ENDOCRINES, STRESS, AND HEREDITY ON ATHEROSCLEROSIS 



exercised group. However, he also found an increased 

 incidence of myocardial infarction in the exercised 

 group despite the decrease in gross aortic and coronary 

 atherosclerosis. Serum cholesterol levels were found 

 to be significantly lower in rats forced to swim than 

 in sedentary controls and in pair-gained sedentary 

 controls (69). Orma (1 17), Warnock et al. (168), and 

 Wong et al. (180) reported that exercise decreased 

 hypercholesterolemia and atherogenesis in cholesterol- 

 fed cockerels. McAllister et al. (92), on the contrary, 

 found more severe atherosclerosis in exercised, 

 cholesterol-fed, hypothyroid dogs as compared to 

 sedentary ones. Their findings are complicated by 

 the fact that the exercised dogs were ingesting their 

 rations as meals while the sedentary hypothyroid 

 animals, with the usual depression of appetite, ate 

 their food slowly over the entire 24-hour period. 

 Such differences in feeding pattern in chicks have 

 been shown to influence the atherogenicity of the 

 diet per se (28). 



Data in man relating physical activity to anatomical 

 atherosclerosis or clinical coronary disease are even 

 more difficult to evaluate. Several investigators (78, 

 97, 160) observed that increasing the caloric intake 

 did not produce the expected increase in serum 

 lipoprotein and cholesterol levels when the subjects 

 were exercised intensely enough to prevent weight 

 gain. They concluded that only a positive caloric 

 balance over a long-time period could elevate serum 

 lipid levels. 



Pomeroy & White (125) reviewed the life history 

 of former football players and found fewer deaths 

 from cardiovascular disease among those who con- 

 tinued a program of regular exercise into the middle 

 years than among those who stopped physical activity 

 after their school years. 



The most indicative data relating the amount of 

 physical activity at work with a decreased incidence 

 of death from atherosclerotic vascular disease, 

 particularly ischemic heart disease, come from the 

 studies of Morris in Great Britain (104, 106). His 

 data were obtained from a relatively homogenous 

 population of a similar socio-economic group : by a 

 comparison of sedentary bus drivers with physically 

 active conductors, by a comparison of sedentary 

 telephone operators with active postmen, and by 

 other comparisons of a similar nature. His findings 

 indicate that the incidence of ischemic heart disease 

 in middle age tends to be lower in the groups habit- 

 ually engaged in a greater amount of physical 

 activity. These investigations, although indicative, 

 are not to be taken as final proof, because, in the bus 



workers at least, there was a difference in obesity — 

 the drivers were more obese from the start than the 

 conductors, as judged by the size of the uniforms 

 (103). This leaves open the question of whether the 

 difference between jobs was fortuitous, dependent 

 upon self-selection, which in turn was dependent on 

 temperament and body build of the individual 

 worker. Studies from other countries, i.e., Sweden, 

 Finland, and Italy (74), with a generally high 

 morbidity and mortality rate from atherosclerotic 

 heart disease, are not so clear cut as the British 

 studies. Studies from the United States show no 

 difference between active and sedentary groups in an 

 urban population (153, 157); however, farmers have 

 less atherosclerotic heart disease than city dwellers. 

 Some authors suggest that continued physical activity 

 through middle age may be of possible benefit in the 

 prevention of atherosclerotic disease ( 1 79). 



One fact clearly emerges from these studies: that 

 no difference between physically active and inactive 

 groups can be observed in populations with a low 

 incidence of atherosclerotic heart disease and low 

 mean serum lipid levels. In populations with a high 

 incidence, however, there is a difference in some 

 but not in all countries. Furthermore, even where a 

 difference has been well documented, as in Great 

 Britain, this is only relative; the absolute incidence of 

 this disease in the physically active is still high com- 

 pared to all groups in a country with a low incidence. 

 Therefore, physical activity must play a minor role 

 compared to other factors such as diet. 



The mechanism by which physical activity might 

 influence atherosclerosis is not clear. The data 

 regarding serum cholesterol and lipoprotein levels 

 suggest an influence via metabolism. Other data also 

 indicate that the factors preventing blood coagulation 

 and aiding fibrinolysis are favorably influenced by 

 heavy physical work (11, 12, 77). This was pointed 

 out in human studies, and Warnock et al. (168) report 

 the same effect in chicks. These latter effects may be 

 important, particularly since Morris' work points to 

 a decrease of coronary thrombosis and major occlusion 

 in active middle-aged men, without any noticeable 

 decrease in vascular atheroma and diffuse, nonfatal 

 myocardial fibrosis (104). 



Furthermore, physical work may have another 

 effect. There are several studies indicating a stimula- 

 tion of the production of intercoronary anastomoses by 

 physical work (13, 105, 184). Nor must it be over- 

 looked that physical activity is a form of training 

 which permits the body to adjust more readily to 

 periods of stress. 



