obese overeat because they have excess body fat. Over recent years, research has focused
on the eating behaviour of the obese not in terms of calories consumed, or in terms of
amount eaten, but more specifically in terms of the type of food eaten.
Population data indicates that calorie consumption has decreased since the 1970s
and that this decrease is unrelated to the increase in obesity (see Figures 15.5 and
15.6). However, this data also shows that the ratio between carbohydrate consumption
and fat consumption has changed; whereas we now eat less carbohydrate, we eat
proportionally more fat (Prentice and Jebb 1995). One theory that has been developed
is that, although the obese may not eat more than the non-obese overall, they may eat
proportionally more fat. Further, it has been argued that not all calories are equal
(Prentice 1995) and that calories from fat may lead to greater weight gain than
calories from carbohydrates. To support this theory, one study of 11,500 people in
Scotland showed that men consuming the lowest proportion of carbohydrate in their
diets were four times more likely to be obese than those consuming the highest pro-
portion of carbohydrate. A similar relationship was also found for women, although
the difference was only two- to three-fold. Therefore, it was concluded that relatively
lower carbohydrate consumption is related to lower levels of obesity (Bolton-Smith and
Woodward 1994). A similar study in Leeds also provided support for the fat proportion
theory of obesity (Blundell and Macdiarmid 1997). This study reported that high fat
eaters who derived more than 45 per cent of their energy from fat were 19 times more
likely to be obese than those who derived less than 35 per cent of their energy from
fat. Therefore, these studies suggest that the obese do not eat more overall than the
non-obese, nor do they eat more calories, carbohydrate or fat per se than the non-
obese. But they do eat more fat compared with the amount of carbohydrate; the
proportion of fat in their diet is higher. So how might a relative increase in fat
consumption relate to obesity?
As a possible explanation of these results, research has examined the role of fat and
carbohydrates in appetite regulation. Three possible mechanisms have been proposed
(Blundell et al. 1996; Blundell and Macdiarmid 1997):
1 The benefits of complex carbohydrates to energy use. First, it has been suggested
that it takes more energy to burn carbohydrates than fat. Further, as the body
prefers to burn carbohydrates than fat, carbohydrate intake is accompanied by an
increase of carbohydrate oxidation. In contrast, increased fat intake is not accom-
panied by an increase in fat oxidation. Therefore, carbohydrates are burned, fat is
stored.
2 The benefits of complex carbohydrates to hunger. Second, it has been suggested
that complex carbohydrates (such as bread, potatoes, pasta, rice) reduce hunger and
cause reduced food intake due to their bulk and the amount of fibre they contain. In
addition, they switch off the desire to eat. Therefore, carbohydrates make you feel
fuller faster.
3 The costs of fat to hunger. Third, it has been suggested that fat does not switch off
the desire to eat, making it easier to eat more and more fat without feeling full.364 HEALTH PSYCHOLOGY