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Physiological theories
Page 356 Black blue 356 HEALTH PSYCHOLOGY In addition, Rand and MacGregor (1991) reported that individuals who had lost weight following gastric bypass surgery stated that they would rather be deaf, dyslexic, diabetic, have heart disease or acne than return to their former weight. These studies suggest a relationship between body size and depression. However, it is possible that depressed obese individuals are more likely to seek treatment for their obesity than those who are not depressed and that there may be many obese individuals who are quite happy and therefore do not come into contact with health professionals. In contrast to the above studies, Halmi et al. (1980) reported that although just over 28 per cent of a group of 80 patients waiting to have a gastric bypass operation were (or had been) clinically depressed, they argued that this is compatible with the prevalence of depression in the general population. Therefore, although some obese people may be depressed there is no consistent support for a simple relationship between body size and psychological problems. WHAT CAUSES OBESITY? The theories relating to the causes of obesity include both physiological theories and behavioural theories. Physiological theories Several physiological theories describe the possible causes of obesity. Genetic theories Size appears to run in families and the probability that a child will be overweight is related to the parents’ weight. For example, having one obese parent results in a 40 per cent chance of producing an obese child and having two obese parents results in an 80 per cent chance. In contrast, the probability that thin parents will produce overweight children is very small, about 7 per cent (Garn et al. 1981). However, parents and children share both environment and genetic constitution, so this likeness could be due to either factor. To address this problem research has examined twins and adoptees. Twin studies Twin studies have examined the weight of identical twins reared apart, who have identical genes but different environments. Studies have also examined the weights of non-identical twins reared together, who have different genes but similar environments. The results show that the identical twins reared apart are more similar in weight than non-identical twins reared together. For example, Stunkard et al. (1990) examined the BMI in 93 pairs of identical twins reared apart and reported that genetic factors accounted for 66–70 per cent in the variance in their body weight, suggesting a strong genetic component in determining obesity. However, the role of genetics appears to be greater in lighter twin pairs than in heavier pairs. Page 356 Black blue Page 357 Black blue OBESITY AND CORONARY HEART DISEASE 357 Adoptee studies Research has also examined the role of genetics in obesity using adoptees. Such studies compare the adoptees’ weight with both their adoptive parents and their biological parents. Stunkard et al. (1986b) gathered information about 540 adult adoptees in Denmark, their adopted parents and their biological parents. The results showed a strong relationship between the weight class of the adoptee (thin, median weight, overweight, obese) and their biological parents’ weight class but no relationship with their adoptee parents’ weight class. This relationship suggests a major role for genetics and was also found across the whole range of body weight. Interestingly, the relationship to biological mother’s weight was greater than the relationship with the biological father’s weight. Research therefore suggests a strong role for genetics in predicting obesity. Research also suggests that the primary distribution of this weight (upper versus lower body) is also inherited (Bouchard et al. 1990). However, how this genetic predisposition expresses itself is unclear. Metabolic rate, the number of fat cells and appetite regulation may be three factors influenced by genetics. Metabolic rate theory The body uses energy for exercise and physical activity and to carry out all the chemical and biological processes that are essential to being alive (e.g. respiration, heart rate, blood pressure). The rate of this energy use is called the ‘resting metabolic rate’, which has been found to be highly heritable (Bouchard et al. 1990). It has been argued that lower metabolic rates may be associated with obesity as people with lower metabolic rates burn up less calories when they are resting and therefore require less food intake to carry on living. Research in the USA has evaluated the relationship between metabolic rate and weight gain. A group in Phoenix assessed the metabolic rates of 126 Pima Indians by monitoring their breathing for a 40-minute period. The study was carried out using Pima Indians because they have an abnormally high rate of obesity (about 80–85 per cent) and were considered an interesting population. The subjects remained still and the levels of oxygen consumed and carbon dioxide produced was measured. The researchers then followed any changes in weight and metabolic rate for a four-year period and found that the people who gained a substantial amount of weight were the ones with the lowest metabolic rates at the beginning of the study. In a further study, 95 subjects spent 24 hours in a respiratory chamber and the amount of energy used was measured. The subjects were followed up two years later and the researchers found that those who had originally shown a low level of energy use were four times more likely to also show a substantial weight increase (cited in Brownell 1989). These results suggest a relationship between metabolic rate and the tendency for weight gain. If this is the case, then it is possible that some individuals are predisposed to become obese because they require fewer calories to survive than thinner individuals. Therefore, a genetic tendency to be obese may express itself in lowered metabolic rates. However, in apparent contrast to this prediction, there is no evidence to suggest that obese people generally have lower metabolic rates than thin people. In fact, research suggests that overweight people tend to have slightly higher metabolic rates than thin Page 357 Black blue