The London Busmen Study

Exercise in disease prevention:

This time I am telling you about one of the landmark studies to investigate the role of exercise in disease prevention. A team of investigators at the London School of Hygiene and Tropical Medicine proposed that regular exercise might reduce the risk of developing coronary disease.  The investigation was the famous London Busmen Study whose leader was Professor Jerry Morris. I was fortunate enough to know Jerry Morris and to work with him as one of the directors of the Coronary Prevention Group.

The study compared the rates of coronary heart disease (CHD) between drivers and conductors of the London Transport Executive. The investigators proposed that: “Men in physically active jobs [conductors] have a lower incidence of coronary heart-disease in middle age men than have men in physically inactive jobs [drivers]. More important, the disease is not so severe in physically active workers, tending to present first in them as angina pectoris and other relatively benign forms, and to have a smaller early case-fatality and a lower early mortality-rate.”

Methods:

Special arrangements were made to examine the medical and absence records of the employees between the investigators, the Chief Medical Officer and the Staff Administration Officer of the London Transport Executive.  About 31,000 men aged 35-64 employed as bus, trams and trolleybus drivers and conductors, motormen and underground railway guards were observed. These employees were chosen “because the numbers in each grade were large and the groups were homogeneous as regards occupation.” All sickness absences were recorded from the Central Record of Staff Statistics, and subsequent medical diagnoses were obtained from the employees’ general practitioners, hospitals and London Transport medical officers. Cases of heart disease with absences of any duration were flagged and coded. Copies of death certificates were also checked for any indication of coronary heart disease deaths.

Results:

The investigators found 80 cases of coronary heart disease (angina pectoris, myocardial infarction and immediate mortality from CHD) in 30,726 person-years for drivers, and 31 cases in 19,166 person-years for conductors. The annual rate of CHD for drivers was 2.7 per 1,000, and 1.9 per 1,000 for conductors. The investigators did not provide additional statistical analysis on these results, but did make note that the differences were likely significant: “Is this a chance phenomenon? Statistical tests suggest that it is unlikely. It is also worth recollecting that, although numbers of cases are too small for many rates to be calculated, the distribution of the various types of first presentation of coronary heart disease differs in the conductors of central buses from that of the drivers of central buses; likewise, the distribution of cases in the tram and trolleybus conductors differs from that in their drivers.”

Conclusions:

The investigators concluded that employees in positions that required high physical activity had lower rates of coronary heart disease. They then developed several research topics to examine the differences found, including early experiences of drivers in terms of health and occupational choices, occupational mental strain and the effect of greater physical activity among workers in different social classes. A subsequent analysis was made on postal workers and telephonists in the British Civil Service soon after the conclusion of the transport workers study in 1950. In 1952, with the data obtained from this study, Morris and other investigators examined physique and coronary heart disease by observing uniform size of conductors and drivers. A strength of the initial study was that it was one of the earliest that examined an entire population of men employed in occupations contrasting in physical activity. However later interrogation of the data found that at entry to the jobs the drivers had larger uniform requirements than conductors. So maybe the drivers were fatter and at greater risk of coronary disease before they took up their jobs?

Is there a threshold?

Another conclusion of this and subsequent studies was that there was a threshold of exercise needed to be taken to provide protection. This was about 40 minutes of moderately vigorous exertion on three to four days per week. Further work has argued against this hypothesis and it is now believed that much lower levels of exercise are also protective though the effectiveness increases with increasing doses of exercise.

Indeed, the greatest impact of exercise on disease protection is gained from increasing from doing nothing to doing something! – though it is still true that the more the better.