There is a common notion in the fitness and nutrition industry that your body has a specific set point for bodyweight that you cannot deviate from by more than 10-20 pounds. Right off the bat this should set off alarm bells because it does not consider different heights. How could this 10-20 number apply both to a 5’2’ woman, and a 6’5 man. Therefore if this theory has any merit it must be in relation to height and more specifically as a percentage of body size. There are however other problems with set point theory as well.
Set point theory only works in one direction, specifically as you become lighter and smaller, and it only applies to lean body mass.
There is a lower limit of lean body mass that once you dip below it will cause death. Your cardiovascular system will begin to fail, heart, lungs and internal organs will all fail…bones will become brittle and your muscle will cease to be able to even support the act of breathing. The immune system will become severely compromised and your ability to fight infection and survive them will be diminished.
These fatal changes will be due to the complete lack of macro and micro nutrients that would have brought you to this dangerously low bodyweight in the first place (this is the unfortunate circumstance of an anorexic who finally succumbs to their disorder).
Based on anthropological data we have that relates skeletal muscle mass to skeletal height we can most likely determine what the lower lean mass threshold is beyond which the risk of fatality is extremely increased. For example a 5’2” woman can probably go down to a bodyweight closer to 60-70 pounds before it becomes fatally low, whereas a 6’8” man would die at a much higher minimum weight closer to 120-130 pounds. This is how set point theory may be able to in part help define what our lowest healthy and manageable bodyweight is. What then is the set point for our upper limit to a manageable healthy bodyweight?
On the opposite end there is no definable size of fat mass that approaches a fatal limit per se…it could be postulated that 50 pounds overweight can results in a series of increased risk factors that result in potential fatal complications 30 years down the road. Similarly 100 pounds overweight might result in potential fatal complications 15 years down the road and so on…but there is no apparent limit to the size of fat mass the human body can accumulate as is evidenced by people who have managed to exceed bodyweights of 600-700lbs regardless of their height. Therefore we have no definable upper limit of body fat mass that causes immediate or very predictable death.
Set point theory therefore only applies to LEAN MASS, and has nothing to do with FAT MASS. And more specifically a percentage of your lean mass, which is in turn based on your height. Underneath your body fat is your true lean body mass which is tightly correlated to your height and how much you exercise your muscles. Therefore a 5’3” woman who weighs 120lbs would have a much smaller range of potential lean mass change compared to a 6’8” male weighing 250lbs. In both these cases a 10% change in body weight represents 10lbs to the woman but 25lbs to the man. Therefore it is incorrect to refer to set point in terms of pounds, but rather it should be viewed in terms of percentage of body weight. Paradoxically both of these individuals have the same theoretical ability to gain fat mass as there is no scientific measurable limit to the amount of fat a human can accumulate.
This is why set point theory does not work to define an upper body weight limit, and specifically an upper amount of body fat.