Why Are Thin People Not Fat? In experiments where particpants  eat whatever they feel like, all of them stay with a few percent of body weight all the time. So there’s definitely a built-in feedback loop between appetite and weight.

The documentary begins by mentioning a similar experiment done on Vermont prison inmates in 1967. The inmates were grossly overfed with the purpose of studying the hormonal changes that happen when a person becomes obese. The prisoners who signed up were promised an earlier release.

Each inmate was supposed to increase their body weight by 25 percent. However, as the experiment progressed, it turned out that no matter how high the energy intake got, some of the inmates could not reach their targets. Despite eating and eating, they just didn’t gain enough weight. One of them could not increase his body weight more than 18%, even though his daily calorie intake reached a whopping 10,000 kcal

So how do naturally thin people stay thin?

• Appetite has a genetic basis
• Age, weight, and diet of the mother during pregancy influence the child’s weight
• Eating habits learned during childhood carry on until adulthood
• Naturally thin people avoid excess calories instinctively
• People have a certain “natural weight” towards which the body aims
• Basal metabolic rate plays a strong role in energy expenditure
• The feeling of hunger is related to the number of fat cells
• The number of fat cells can grow but never diminish

It also looks like in some people, the mechanisms to preserve the natural weight setpoint are stronger than in others. Increased heat production is obviously one way to maintain weight during increased energy intake. Some people have also argued that as the number of calories eaten increases, the body starts to burn them by increasing small, almost involuntary movements such as tapping your fingers, moving your legs, etc. – physical activity which is not considered exercise but still uses up extra energy.

hat-tip Matthew