In people without diabetes, a precise endocrine response ensures that the energy needs of the exercising muscle are met and glucose homeostasis is maintained. This metabolic response is ameliorated in Type 2 diabetes and lost in Type 1 diabetes and the challenge is to reproduce the physiological state of the non-diabetic individual. A brief review of the metabolic, hormonal and physiological responses to exercise is given below.
Metabolic changes provide the energy required for exercise. Glucose uptake by exercising muscle increases and at the onset of exercise, muscle glycogen is converted to lactate to provide the energy substrate. When muscle glycogen is exhausted, energy is provided by glucose from the liver following glycogen-olysis and eventually from metabolism of free fatty acids in adipose tissue. The metabolic response depends on a number of factors:
• Intensity and duration of exercise. High intensity/short duration of exercise, e.g. sprinting, will utilise carbohydrate (glycogen) as energy substrate and low intensity/long duration, e.g. marathon running, will utilise carbohydrate (glycogen) initially, but predominately fat (FFA).
• Exercise timing. The amount of available glucose will depend upon whether exercise is taken in the fasting or post-prandial state.
• Level of fitness. Athletes who train for endurance events show reduced rates of glycogen breakdown and are able to utilise FFA more efficiently.
• Dietary intake. The amount and type of carbohydrate consumed routinely and pre- and post-events will affect performance.
Hormonal responses to exercise are characterised by the action of insulin and its counter regulatory hormones glucagon, cortisol and catecholamines. The key response to exercise is the suppression of insulin secretion, accompanied by a rise in catabolic hormones (Figure 2.1). This stimulates release of glucose from the liver and FFA from adipose tissue and maintains energy levels during exercise.
Physiological responses are shown by the increased heart rate, stroke volume and cardiac output which accompanies exercise and which facilitates increased oxygen delivery and removal of carbon dioxide.
At the end of a period of exercise, recovery takes place as hormone concentrations return to their pre-exercise levels and glycogen stores are replenished. There is continued oxidation of fat, which limits the use of glucose as an energy substrate and allows replenishment of liver and muscle glycogen. Muscle uptake of glucose is enhanced through increased glucose transporters (GLUT-4) and this process of glycogen storage can last up to 12-18 h after exercise. The speed of this process depends upon the type and duration of exercise.
People with Type 2 diabetes usually have sufficient circulating insulin to precipitate the normal metabolic response to exercise and any risk of hypoglycae-mia is associated with the use of either insulin or oral hypoglycaemic agents.
People with Type 1 diabetes depend upon exogenous supplies of insulin and any exercise may pose some risk of hypoglycaemia. When a person with Type 1 exercises with too little insulin the counter-regulatory hormonal response may elevate both circulating glucose and ketone levels. On the other hand, too much insulin increases the risk of hypoglycaemia by blocking the exercise-induced increase in glycogenolysis and gluconeognesis. The risk of hypoglycaemia can continue for 6-14 h after strenuous exercise as glucose is synthesised to replace lost glycogen stores and insulin sensitivity is increased during the recovery period.
Advice to people with diabetes who wish to increase their physical activity or improve performance will cover the whole range of activities from a gentle stroll to competing at the top level. Advice should take into consideration the following:
• Perceptions and beliefs
• Type, intensity, frequency and duration of exercise
• Contraindications to exercise
• Dietary intake
Barriers to increasing physical activity include time constraints, usually linked to domestic or work commitments, health status, age, performance expectations and lack of will power. The emphasis should be on adapting advice to fit in with the individual's lifestyle and avoiding prescriptive advice, which may fail to address the individual's perceptions and beliefs.
People with Type 2 diabetes who wish to increase their levels of general activity will need encouragement, motivation and the choice of a physical activity or sport which matches their individual needs. For the majority of this middle-aged sedentary population, an increase in gentle exercise, e.g. walking, can be safely recommended. Exercise at this level has been shown to have health benefits for people with Type 2 diabetes (27). It is feasible for people with Type 2 to increase physical activity at a higher intensity, but recommendations should include checking with their physician before beginning a programme of vigorous exercise and exercising under supervision.
Type, Intensity, Frequency and Duration
Advice to people with diabetes will depend upon the type of exercise they wish to do. Gentle exercise at light or moderate intensity does not place the same demands upon diabetes management as that caused by vigorous exercise. People with Type 1 diabetes may be at risk of hypoglycaemia and it is important to consider the effect of the type, intensity, frequency and duration of exercise and its effect on blood glucose. Timing of meals and sessions of physical activity should be planned wherever possible and exercise should avoid the peak action of insulin as this has been shown to precipitate hypoglycaemia. The majority of people with Type 1 who take exercise find that they may have to adjust their insulin, their carbohydrate intake or a combination of both. People who are taking part in competitive sports or who wish to take part in endurance races or increase their activity to a vigorous level are strongly recommended to consult a specialist diabetes professional.
People with Type 2 diabetes who are controlled by diet alone, or who take metformin or acarbose, are able to exercise without risk of hypoglycaemia. They do not need to make any adjustment to their medication when increasing physical activity. Those taking insulin or oral hypoglycaemic agents will need to time exercise sessions to avoid peak action of agents and may need to take extra carbohydrate during training to prevent hypoglycaemia. It is difficult to adjust insulin for exercise in people taking an insulin mixture twice daily and in Type 2 diabetes this is further complicated by an inability to measure accurately insulin resistance. Because of the action of increasing insulin sensitivity, people with Type 2 diabetes who increase their physical activity may find that their insulin requirements decrease over time and that they may need less insulin. This is best measured on an individual basis and should be monitored by frequent blood glucose tests.
People with Type 1 diabetes will need to plan ahead for exercise and make appropriate insulin adjustment. One study has shown that moderately intensive exercise sessions lasting 45min can lead to hypoglycaemia and this was effectively overcome by a reduction in insulin by 30-50% before exercise (28). The effects of exercise are mediated by the timing of exercise (pre- or post prandial) and the blood glucose levels at the start of exercise. It is impossible to formulate precise guidelines for insulin adjustment for those with Type 1 wishing to exercise, but consultation and experimentation accompanied by frequent blood glucose tests can help to identify the most effective strategies in individuals with Type 1 diabetes.
The absorption of insulin injected subcutaneously can be accelerated by exercise and may precipitate hypoglycaemia. To prevent this, it is advised that insulin is injected away from the site of exercising muscle, e.g. into the abdomen in the case of runners and cyclists, and that exercise does not take place at the time of peak insulin action.
Although the health benefits of exercise are well established, there are certain areas where caution must be observed. Encouragement to increase physical activity for people with diabetes will be affected by some underlying features and physical exercise is not without risk in diabetes. All people with diabetes are advised to have a medical examination prior to beginning an exercise programme to identify any complications which may be adversely affected by exercise. Those wishing to take part in strenuous activity will find the following are relative contraindications: poor glycaemic control, the presence of ketones, proliferative retinopathy, microangiopathy, neuropathy, nephropathy and cardiovascular disease. Although strenuous activity may be contraindicated, some individuals may be able to increase general everyday physical activity.
Practical implications of increasing physical activity include consideration of possible damage to the soft tissues and joints, especially in the feet, and it is recommended that those with diabetes consult a podiatrist and purchase good, supportive footwear.
Precipitation of cardiac events is possible, especially in those with Type 2 diabetes, and exercise should not be encouraged without giving the following guidelines: avoid irregular, strenuous exercise, do not exercise when unwell and stop exercise immediately if any pain, especially chest pain, is experienced. It is also important to warm up and cool down thoroughly before and after exercise sessions.
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