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
OBESITY AND CORONARY HEART DISEASE 357