The physiological and psychological rewards of being physically active are well established.
The number of important benefits of exercise is extensive, but broadly fall into four major categories:
Maintaining reserve capacities
Ameliorating the effects of age and chronic disease
It is well known that physical inactivity or a sedentary lifestyle is related to an increased risk of cardiovascular disease and other chronic disease states, such as hypertension, diabetes, obesity, osteoporosis and certain forms of cancer.
Therefore, the recommendation that everyone should accumulate 30 minutes or more of physical activity over the course of most days of the week is fully justified. However despite the health benefits of physical activity and the fact that is it potentially accessible to all, it is estimated that over 60% of the world population is not active enough.
The groups that have been identified as being particularly inactive are: people living in urban areas, people from poor communities, and females. Physical inactivity is an issue for both developed and developing countries. In the UK, less than 1 in 5 men and l in 10 women participate in the recommended amount of activity, with the figures looking worse with increasing age.
The preferred energy fuel for the muscles is glucose, especially as the amount and intensity of physical activity increases. Glucose is formed from the breakdown of carbohydrates (sugars and starches) in the diet and is stored as glycogen. However, the body can only store a limited amount of glycogen, so the stores need to be kept topped-up to avoid fatigue. Consequently, a lack of carbohydrates in the diet often causes people to give up their exercise programme.
No matter what type of exercise, the body will always use some glucose for energy, and the more glucose it uses, the more carbohydrates need to be consumed to replenish the stores.
Physically active people need to consume a diet that contains 60-70% energy from carbohydrates. Daily carbohydrate requirements are dependent on the amount of exercise and range from about 4g of carbohydrate per kilogram of body weight per day for a person who is physically active 3 hours a week to 10g carbohydrate per kilogram of body weight per day for a person training more than 4 hours a day. Carbohydrate foods have a very significant impact on exercise performance and so are particularly relevant for athletes.
Most carbohydrate foods are eventually broken down into glucose and therefore one type is not intrinsically better than the other. In relation to exercise it is more important how quickly the carbohydrate is converted to glucose, ie the glycaemic index (GI) of the food. The recommendation is to eat moderate- to high-GI foods just before, during, and immediately after exercise to help stimulate glycogen synthesis.
Physical activity in health and disease
Physical inactivity is recognised as a significant common and preventable risk factor for non communicable diseases, which account for almost 60% of global deaths and 43% of global burden of chronic disease. Maintaining regular physical activity helps avoid positive energy balance and obesity.
Physical activity reduces the risk of developing heart disease, type II diabetes, osteoporosis, colon cancer, and may also play a protective role against breast cancer. Physical activity also plays a role in a person’s well being by reducing stress, anxiety and feelings of depression.
Maintenance of energy balance is dependent both on energy intake and energy expenditure. If energy intake from food and drink exceeds the amount used for physical activity and bodily functions, the excess is stored as body fat.
The FAO/WHO Expert Consultation (1998) recommended that the optimal combination to avoid positive energy balance and obesity was a low-fat, high-carbohydrate diet and regular physical activity. There is an increasing realisation that the emerging epidemic of obesity cannot simply be attributed to dietary factors. One of the clearest indications to support this is the fact that, in general, energy intakes are declining while average body weight is increasing. This indicates that physical activity levels are probably declining even faster, possibly as a consequence of increasing mechanisation and transport use. There is a growing body of evidence that shows physical inactivity to be a better predictor of obesity than measures of diet quantity or quality.
In addition, exercise is important in the treatment of people who are already obese and has a particularly useful role in the long-term maintenance of any weight loss. Physical activity is associated with improved motivation and therefore with better dietary compliance, increased lean body mass, and an amelioration of the usual suppression of metabolic rate which accompanies weight loss.
The first step for people who are obese is simply to modify their lifestyle to build in more physical activity. Simply moving around is difficult for someone who is very obese, but expends a lot of energy. Simple lifestyle changes are about becoming less sedentary, so include advice to decrease the amount of time spent watching television, surfing the internet, and playing computer games.
The next stage is to become more active, for example taking the stairs instead of the lift/escalator, parking further away from the shops, getting off the bus a stop earlier, or walking to the shops. The aim is to gradually build up to 30-40 minutes of sustained exercise, such as brisk walking, cycling or swimming, at least five days a week.
Physical activity protects against the development of type II diabetes. Regular moderate intensity exercise is associated with a reduced relative risk of developing diabetes of about 0.6-0.7 and this figure still applies in overweight individuals. Physical activity improves insulin sensitivity and acutely lowers blood glucose, helps to favourably influence lipid profiles, facilitates weight control and maintains muscle mass.
People with diabetes should be encouraged to engage in moderate physical activity for at least 20-30 minutes most days of the week. Moy et al. (1993) showed a three-fold reduction in mortality over seven years in patients with type 1 diabetes who were physically active compared with sedentary patients. Some types of exercise will mean that additional carbohydrate is required to prevent hypoglycaemia in individuals with diabetes.
Heart disease accounts for a third of all global deaths. Physical inactivity is linked to an increased risk of coronary heart disease, but only a minority of adults take sufficient exercise to benefit health. The mechanism by which physical activity might help protect against coronary heart disease may involve effects on coagulation and thrombosis as well as an influence on lipoprotein metabolism. Individuals who frequently exercise often have a lower concentration of plasma fibrinogen.
Physical activity may also produce a reduction in plasma triglycerides and an increase in HDL cholesterol (good cholesterol), and therefore a reduction in risk from developing coronary heart disease. Habitual physical activity also reduces the risk of arterial hypertension, particularly among those who are overweight, and therefore can be a useful adjunct with pharmacological treatment. Increased levels of aerobic exercise can also produce a reduction in both systolic and diastolic blood pressure.
To confer a benefit to the cardiovascular system the physical activity does not need to be carried out at a high intensity. Gentle exercise such as walking, cycling and swimming, can be effective in reducing the risk of coronary heart disease and contribute to the maintenance of a desirable body weight, and can be modified to suit most age groups and abilities.
Physical activity is important for bone health and helps protect against the risk of fracture by maintaining and improving bone density and neuromuscular competency. There is evidence that a lack of physical activity plays a major role in the deterioration of bone quality that accompanies osteoporosis.
The loading of bone either from gravitational forces, or from muscular tension influences its functional strength, particularly at the loaded regions of the skeleton. Both factors are involved during weight-bearing activities such as running, jumping and climbing stairs, whereas activities such as weight-training and swimming involve muscular tension alone. Although there is unlikely to be any improvement in bone density from low impact activities such as walking at a normal pace, they may contribute to improved balance and coordination, which in turn might help prevent the falls which precipitate fractures.
Participation for about 30 minutes per day in varied physical activities with a weight bearing component would be expected to promote stronger bones in all ages, however the activities need to be carried out at an intensity that is appropriate for both the age and capability of the individual.
Due to the relationship of physical activity with energy expenditure, obesity, changes in sex hormone levels and age at menarche, there are certain cancers for which it is relevant to consider the role of physical activity.
Regular physical activity confers a statistically significant, albeit small, measure of protection against the development of breast tumours, although the relationship is often stronger in young women than older women.
Several studies, both case control and cohort, have demonstrated a protective effect of increased physical activity against colon cancer.
Since physical activity may have a role to play in protecting against cancer, the recommended 30 minutes or more of physical activity most days of the week is also appropriate for cancer prevention.