Jeremy Morris on Physical Activity and Coronary Disease
Morris reviews his tests of the hypothesis about physical activity and protection from coronary heart disease from the early 1950s through the 1980s, starting with this colorful description: “Conductors on London’s double-decker buses (up and down stairs 11 days a fortnight, 50 weeks a year, often for decades) experienced half or less the incidence of acute myocardial infarction and ‘sudden death’ ascribed to coronary heart disease [CHD] in the sedentary bus drivers.” He indicates that postmen compared to clerks show the same protection. He soon detected selection bias of body mass at entry in these employees, but found that the conductors, whether they were “slim, average, or portly suffered about half or less the incidence of the drivers” (Morris 2005, 275).
Morris philosophized that because occupations were becoming increasingly sedentary, free of any great physical demand, that any future role of physical activity in protection from vascular disease would have to be related to leisure time activity outside the occupation and thus proceeded to set up a number of studies in this regard, the first being among UK civil servants in the Whitehall study.
Morris was surprised and disappointed to find that this study showed no relationship of leisure time activity to coronary disease until very high levels of vigorous exercise were attained, in leisure activities at the level of 6 mets or 7.5 kcal per minute. Since this would entail a vast difference in public health recommendations concerning total activities or vigorous ones, he undertook a new test of the hypothesis in other large civil service populations. His fresh hypothesis was that only vigorous, habitual, frequent aerobic exercise in sports or “getting about” would offer substantial protection against CHD and that other physical activity would not be protective, including high totals of energy expenditure. This was generally confirmed in his next series of studies in which he found the value of vigorous exercise to be quite independent of all other risk factors.
Morris raises the dilemma of excessive statistical adjustment for those risk factors: “These factors themselves are likely to be lowered by exercise, and may indeed be mechanisms of the effect of exercise on CHD. Therefore, their introduction into the multivariate analysis may misleadingly reduce the value of the exercise factor itself in the outcome” (Ibid., 279).
Morris then compares the British with American and other studies that tended to show a greater gradient for levels of activities in which a larger proportion of healthy individuals could and did engage in. He speculated it “could it be that the American cohort is basically less active and less fit than the British and thus capable of benefiting from less intense exercise?” (Ibid., 280). He additionally points out that the British men were a healthy cohort in comfortable civil service jobs and not as representative as the general populations examined elsewhere.
Then Morris restates his hypothesis: “The initial hypothesis has undergone several transformations in the course of its 40 years. It can now be stated as follows: Adequate aerobic exercise in leisure time, which is habitual and ongoing, and the training and improved cardio-respiratory fitness and performance this produces, confers substantial protection against occurrence of CHD in middle-aged and elderly men. The total death rate is also lowered. This is the case whatever the risk status of the men with respect to other [risk] factors. Protection by exercise is effectual mainly in the acute phase of the disease, in particular against thrombosis, though there is also some benefit from reducing and counteracting standard risk factors and the build up of chronic coronary atherosclerosis” (Ibid., 281-2).
Morris has, nevertheless, over the years been the principal public advocate of vigorous physical activity for the population and within a population strategy because “only a minority of the population takes anything worthy of the name of exercise, and only a small minority of the lower social classes that are most vulnerable to heart attack … The return of physical activity as a norm in everyone’s everyday life – the restoration of biological normality in Rose’s words – would require cultural change on the scale similar to that which has occurred with smoking” (Ibid., 283). (Henry Blackburn)
Morris, J.N. 2005. Exercise versus heart attack: History of a hypothesis. In Coronary heart disease epidemiology: From etiology to public health, 275-90. 2nd ed. Oxford: Oxford University Press.