Metab 2000; 26: 22, 27.
Diabetes and Physical Activity PAMELA DYSON Oxford Centre for Diabetes, Endocrinology and Metabolism, Oxford, UK
INTRODUCTION
It is now widely accepted that increasing physical activity leads to great health benefits whether or not people have diabetes (1).
Increased physical activity has been associated with physical, mental and social benefits including the following: .
Reduction in all-cause mortality (2,3). .
Reduction in cardiovascular disease (CVD) including coronary heart disease (CHD), stroke and heart attack (4). .
Blood pressure reduction (5). .
Improved weight loss in the obese and weight maintenance in those of normal weight (6). .
Prevention of Type 2 diabetes and improved glycaemic control (7, 10). .
Prevention of osteoporosis (11). .
Improved flexibility and strength (12). .
Increased self-esteem and confidence (13).
These benefits apply to all people whether they have diabetes or not, but the benefits of improved glycaemic control are especially appropriate to people with diabetes.
In addition, a reduction in the incidence of CVD and the positive effect on body weight associated with physical activity can only benefit the health of people with diabetes.
Nutritional Management of Diabetes Mellitus. Edited by G. Frost, A. Dornhorst and R. Moses & 2003 John Wiley & Sons, Ltd. ISBN 0 471 49751 7
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PREVENTION OF TYPE 2 DIABETES
There is now unequivocal evidence that physically fit people are less likely to develop Type 2 diabetes and some intervention trials have shown that encouraging people with impaired glucose tolerance (IGT) to increase their physical activity significantly reduces their risk of developing diabetes (14, 16). This benefit is independent of body mass index (BMI) and there is some evidence that physical activity has a greater protective effect as BMI increases (17). It may be of more importance for people at risk of Type 2 diabetes to increase their physical fitness rather than concentrate on weight reduction.
GLYCAEMIC CONTROL
The benefits associated with improved glycaemic control are related to changes in insulin sensitivity, and are more pronounced in people who have Type 2 diabetes (18) or who are overweight (19). For many people with Type 1 diabetes, who do not exhibit insulin resistance, the main benefits of exercise may be related to improvements in dyslipidaemia, enhanced cardiovascular function and blood pressure reduction (10).
CARDIOVASCULAR RISK FACTORS
The role of exercise and the prevention of coronary heart disease in the general population have been well documented, but there is less evidence of a similar effect in people with diabetes. Modification of risk factors for CHD, including decreased total and LDL (low-density lipoprotein) cholesterol and triglyceride concentrations, have been demonstrated in Type 1 diabetes (20). People with Type 2 diabetes have two to four times the cardiovascular risk of those without diabetes and low cardiorespiratory fitness has been shown to be a predictor of mortality in men with diabetes (21).
BODY WEIGHT MANAGEMENT
Physical activity has a role in weight reduction and aids weight maintenance in those of normal weight. As 80% of people with Type 2 diabetes are overweight, most individuals would benefit from weight reduction.
DIABETES AND PHYSICAL ACTIVITY 21
EXERCISE AND DIABETES
Despite the widespread beliefs of the benefits of physical activity and the promotion of exercise by many health professionals, people with diabetes are reluctant to increase their physical activity (22).
This is not restricted to those with diabetes as it applies to the British population as a whole.
A UK study in 1990 showed that only 15, 30% of British adults are taking sufficient exercise for optimum health and that there is a large discrepancy between people’s perception of their fitness and the amount of exercise they actually take (23).
A recent Canadian study has shown that while 84% of people with diabetes thought they should be exercising, only 45% were actually doing so (24).
Against this background of reluctance to exercise there is also a lack of knowledge of the physiology of exercise.
In order to maximise the advice given to people with diabetes who wish to increase their physical activity, it is essential to gain an understanding of the physiology of physical activity, exercise and sport.
PHYSICAL ACTIVITY, EXERCISE AND SPORT
DEFINITIONS Physical activity refers to any body movement made by the skeletal muscles and resulting in energy expenditure, e.g. walking, gardening, housework.
Exercise is planned, structured repetitive body movements usually taken as a leisure time pursuit, e.g. aerobics, jogging, swimming.
Sport is physical activity which involves competitive situations which are usually governed by rules, e.g. football, rugby, netball.
The effects of physical activity programmes depend upon the intensity, frequency and duration of exercise.
INTENSITY Physical activity, exercise and sport can all be classified as either light, moderate or vigorous.
Light activities require little exertion and do not cause a significant change in breathing.
Moderate activities require sustained muscular movements and will result in heavier breathing and a feeling of warmth.
Vigorous activities require sustained muscular movements and result in a feeling of being sweaty or out of breath.
Examples of different activities and their intensity are shown in Table 2.1.
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Table 2.1 Intensity of various activities
Intensity of activity Examples
Light Slow walking, light gardening (weeding, mowing with power mower), light housework (dusting, hoovering), light DIY (decorating), bowls, golf, snooker Moderate Brisk walking, heavy housework (scrubbing, spring cleaning), heavy gardening (digging), heavy DIY (sawing, mixing cement), football, tennis, cycling, swimming, aerobics, all at a level to produce some breathlessness and a feeling of warmth, working as a labourer, roofer or refuse collector Vigorous Sport and exercise at a level to induce sweating and breath- lessness, e.g. squash, running, football, rugby, swimming, tennis, aerobics, cycling, gym work, any work or occupation involving frequent climbing, lifting, carrying, e.g. mining, forestry
FREQUENCY AND DURATION The exercise guidelines issued by the American College of Sports Medicine (ACSM) in 1978 recommend at least three sessions of 20, 40 min of vigorous activity each week.
This was revised in 1990 and 30 min of moderate activity daily is now recommended (25).
In 1994, the UK Health Education Authority (HEA) adopted an international consensus statement and recommended the following: moderate intensity activity; of 30 minutes duration or more; at a frequency of 5 or more days each week (1).
The majority of research has concentrated upon the effect of physical activity on CHD rates and as a result the emphasis has been on increasing aerobic or vigorous activity.
Recent research has shown the benefit of moderate activity and for many people with diabetes the greatest health benefit may be in changing from a sedentary lifestyle to a moderately active lifestyle (26).
PHYSIOLOGY OF EXERCISE 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.
DIABETES AND PHYSICAL ACTIVITY 23
Figure 2.1
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 carbo- hydrate (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.
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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 .
Medication .
Contraindications to exercise .