What are the effects of exercise in a diabetic person

Physical activity has both acute and chronic effects in the diabetic person, which differ depending on the type of DM (Type 1 or 2).

During exercise in normal persons, pancreatic insulin secretion decreases (as already mentioned) in response to the increased insulin sensitivity brought about by exercise. In persons with Type 1 DM, however, that use insulin injections for their treatment, this physiological decrease in insulin secretion cannot happen, because insulin has been exogenously administered. Circulating insulin suppresses the expected rise of hepatic glucose production and, on the contrary, aggravates the exercise-induced glucose uptake by the exercising muscles. This hyper-insulinaemia also hinders the normal mobilization of triglycerides from the adipose tissue during exercise, which results in a decreased supply of FFAs for use as a fuel from the muscles.

Metabolic and hormonal response to exercise in Type 1 DM varies to a great extent and depends on multiple factors, including intensity and duration of exercise, glycaemic control before exercise, the kind and dose of insulin administered before the exercise session, the injection site and the temporal relation of the insulin injection with the last (before exercise) meal. Consequently, blood glucose levels can increase, decrease or remain unchanged. Specifically, plasma glucose tends to decrease if there is hyperinsulinaemia during the exercise session, if the exercise is prolonged (> 30 minutes) and if carbohydrates are not consumed before or during the exercise. Hyperinsulinaemia during exercise can be due to the fact that peak action of the administered insulin (short or medium duration) can coincide with the time of the exercise or because insulin injection was administered to an exercising part of the body, resulting in faster absorption. Blood glucose levels usually remain stable when exercise is of short duration and mild to moderate intensity and the appropriate quantity of carbohydrates is administered before or during a moderate intensity exercise session. Finally, blood glucose levels tend to increase if there is hypoinsulinaemia during exercise, the exercise is very intense or an excessive quantity of carbohydrates is consumed before exercise. The final result is hypergly-caemia, with increased mobilization of lipids and increased hepatic ketogenesis. The practical conclusion, therefore, is that in Type 1 DM glucose control during exercise is very complex and dependent on multiple factors.

Despite this complexity, a well-informed diabetic individual can participate in exercise programmes, even competitive ones (marathon running, etc.), successfully and without complications provided they follow some rules of good glycaemic control before the exercise session (the usual desirable blood glucose limits for avoidance of hypo- or hyper-glycaemia are 100-250 mg/dl [5.6-13.9 mmol/L], respectively). It is essential that they also regularly check their blood glucose levels (especially in the cases of prolonged exercise, every 30-60 minutes) and consume extra carbohydrates if needed (usually an additional 1015 g of carbohydrates are required for each hour of moderate intensity exercise, in a 70 kg person). It is important that after a bout of prolonged exercise, these patients monitor their blood glucose levels for possible occurrence of hypoglycaemia several hours later (usually 3-15 hours) or even the next day, owing to the increased uptake of glucose from the exercised muscles for replenishment of the exercise-induced depleted glycogen stores. Table 9.2 shows general instructions for safe exercise in Type 1 diabetics. It should also be emphasized that most intervention

Table 9.2. Instuctions for safe exercise in Type 1 DM

• Adequate metabolic control before starting the exercise programme

• Self monitoring of blood glucose is essential

• Avoidance (or delay) of exercise when blood glucose level is < 100mg/dl (5.6 mmol/L) or > 250mg/dl (13.9mmol/L) with ketosis or > 300mg/dl (16.7mmol/L) even without ketosis

• Carbohydrate intake should increase before exercise, depending on blood glucose levels before its initiation, and for the duration and intensity of exercise. Generally, 10-15 g of carbohydrates should be consumed one hour before or after a moderate intensity exercise session. Further carbohydrates should be consumed when blood glucose levels are < 100 mg/dl (5.6 mmol/L) before exercise

• Be prepared for possible hypoglycaemia a few hours after the exercise (usually 3-15 hours, until even the following day)

