Exercise and diabetes: The benefits of exercise on diabetes

This article discusses the broad range of issues of exercise in people with established diabetes and also considers the question of exercise in the prevention of diabetes itself and its role in improving metabolic control.

What is Diabetes?

Diabetes mellitus is a group of disorders characterised by hyperglycaemia and disturbances of carbohydrate, fat and protein metabolism. In Type 1 diabetes, the main pathological defect is an absolute deficiency of insulin, the principal hormone involved in maintaining glucose balance. Although the disorder may occur at any age, it usually presents before the age of 30. It is the predominant form of diabetes in children and young adults who usually require injections of insulin to maintain metabolic control. Type 2 diabetes, by contrast, is primarily a disorder of resistance to the effects of insulin and tends to develop in mid to late life. People with this type of diabetes are not dependent upon insulin, although some may be treated with it. Type 2 diabetes is a slowly progressive disease and is more common than Type 1 diabetes. Because its onset is asymptomatic, slow and insidious, many people who fulfil the biochemical criteria for diabetes are not clinically recognised. It has been shown that there are almost as many people with undiagnosed Type 2 diabetes as there are those whose disease has been diagnosed.1 This form of diabetes occurs as a result of an interaction between an inherent susceptibility to diabetes, most probably genetic in origin, which becomes manifest when people become obese and physical inactive. In certain societies where people have abandoned traditional lifestyles, the prevalence of diabetes is high. The rapid social transitions that are occurring in many previously underdeveloped countries means that the expected global number of cases of this form of diabetes is rapidly increasing, presenting a major public health challenge.2

Prevention of Type 2 diabetes

The major risk factors for developing Type 2 diabetes are a family history, increasing age, physical inactivity and obesity. The strength of the association with obesity shows that men who are obese have approximately 10 times the risk of developing diabetes compared to those with a low body mass index. Although physical inactivity may play a role in leading to diabetes simply through its effects on energy balance, there are now several studies which show that some of the effect of physical activity occurs through other pathways. Perhaps more importantly, intervention studies have been effective in preventing the development of diabetes in people with the biochemical risk factor, impaired glucose tolerance (IGT). A recent randomised controlled study in China described the effects of increased physical activity and/or diet modification in 530 people with IGT4. The subjects in the dietary modification group were advised to reduce total calorie intake if they were obese, or to replace saturated fats with unrefined carbohydrate if they were not. Another group received physical activity advice and the final group received advice both on physical activity and on appropriate diet. The physical activity advice was to increase leisure time physical exercise by at least one unit per day and if possible by two units per day if they were under 50 years of age. One unit of exercise was defined as either 30 minutes of mild exercise such as walking or housecleaning, 20 minutes of moderate exercise such as faster walking or cycling, 10 minutes of strenuous activity such as slow running or climbing stairs or five minutes of very strenuous activity such as swimming or playing basketball. After six years of follow-up, the incidence of diabetes was 68% in the control group compared with 44% in the diet group, 41% in the exercise group, and 46% in the diet-plus-exercise group. This study provides powerful evidence that physical activity is involved in the causation of Type 2 diabetes and that changing this lifestyle can reduce the risk of developing the disease. However, as with many studies of this type, the critical unanswered question is how to help people make substantial and long-term changes to their physical activity patterns.

Benefits of physical activity in established diabetes

The major goal of treatment of Type 2 diabetes is to reduce the risk of the complications of the disease by maintaining good metabolic control, not only by reducing the blood glucose level but also by treatment of abnormalities of blood lipids and hypertension linked to the underlying abnormality of insulin resistance, which is a key feature of this condition.

Dietary change, weight loss and physical activity are usually the first line of treatment, with oral hypoglycaemic drugs and insulin being introduced if metabolic control cannot be achieved. The evidence that physical activity is beneficial to people with established Type 2 diabetes is clear. Exercise improves insulin sensitivity and reduces blood glucose. It also reduces many of the lipid abnormalities and raised blood pressure. As ischaemic heart disease is the major adverse outcome for people with diabetes, these beneficial effects of exercise on the pattern of cardiovascular risk factors are likely to be of considerable importance. At present the American Diabetes Association recommends aerobic exercise at a moderate intensity, lasting between 20 to 45 minutes on three days a week.5 It should, as with all exercise, be preceded by a warm-up period and followed by a cool-down period. As heart disease and diabetes often co-exist, the exercise should be tailored to the individual''s physical condition and particular attention should be made to the assessment of cardiac function.

Although we know a considerable amount about the physiological and biochemical effects of exercise, major uncertainties surround the choice of appropriate psychological techniques to help individuals make these important behavioural changes. Those individuals who stick with a programme and manage to change their behaviour usually represent only a small minority of the total group of people with diabetes who could benefit from increasing exercise. The challenge is to develop techniques that maximise the proportion of people who engage in and maintain some form of behavioural change.

In Type 1 diabetes, exercise is less clearly directly beneficial to metabolic control as this is a disorder characterised by lack of insulin rather than resistance to its effects. The mainstay of treatment is replacement of this insulin deficiency and interventions to improve insulin sensitivity have little effect. However, exercise should be encouraged in people with Type 1 diabetes not only because of the beneficial cardiorespiratory and psychological effects, but also because exercise is a normal part of social interaction and recreation from which people with diabetes should not be excluded.

Special considerations of exercise in Type 1 diabetes

As this is a lifelong chronic disorder for which the patient has to take responsibility for their own treatment, people with Type 1 diabetes usually have considerable knowledge of diabetes, and are particularly aware of how changes to their lifestyle can alter their metabolic balance and what adjustments to their treatments need to be made. This knowledge arises out of experience and by dialogue with the diabetes nurse specialists who provide support in most areas of the country. Therefore, the management of the adjustment of dietary intake and insulin dose that are required to maintain metabolic balance in people with Type 1 diabetes who undertake vigorous activity is usually best left to the person themselves. The balance is between taking in too little carbohydrate before exercising and failing to reduce the insulin dose which risks the development of hypoglycaemia or low blood glucose. This is usually manifest as muscular weakness and uncoordination, mental confusion and sweating. Patients can usually recognise these symptoms themselves and correct the glucose by eating. However, if left untreated or unrecognised, hypoglycaemia can result in unconsciousness. The observation of any of the warning symptoms during exercise should be taken as a cue for action and all people with diabetes should have quick-acting, rapidly absorbed carbohydrate available during exercise in case of hypoglycaemia.

At the other extreme, people can develop metabolic disturbance during exercise if they overdo the addition of carbohydrate and reduce their insulin dose too much, leading to hyperglycaemia (high blood glucose). As the appropriate adjustment of the dietary intake and insulin regimen depends upon the type and timing of the activity that is planned, and is variable between individuals, it is unwise to give general recommendations for responding to planned exercise. However, as with many aspects of diabetes care, finding the right balance is a process of learning by experience, supported by experienced and accessible professional advice. A key factor in this process of developing an individual''s ability to care for themselves is confidence. The knowledge that there are role models in elite athletes who have diabetes and manage to perform at the highest level of professional sport is likely to help.

Further information about diabetes can be obtained from

Diabetes UK, Macleod House, 10 Parkway, London NW1 7AA

Telephone: 020 7424 1000

Courtesy of Fitness Professionals UK

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