The bodyweight set point theory states that we each have a certain weight, or ‘set point’, which is difficult to deviate from. It is a bit of a misnomer, as our bodies do not actually keep our bodyweight constant. Instead, our metabolism changes in response to altering weight.
At its most basic level of understanding, body weight is determined by the balance of energy intake (calories in) on one side of the equation, and energy expenditure (calories out) on the other. If the ‘calories in’ side is higher than the ‘calories out’, weight gain ensues, and vice-versa. This is a very basic explanation of the relationship between intake, output and weight gain. Within our wonderfully complicated bodies, there is much more to the story. Food intake (appetite; calories in) is under the control of the central nervous system via an extremely complex (and not fully elucidated) system of interconnected neuroendocrine circuits. Energy expenditure (calories out) is controlled by numerous endocrine hormones such as leptin and many thyroid hormones. It is these hormones responsible for intake and output which can be manipulated, and through which the bodyweight set point is kept.
One (all too common) example of the way that this works is seen in people who are starting on a weight loss journey. They seem to easily lose the first 10 or 15 pounds due to better nutrition and increased exercise, but then they hit a plateau, after which it becomes very difficult to continue losing weight. Why does this occur? They are eating and exercising the same way that they had been during the initial weight loss period, but no further weight loss occurs. According to the bodyweight set point theory, it is because those metabolic components which control food intake and energy expenditure are altered in order to make it more difficult to lose weight. In this particular example, this means that energy expenditure is controlled much more tightly and the body errs on the side of conserving energy; an activity which may have once burned 600 kcals, may now take only 500 kcals to complete. Also appetite may increase, adding more to the ‘calories in’ part of the equation. As you can see, this means that the individual would have to work much harder just to have the same results as in the beginning. This is the point at which many people become frustrated and may give up on eating right and exercising.
So the question is, what can we do about this? Are we essentially enslaved to the ‘set point’ that our genes choose? The answer is, fortunately… not exactly. Some studies have shown that the ‘set point’ at which the body feels comfortable can be reprogrammed after long term weight maintenance. Also, even though the actual factors may change, the basic body weight equation (calories in vs. calories out) still applies. Therefore, if we consume fewer calories than we use, we will lose weight. The key is certainly a long term lifestyle change outlook on weight loss, rather than short term crash diets. This is not to say that it will be easy to see a long term change in bodyweight, especially after hitting a plateau, but it is certainly not impossible.
NOTE: The same applies for those trying to INCREASE their bodyweight, except in reverse.