• A relatively prolonged exercise session may require decrease in the insulin dose in order to avoid hypoglycaemia

• Injection site should preferably be away from an exercising part of the body

• Consumption of fluids is very important and should be encouraged

• All people with DM should carry an identity card and carbohydrate-containing food when they exercise physical activity studies in Type 1 diabetes have not shown an improvement of glycaemic control with exercise. The main reason for this is possibly the excessive decrease in the insulin dose or the excessive carbohydrate intake before exercise, in an attempt to avoid hypoglycae-mias. Type 1 diabetics who participate in competitive sports may observe that their glycaemic control deteriorates, possibly due to the irregular intense exercise schedules, the decrease of insulin doses and the increased carbohydrate consumption. Nevertheless, the long-term benefits of regular physical activity regarding cardiovascular morbidity and mortality that apply to the general population, and those with Type 2 DM (examined below), are applicable to these patients as well.

In Type 2 diabetics (characterized by hepatic and peripheral insulin resistance and hyperinsulinaemia during the initial stages), peripheral glucose utilization during moderate intensity exercise increases more than its hepatic production, resulting in a tendency for blood glucose levels to decrease. At the same time, however, plasma insulin levels decrease, and thus hypoglycaemia risk in these patients (who do not use exogenous insulin) is relatively small. Despite that, for patients who use insulin or insulin secretagogues (sulfonylureas or meglitinides), the risk of hypoglycaemia is real (albeit less than in Type 1 DM).

Physical activity (together with proper nutrition) represents the cornerstone of Type 2 DM treatment. Exercise improves insulin sensitivity, which is accompanied by improvement in glycaemic control (decrease in HbAlc by 0.7 percent in a meta-analysis) and all the other atherosclerotic risk factors already mentioned for non-diabetic people (hypertension, dyslipidaemia, obesity) that very frequently accompany Type 2 diabetes as part of the metabolic syndrome. These improvements are reflected in the amelioration of morbidity and mortality of Type 2 diabetics that exercise regularly, compared with those diabetic persons who do not exercise. During the last few years several prospective and retrospective studies have proven the reverse relationship between muscular activity level and cardiovascular morbidity and mortality in diabetic patients (Type 1 and 2). Selection of timing of exercise may be an especially important parameter for Type 2 diabetics. Exercise late in the evening has been shown to decrease hepatic glucose production and fasting plasma glucose levels the following morning. Furthermore, postprandial exercise can be beneficial since it decreases the frequently observed postprandial hyperglycaemia. Apart from this beneficial effect of exercise in treating DM, well planned clinical studies during the last few years have proven that physical activity programmes, with or without dietary interventions, decrease the risk of developing Type 2 DM in high-risk people (for example, persons with impaired glucose tolerance). Specifically, in two of the best studies performed (one in the USA. and one in Finland) the risk of developing Type 2 DM was decreased by 58 percent in the exercise and diet group compared to the control group (see also Chapter 30 'Prevention of diabetes mellitus').

The type and intensity of exercise necessary for cardiovascular protection has not been definitely elucidated. The American College of Sports Medicine (Albright, et al., 2000) recommend that all diabetic persons should strive to expend at least 1000 kcal/week in leisure time physical activities, at a moderate intensity level (40-70 percent VO2max), 3-5 times per week, with a minimum duration of at least 30 minutes per day. Higher intensity exercise does not seem to contribute to a further decrease in cardiovascular risk and is accompanied by a higher chance of complications. The value of anaerobic exercise (resistance training) -for example weightlifting - in healthy people of any age, regarding cardiovascular benefits, has been recognized in the last 10-15 years.

Traditionally there has been a reluctance to recommend anaerobic exercise, especially in elderly diabetics, due to the fear of abrupt increases in blood pressure. Lately, however, many studies have shown that anaerobic exercise is quite safe and effective in improving glycaemic control, even in elderly diabetic people.

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