Some degree of obesity is almost invariable in the Western World and almost all people develop some obesity as they get older. Obesity implies the excess storage of fat and this can most easily be detected by looking at the undressed patient.
Tables of desirable weights for a given height can be found in the Appendices: 10% greater than these desirable weights is described as overweight; 20% or more than the ideal weight as morbid obesity. Another way of classifying grades of obesity is the body mass index (BMI) Information box 3.3):
BMI = weight (kg)/(height in metres)”
CAUSES OF OBESITY
Most patients suffer from simple obesity, but in certain conditions obesity is an associated feature;
even in these situations, intake of calories must exceed expenditure. Hormonal imbalance is often incriminated in women, e.g. postmenopause or when taking contraceptive pills, but most weight gain in such cases is usually all and due to water retention.
Not all obese people eat more than the average, but all viously eat more than they need.
GENETIC AND ENVIRONMENTAL FACTORS. These have always been difficult to separate. However, refeeding experiments in both monozygotic and dizygotic twins, reared together or apart, suggest that genetic influences account for 70% of the differences in BMI later in life and that the childhood environment has little or no influence. These overfeeding experiments also showed that weight gain did not occur in all pairs of twins, suggesting that in some a facultative increase in thermogenesis occurred so that part of their extra dietary energy was expended inefficiently.
FOOD INTAKE. Many factors related to the home environment, e.g. finance and the availability of sweets and snacks, will affect food intake. Some patients eat more during periods of heavy exercise or during pregnancy and are unable to get back to their former eating habits. The increase in obesity in social class 5 can usually be related to the type of food consumed, i.e. food containing sugar and fat. The underlying mechanisms for controlling satiety are ill-understood; psychological factors and how food is presented may override complex biochemical interactions.
It has been shown that obese patients eat more than they admit to eating and over the years a very small daily excess can lead to a large accumulation of fat.
CONTROL OF APPETITE. This is complex and partially depends on external stimuli, such as the company, the type of food, the surroundings and the usual habitual behaviour.
Appetite is the desire to eat and this usually initiates food intake. Following a meal, satiation occurs. This depends on gastric and duodenal distension and the release of many substances peripherally and centrally. Cholecystokinin (CCK), bombesin and somatostatin are released from the small intestine and glucagon and insulin from the pancreas following a meal. All of these hormones have been implicated in the control of satiety. Centrally the hypothalamus, particularly the paraventricular nucleus, and the ventromedial wall of the hypothalamus are thought to be the main satiety centres. Numerous neurotransmitters, e.g. CCK, opioids, serotonin and corticotrophin-releasing hormone, playa role in the central control of satiation. In obesity, no single abnormality involving appetite control has been identified although the obese eat more than the non-obese.
ENERGY EXPENDITURE. Obese patients tend to expend more energy during physical activity as they have a larger mass to move. On the other hand, many obese patients decrease their amount of physical activity. The energy expended on walking at 3 miles per hour is only 15.5 k] min-I (3.7 kcal min-I) and therefore increasing exercise plays only a small part in losing weight. Nevertheless, as increased body fat develops insidiously over many years, any discrepancy in energy balance is important.
THERMOGENESIS. Brown adipose tissue in animals when stimulated by cold or food dissipates the energy derived from ingested food as heat. This can be a major component of overall energy balance and it has been suggested that this may also apply to humans. A defect in thermogenesis would explain why some obese patients require a very low calorie intake to maintain any weight loss achieved and gain weight easily after only small calorie increases. This mechanism may play some role in the development of obesity.
Most patients recognize their own problems, although often they are unaware of the main foods that cause obesity. Many symptoms are related to psychological problems,e.g. in women who cannot find fashionable clothes to wear. Social pressures are also important.
The degree of obesity is assessed by comparison with tables of ideal weight for height (see Appendices), the BMI and also by measuring skinfold thickness. This should be measured over the middle of the triceps muscle; normal values are 20 mm in a man and 30 mm in a woman. The conditions and complications that are associated with obesity. The relationship between cardiovascular disease (hypertension or ischaemic heart disease), hyperlipidaemia, smoking, physical exercise and obesity is complex. Difficulties arise in interpreting mortality figures because of the number of factors involved. Many studies of obesity do not, for instance, differentiate between smokers and non-smokers or between the types of physical exercise that are taken. Many do not take into account the cuff-size artefact in the measurement of blood pressure; an artefact will occur if a large cuff is not used in patients with a large arm. Nevertheless, obesity almost certainly plays a part in all of these diseases and should be treated. The only exception is that stopping smoking, even if accompanied by weight gain, is more important than any of the other factors.