160 Obesity
this theory. In particular, there is no evidence to suggest that obese people
generally have lower metabolic rates than thin people. In fact, research sug-
gests that overweight people tend to have slightly higher metabolic rates than
thin people of similar height (Garrow, 1987). The obese also expend more
energy than thin people for a given activity (Prentice et al., 1989). 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). This view has not gone unchallenged,
however, and Garrow and Webster (1985) have argued that this is an unlikely
explanation which is unsupported by the evidence. In sum, the “slow
metabolism” theory of obesity may be held by many laypeople. But it is
no longer considered to be backed by research.
Fat cell theoryA 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 to
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). It was originally believed
that the number of fat cells was determined by genetics and early infancy and
remained constant throughout life (Hirsch, 1975). It is now recognized that
when the existing number of cells has been used up, new fat cells are formed
from preexisting preadipocytes, and that preadipocytes can be turned into
adipocytes across the life span (Roncari, Kindler, and Hollenberg, 1986).
Therefore, it would seem that if an individual is born with more fat cells,
then there are more cells immediately available to fill up, making excessive
weight gain easier in some people than others. In addition, research sug-
gests that once fat cells have been made, they can be emptied but never
lost (Sjostrom, 1980). However, even a person born with a lower number
of fat cells can create them if they are needed for the storage of fat.
Appetite regulationA genetic predisposition may also be related to appetite control. Over recent
years researchers have attempted to identify the gene or collection of genes