Physiological theories

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Physiological theories
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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.
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.
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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
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