There are many studies on the independent effects of diet and exercise on chronic disease. But a new area of interest currently emerging is the consideration of how exercise and nutrition can work together in preventing and treating illness. To date, clinical studies show a possible synergistic effect (ie, greater than simply additive) of diet and exercise on the prevention and treatment of diabetes, heart disease and osteoporosis. This article presents a summary of research for each of these areas - focussing particularly on how exercise can have an impact on metabolic factors..

Diabetes Adult onset diabetes (also known as Type 2 diabetes, or Non Insulin Dependent Diabetes Mellitis, NIDDM) is associated with an increased risk of heart disease, stroke, kidney failure and blindness. The strongest predisposing risk factors for developing NIDDM are obesity, and a family history of diabetes..

People with this type of diabetes share a number of common disturbed metabolic responses. One is impaired glucose tolerance - when a standard amount of glucose is given after an overnight fast, blood glucose rises to an excessively high level, such that some sugar is lost in the urine. A linked phenomenon is insulin resistance. Body cells have receptors on their surface walls which get triggered when insulin comes by, leading to various messages being passed on to the cells. The most important are messages to take up glucose and fats into the cell from the bloodstream. For some reason, people with NIDDM have less responsive insulin receptors - so, for these individuals, the messages do not get through loud and clear, and higher levels of glucose and fats are left coursing round in the blood. This in turn has detrimental effects on many body systems. Thus, in NIDDM, the problem is not a lack of insulin (in fact, typically blood levels of insulin are elevated), but a dampened response..

There has been a lot of research focus on how nutrition may be used to help to prevent and manage NIDDM. For example, populations with higher fat intakes have been found to have a higher incidence of diabetes. Dietary fats appear to affect both glucose tolerance and insulin sensitivity. In a similar way to the effects of fats on blood cholesterol, saturated fats appear to be detrimental, while mono-unsaturated and polyunsaturated fats (especially the type found in oily fish) are beneficial. In contrast to the fat story, there are no clear answers as to whether eating a lot of refined sugar increases risk of developing the disease. Being overweight is also a clear risk factor in developing NIDDM - it increases the likelihood of suffering from insulin resistance..

The role of exercise is less clear, but there is plenty of suggestive evidence for physical activity helping to prevent adult onset diabetes. For example, physically active societies have less NIDDM than less active communities, and as populations have become more sedentary the incidence of NIDDM has increased. Why should this be? One obvious route is via an effect on body weight - active exercisers tend to be less portly than less active people. But a number of recent studies have found that exercise has a beneficial effect even when no difference in body weight occurs. A large-scale American study followed 87,253 women for eight years. Those who exercised vigorously at least once a week were much less likely to develop diabetes than the women who exercised less, even when differences in body weight were taken into account. The exercise-related reduction in risk was seen amongst both overweight and non-obese women..

In another American study, 5,990 male graduates of the University of Pennsylvania were followed up for14 years. Levels of leisure-time activity were found to be inversely related to the development of NIDDM. When leisure-time activity was quantified in terms of how many calories were burned up, analysis revealed that for every 500 calorie increase in energy expenditure per week, the risk of developing adult-onset diabetes was reduced by 6 per cent The reduction in risk held even when obesity and parental history of diabetes were taken into account. To give an idea of the levels of activity involved, for the average man weighing 74kg, half an hour's moderate pace cycling uses up 222 calories; half an hour's cross-country running uses up 363 calories, and half an hour of squash uses up 471 calories ('Physical activity and reduced occurrence of NIDDM', Helmrich et al, New England Journal of Medicine, Vol 325, pp147-52)..

When physical changes in response to exercise are put under the microscope, some interesting effects are found. Exercise has a direct effect on some of the variables also affected by diet. Regular activity has been shown to increase insulin sensitivity and lower blood insulin concentrations. It also improves the body's ability to shunt glucose into the cells. Conversely, reduced activity leads to reduced insulin sensitivity and glucose tolerance. Thus, exercise not only helps to reduce the risk of NIDDM, but for individuals who have already developed the disease, activity can help manage its effects. Exercise has been found to improve insulin sensitivity in patients with NIDDM for up to 72 hours after ceasing exercise..

Therefore the optimum strategy for anyone wishing to prevent development of NIDDM (particularly relevant to those with a family history of the disease) would be to follow standard recommendations for a healthy diet, while also taking regular exercise - they will then benefit from an additive effect. Similarly, those who have already developed the condition would benefit from the dual strategy of diet and activity..