Duration and frequency of the activity are also important. The longer and more often classes are taken, the more benefit is likely. But this should not be seen as providing enough physical activity by itself. For many people, particularly the elderly, participating in an organised aerobics class can often result in a reduction in other forms of spontaneous physical activity which may have been done during the rest of the day. Total net increase in energy expenditure may actually be less than before aerobics classes were undertaken. It is important therefore for the fat loss leader to monitor all forms of physical activity, not just that which occurs in organised classes.

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Within any particular environment, however, the response of individuals to that environment will vary. Some may respond to a high fat diet with only a small gain in weight, whereas others will gain a lot. How much an individual responds to the environment is largely dependent on his or her genetic makeup. The most convincing evidence for this has come from a series of studies performed by Dr Claude Bouchard and others from Laval University in Quebec, on identical twins. They increased the calorie intake of twins by 1000kcal/day for 3 months under strictly controlled conditions. Naturally, everyone gained weight, but some gained much more than others. If one twin only gained a few kilograms, the other twin tended to have a small weight gain as well. If one twin gained a lot of weight, so did the other twin. In other words, under identical environmental conditions, people gain weight at different rates and the amount they gain seems to be genetically determined.

This can be viewed in another way using the population distribution of body size . Consider a population that lives in an environment which does not promote obesity—such as a remote Pacific island with little contact with the outside world, where the food has to be gathered, and where deep-fried chicken is still unknown (environment A). There will be a variety of body sizes, but very few will be classified as obese. Then an airport is built, the tourists arrive, motorised transport takes over from legs and paddles and the fried chicken takeaways set up business. Ten years later, the population distribution of body size has shifted to the right and more people are now classified as obese (environment B). However, the distribution is skewed, so not everyone gained weight at the same rate. A person X who is ‘genetically lean’ and is at the left hand side of the curve gained only a small amount of weight as the environment changed from A to B. Person Y happens to be genetically more predisposed to weight gain and is on the right hand side of the curve. He or she gained considerable weight and in environment B is now obese (Y).

This shift in population levels of obesity also provides evidence for approaches to deal with the problem. Individual, one-on-one approaches (the ‘high risk’ approach) are obviously necessary. But these do little to affect the total population. A small shift in the total curve shown in Figure 1.3 to the left on the other hand, will have a major effect on public health. In other words, small fat losses in a large number of people will have a greater impact on the health of the population as a whole than larger losses in a small number.

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