Part of the problem is the relatively low level of physical activity of many people in Western countries. The dramatic increase in overweight and obesity between 1980 and 1998 shown in Figure 6.3 occurred at a time when average food consumption was static or decreasing. As discussed in section 18.104.22.168, the average physical activity level in Britain is only 1.4; physical activity accounts for only 40% more energy expenditure than basal metabolic rate. At the same time, food is always readily available, with an ever-increasing array of attractive snack foods, which are easy to eat, and many of which are high in fat and sugar.
Sometimes, the problem can be attributed to a low rate of energy expenditure despite a reasonable level of physical activity. There is a range of individual variation as much as 30% above and below the average BMR (section 22.214.171.124). This means that some people will have a very low BMR, and hence a very low requirement for food. Despite eating very little compared with those around them, they may gain weight. Equally, there are people who have a relatively high BMR and are able to eat a relatively large amount of food without gaining weight.
Lean people can increase their energy expenditure to match their food intake; leptin (section 1.3.2) increases the activity of mitochondrial uncoupling proteins (section 126.96.36.199). The result of this is an increased rate of metabolism of metabolic fuels and increased heat output from the body, especially after meals and while asleep.
Other people seem to be much more energy efficient, and their body temperature may drop slightly while they are asleep. This means that they are using less metabolic fuel to maintain body temperature, and so are able to store more as adipose tissue. Such people tend to be overweight. (This response, lowering body temperature and metabolic rate to conserve food, is seen in a more extreme form in animals that hibernate. During their long winter sleep these animals have a very low rate of metabolism, and hence a low rate of utilization of the fuel they have stored in adipose tissue reserves.)
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