Negative Feedback Loops
The human body constantly attempts to maintain homeostasis, a term which means "same state." Homeostasis maintains a constant state of balance in the body even when there are large changes in external elements like temperature and food intake. The body will try to maintain blood pressure, body temperature, respiratory rate and blood glucose levels around a normal range. These systems are maintained by negative feedback loops. Any change from the normal triggers the negative feedback loops, by which the body tries to avoid large deviation from the norm. Think of a negative feedback loop like changing the temperature in a house. When it is cold outside, the thermostat, which represents the normal homeostatic loops in the body, responds by turning on the furnace. When the temperature reaches the set point, the thermostat turns the heater off until the temperature falls below a set level. Like the thermostat, the negative feedback loop requires a receptor, a control center and effectors. When humans try to lose weight, the negative feedback loop may come into play, especially if caloric intake drops below a point where caloric intake in less than the body needs to function effectively. Many times, people who lose weight on very low calorie diets gain it back again due to a resetting of homeostatic mechanisms to a lower than normal value. This reduction in the body's metabolic rate is an attempt by the body to avoid starvation. After the initial weight loss, patients find they gain weight back, even at the lower caloric intake level, simply because a change in the metabolic set point. Some patients may be able to overcome the metabolism by effective levels of exercise, since the metabolism responds to exercise by providing energy for muscle work based on expenditure needs rather than reduced intake.
It is unlikely that humans are genetically predisposed to metabolic set point reduction. While obesity and weight problems often run in families, no studies to date have associated this issue with genetic problems, except in the very rare cases of congenital abnormality. Rather, it is more likely that patterns of eating, exercise and diet are learned behaviors and this is the cause of similar feedback loops within families as are related to weight loss.
