Explain metabolic rate and individual variations based on factors such as sex, activity level, and age.
The metabolic rate is simply how many calories one's body burns at rest. This number varies very widely from individual to individual, and a number of factors control the base metabolic rate, but we can make some generalizations.
Overall, the more fit and strong one is, the higher the metabolic rate. This is because muscle tissue is very active metabolically, even when it is not working. So the more lean muscle tissue an individual has, the higher their metabolic rate will be in general. For this reason, the average man has a higher metabolic rate than the average woman, and athletic people have high rates than non-athletes.
The metabolic rate is high during periods of growth, so children, teenagers, and pregnant women tend to have high rates. Elderly people generally have lower metabolic rates, although some studies seem to indicate that this is more a function of lower activity levels than of age itself.
Last, women of chilbearing age have a metabolic rate that varies with the menstrual cycle. Their base metabolic rate is lower the first two weeks of their cycle, then it kicks up to a higher rate at the time of ovulation, staying high until the next menstrual period begins.