. The mechanisms by which exercise may influence the risk of cardiovascular disease are not clear but may include effects on other known risk factors. In a meta-analysis of 22 randomised controlled trials of exercise substantial reductions in blood pressure were reported, but these may have been confounded by concomitant weight loss.¹ Falls in body weight probably confound meta-analyses of the role of exercise in reducing blood cholesterol concentration too.² No effects of exercise on blood pressure or blood cholesterol concentration were found in the exercise trials reporting follow up of at least six months; this suggests that exercise interventions lack long term efficacy in reducing risk factors.³

The meta-analysis cited by Batty⁴ is frequently used to support the view that exercise reduces the risk of cardiovascular diseases. Predominantly non-experimental observational studies were included that do not give any indication of the ability of exercise interventions to alter behaviour.

Our recent review of multiple risk factor interventions did not examine pooled changes in exercise because only four of the nine studies that used an exercise component provided information about changes in exercise, and this was inconsistently reported (1). Self reports of health behaviours in necessarily unblinded trials always raise the possibility of bias towards inflation of positive behaviour. Meta-analysis of the results shown in the 1 is meaningless since the studies are concerned with quite different aspects of exercise behaviour, measured in different ways. With the exception of one study the reported effects seem to be small; in the Tromsø study activity levels declined over six years in both the intervention and control groups.

The findings of our review do not support the use of personal or family counselling and education in the general population. Further research is needed to identify more effective interventions in exercise, diet, and smoking for use in primary prevention. Among patients who have clear evidence of cardiovascular disease, personal counselling and educational interventions seem to be beneficial, although those focusing only on exercise may not be as effective in reducing mortality from coronary heart disease as those advising on diet and smoking as well as on exercise.³

Shah Ebrahim, Professor of clinical epidemiology,^b George Davey Smith, Professor of clinical epidemiology ^c

^b Department of Primary Care and Population Sciences, Royal Free Hospital School of Medicine, London NW3 2PF, ^c Department of Social Medicine, University of Bristol, Bristol BS8 2PR

1. Arroll B, Beaglehole R. Does physical activity lower blood pressure: a critical review of the clinical trials. J Clin Epidemiol 1992;45:439-47.
2. Vu Tran Z, Weltman A. Differential effects of exercise on serum lipid and lipoprotein levels seen with changes in body weight. A meta-analysis. JAMA 1985;254:919-24. [Abstract]
3. Ebrahim S, Davey Smith G. Health promotion in older people for cardiovascular disease prevention—a systematic review and meta-analysis. London: Health Education Authority, 1996.
4. Berlin JA, Colditz GA. A meta-analysis of physical activity in the prevention of coronary heart disease. Am J Epidemiol 1990;132:612-28.