Exercise in the prevention of Coronary Heart Disease (CHD)
In the UK
Back in the 1950s and 1960s, studies of bus drivers and Whitehall civil servants by Professor Jerry Morris, a Scottish doctor working at University College, London, became epidemiological classics. In the former, he and his colleagues compared the rates of heart disease in London bus drivers with those of bus conductors. While the drivers sat all day long, fuming at pesky taxi drivers and other road-users, the conductors shinned up and down stairs getting plenty of exercise. They found that the drivers had a more than 40 per cent higher rate of CHD than the conductors.
The Whitehall civil servant study looked at leisure-time physical activity and again found that the rate of fatal heart attacks in those who took vigorous exercise as recreation was about 40 per cent less that of the inactive, while the rate of non-fatal heart attacks was 50 per cent lower. Morris made several discoveries: that intermittent heavy exercise is more effective than lower-level activity, even with equivalent totals of exercise; that there is a threshold for the protective effect; and there is dose-response relationship above this level – i.e. the more exercise you take, the better the effect.
In the US
In the USA, similar studies were carried out on the San Francisco longshore men (stevedores) and on university graduates. In a study of nearly 17,000 Harvard graduates aged 35–74, taking exercise was found to be inversely related to mortality – that is to say, the more exercise taken the greater the reduction in mortality. A further study from the US followed over 26,000 men and women who had performed an exercise test to determine their levels of physical fitness. The subjects were subdivided by fitness level – high, moderate and low. Over an average follow-up of 10 years, the rate of coronary disease was 11 per cent lower in the moderate-fitness group and 25 per cent lower in the high-fitness group when compared to the low-fitness group.
Since then numerous studies have confirmed the association between regular exercise, physical fitness and protection from CHD. They have shown that the greater the total of physical activity, running, weight training and rowing, the greater the reduction in the risk of CHD. Also, improving fitness level from unfit to fit nearly halves risk compared with remaining unfit. Most tellingly, a meta-analysis of 33 trials, which included over 100,000 subjects followed up for an average of 11 years after an exercise test, divided them into low-, intermediate- and high-fitness categories. The low-fitness group had a 56 per cent greater chance of suffering a heart attack than the high-fitness group.
Even the rather limited doses of exercise recommended by national bodies are protective. In a study of nearly half a million adults in the USA, compliance with exercise guidelines was compared with mortality over nine years. For those following the recommendations for strength-building, the death rate was 82 per cent of that expected; for those following the aerobic exercise recommendations it was 65 per cent; and for those following both it was 50 per cent. One development since the original Studies of Jerry Morris is that the “threshold” effect has not been confirmed. Any amount of exercise has an effect. It is far safer to do a bit than to do nothing.
How does that work?
Some of the ways in which exercise prevents CHD are obvious. Most of the reversible risk factors for coronary atheroma are reduced. Regular exercisers are thinner than non-exercisers, have lower blood pressure, lower blood cholesterol and are less likely to develop diabetes. Apart from stopping smoking, if you do, there is no more effective way of avoiding a heart attack than regular vigorous exercise.
Next week I will talk about exercise as a treatment for heart disease.
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