The Metabolic Basis Of Obesity
Obesity exists when fat makes a greater than normal contribution to total body weight. In a normal woman, approximately 20-25% of her body weight is fat, and obesity is defined as a body-fat content greater than 30% of total body weight. In contrast, a normal young man will have less body-fat, 15-18% of his total body weight. For a man a definition of obesity would be a body-fat content greater than 25% of total body weight.
However, in practice the condition is rarely identified by measuring the fat content of the body. In most instances the relation between a person's height and weight is used to classify him or her as overweight or obese.
Health effects of over-weight and obesity
Overweight and obesity are considered undesirable because of the link between them and a variety of health problems. The disease most frequently associated with obesity in the public's minds is coronary heart disease (CHD), although the effect of weight on the two important risk factors, blood pressure and serum cholesterol, is more important than the effect of body weight alone. It is also well accepted that obesity increases the risk of diabetes mellitus and the formation of gallstones and that varicose veins are much more common amongst obese people. Clearly excess weight makes life more difficult for people with conditions that affect their mobility, like arthritis, and it places an additional burden on people with respiratory disease. Poor respiratory movement due to overweight can also increase the risks of surgery and of post-operative chest infections. Overweight and obesity are undesirable conditions, in the treatment of which diet and exercise play crucial roles.
The metabolic basis of obesity
Obesity results from an excess of energy intake over expenditure. Although this simple statement of the problem is true, it conceals the complexities of the condition, the variety of factors that contribute to it, and the difficulties of treating successfully. The requirement of energy varies enormously between individuals. Two people. similar in age, sex, and level of physical activity, can have widely different energy requirements, so that, despite eating identical amounts of food, one may gain weight while the other remains the same or even loses weight. Obese people do not necessarily eat more than those who are lean, but all obese people have at some time consumed more energy than their own individual requirements.
Everyone requires energy to provide for three types of energy expenditure: basal metabolism, physical activity, and heat production (thermogenesis).
Since nearly three-quarters of a person's energy expenditure in a 24-hour period is accounted for by basal metabolism (the energy requirements for the maintenance of basic physiological processes), there has been considerable speculation that obese people may have a lower basal metabolic rate (BMR). If they did indeed require less energy for these processes, then it would be easier for them to gain weight then people with a higher BMR-who maintained their weight-while eating the same amount of food. There is certainly a wide variation in BMR between individuals, the weight loss achieved by obese people on the same energy has been demonstrated to vary widely under strictly controlled condition people with a higher BMR have an advantage in both avoiding weight game any excess weight more easily. Unfortunately, it also appears that as weight is lost the BMR falls. This metabolic adaptation means that an obese patient will, the goes by, lose less weigh than others on the same energy intake, which can be very discouraging.
The second component of energy expenditure, physical activity, only contributes about 12% of the total, but once again the energy individuals use up while performing the same activity varies greatly. This includes the everyday activities such as sitting, standing, talking, and walking, as well as those demanding more energy.
Yet despite this relatively small contribution to a body's total energy expenditure, increasing physical activity can still play a useful part in any weight control programme. In addition to the calories used up, greater activity may limit the opportunities for a patient to eat, may provide a psychological boost, and may even help to raise BMR.
The final component of energy expenditure is thermogenesis, which describes the energy dissipated in response to food intake, exposure to cold and other environmental challenges. With regard to food intake, the size of the thermogenic response will buffer the change in weight even if energy intake is not finely controlled.
In an obese person the blood sugar, triglycerides (fat) and insulin levels are higher than those in a person of normal weight but, as we have seen in the case of non-insulin-dependent diabetes mellitus, the obese may be resistant to insulin. The higher level of insulin enhances the transport of glucose and triglycerides to the cells, but insulin resistance results in an inability to transfer glucose and triglycerides into the cells.
A calorie-rich diet helps to increase the blood-sugar level, which in turn, further stimulates the insulin. In the obese there is, therefore, always a risk of developing diabetes.