people of similar height. To explain these apparently contradictory findings it has been
suggested that obese people may have lower metabolic rates to start with, which results
in weight gain and this weight gain itself results in an increase in metabolic rate
(Ravussin and Bogardus 1989).Fat cell theory
A genetic tendency to be obese may also express itself in terms of the number of fat cells.
People of average weight usually have about 25–35 billion fat cells, which are designed
for the storage of fat in periods of energy surplus and the mobilization of fat in periods of
energy deficit. Mildly obese individuals usually have the same number of fat cells but
they are enlarged in size and weight. Severely obese individuals, however, have more fat
cells – up to 100–125 billion (Sjostrom 1980). Cell number is mainly determined by
genetics; however, when the existing number of cells have been used up, new fat cells are
formed from pre-existing preadipocytes. Most of this growth in the number of cells
occurs during gestation and early childhood and remains stable once adulthood has been
reached. Although the results from studies in this area are unclear, it would seem that if
an individual is born with more fat cells then there are more cells immediately available
to fill up. In addition, research suggests that once fat cells have been made they can never
be lost (Sjostrom 1980). An obese person with a large number of fat cells, may be able to
empty these cells but will never be able to get rid of them.Appetite regulation
A genetic predisposition may also be related to appetite control. Over recent years
researchers have attempted to identify the gene, or collection of genes, responsible for
obesity. Although some work using small animals has identified a single gene that is
associated with profound obesity, for humans the work is still unclear. Two children have,
however, been identified with a defect in the ‘ob gene’, which produces leptin which is
responsible for telling the brain to stop eating (Montague et al. 1997). It has been argued
that the obese may not produce leptin and therefore overeat. To support this, researchers
have given these two children daily injections of leptin, which has resulted in a decrease
in food intake and weight loss at a rate of 1–2 kg per month (Farooqi et al. 1999). Despite
this, the research exploring the role of genetics on appetite control is still in the very early
stages.Behavioural theories
Behavioural theories of obesity have examined both physical activity and eating
behaviour.Physical activity
Increases in the prevalence of obesity coincide with decreases in daily energy expendi-
ture due to improvements in transport systems, and a shift from an agricultural society358 HEALTH PSYCHOLOGY