Weighty Problem

I saw a documentary that claimed that some people can't gain weight. It was really ridiculous, because the study didn't last long enough.
They did the study in an English(?) prison. If you reached the target weight, they let you out of prison. So there was lots of incentive to gain weight. Everyone in the study gained weight, but very few were able to pass the target weight of 25kg extra (55lbs).
They decided that some people CAN'T get fat.

The human body has all sorts of defensive tendencies, whether it's some sort of natural biological brain, or coincidence.
Fat requires virtually no maintenance to store. Protein, however, requires a constant flow of nutrients.
I don't like hearing that athletes have "a higher metabolism". That sounds too much like athlete's bodies are inefficient, when actually the opposite is true.
If you ever went to see Body World, then you saw how some people had virtually no muscle tissues anywhere on their body. It's horrifying.
The athletes - even the skinny ones - were almost entirely muscle.
Granted, when you exercise, there is a span of days where the body is cranking to recover. But let's say the athlete has a week of inactivity. The athlete will be using more calories sitting still than the obese person. This isn't because their engine is cranked in an inefficient way. This is because fat requires virtually no calories to maintain.
If you ever saw the TV show "The World's Biggest Loser" where teams of contestants try to lose more than other teams, then you know they do it by gaining muscle. Activities themselves really don't burn many calories. It's AMAZING what you can do, with so few calories. It can be frustrating, if you're on an ergonomic machine that counts calories, to see so few calories clicking off. You'd be inclined to think they're under-counting. But they probably aren't. Your body burns most of its calories through maintenance, not activity.
To win "The World's Biggest Loser", you do it by targeting muscle gain, not fat expenditure and starvation. Granted, you have to consume reasonable quantities and healthy foods, but they aren't working-out just to burn-off calories. Muscle weighs twice as much as fat, so as you gain muscle, you have to lose more than twice the fat by-volume in order to lose weight overall. But if you succeed in gaining muscle, your fat-burning engine will coast along, even when you don't work-out.

How long does it take to gain or lose enough muscle? It obviously depends on intensities, diet, but it takes many months just to start gaining weight. Gaining 25kg? Crap, they gotta be kidding! For people who have not been fat, changing their bodies to become fat requires the loss of a LOT of muscle! Even people who aren't athletes tend to have more muscle tissue than obese people. The more solid your muscle, the longer it will take. Saying some people "can't" get fat is preposterous. Anyone who stays sedentary long enough will lose muscle. When you lose muscle, you lose caloric expenditure when sitting and sleeping. If you consume more calories than your body burns, your body will try to burn it off through inefficiencies. This might cause plateaus. But eventually, if you keep up the sedentary lifestyle and over-eating, the math will go against you and your body will store more and more of the calories. This process takes YEARS!

Another thing that makes this prison study preposterous is that if you Google "weigh, gain, prison", you'll get tons of hits on how prison food will make you fat. If you live in a prison, and don't work-out, then you'll definitely get fat.