Several factors determine body weight and composition. These factors can be grouped into genetic, physiological, lifestyle, and psychosocial factors.

Genetic Factors

Estimates of the genetic contribution to obesity vary widely, about 25-40% of an individual’s body fat. More than 600 genes have been linked to obesity, but their actions are still under study. Genes influence body size and shape, body fat distribution, and metabolic rate. Genetic factors also affect the ease with which weight is gained as a result of overeating and where on the body extra weight is added.

If both parents are obese, their children have an 80% risk of being obese; children with one obese parent face a 50% risk of becoming obese. In studies that compared adoptees and their biological parents, the weights of the adoptees were found to be more like those of the biological parents than the adoptive parents, indicating a strong genetic link.

Hereditary influences, however, must be balanced against the contribution of environmental factors. Not all children of obese parents become obese, and normal-weight parents can have overweight children. Environmental factors like diet and exercise are probably responsible for such differences. Thus, the tendency to develop obesity may be inherited, but the expression of this tendency is affected by environmental influences resting metabolic rate (RMR) The energy required (in calories) to maintain vital body functions, including respiration, heart rate, body temperature, and blood pressure, while the body is at rest.


Physiological Factors

Metabolism is a key physiological factor in the regulation of body fat and body weight. Hormones also play a role. A few other physiological factors have been proposed as causes for weight gain, such as carbohydrate craving due to low levels of the neurotransmitter serotonin, but research on this and other theories has so far been inconclusive.

Metabolism and Energy Balance Metabolism is the sum of all the vital processes by which food energy and nutrients are made available to and used by the body. The largest component of metabolism, resting metabolic rate (RMR), is the energy required to maintain vital body functions, including respiration, heart rate, body temperature, and blood pressure, while the body is at rest. As shown in Figure 9.3, RMR accounts for about 65-70% of daily energy expenditure. The energy required to digest food accounts for an additional ±10% of daily energy expenditure. The remaining 20-30% is expended during physical activity.

Both heredity and behavior affect metabolic rate. Men, who have a higher proportion of muscle mass than women, have a higher RMR (muscle tissue is more metabolically active than fat). Also, some individuals inherit a higher or lower RMR than others. A higher RMR means that a person burns more calories while at rest and can therefore take in more calories without gaining weight.

Weight loss or gain also affects metabolic rate. When a person loses weight, both RMR and the energy required to perform physical tasks decrease. The reverse occurs when weight is gained. One reason exercise is so important during a weight-loss program is that exercise, especially resistance training, helps maintain muscle mass and metabolic rate.

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