Why Are Thin People Not Fat?

How do some people manage to stay thin despite eating a lot?

You probably know at least a couple of people who just don't seem to get fat no matter how much they eat. Some of them don't even do much exercise. But is it really true that some of us gain weight more easily than others, or do thin people just eat less calories?

Most of the studies on obesity and weight loss have been done on subjects who are overweight to begin with. A BBC Horizon documentary titled Why are thin people not fat? looked at the obesity problem from a different angle. They chose subjects who were naturally thin and stuffed them with excess calories. None of the participants had watched their food intake before, but their weight had remained roughly the same for years.

The subjects were told to eat at least double their usual calories and to avoid exercise for four weeks. The target energy intake for men was 5,000 kcal and somewhat less for women. The purpose was to find out whether naturally thin people would start gaining weight, given a sufficiently large amount of calories. It was no exercise in healthy eating either: the menu included processed, calorie-dense foods such as cakes and milkshakes. Precisely the kind of thing that should make one fat.

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.

For years, experts argued over the results of the Vermont prison study. According to the classical model of calories in, calories out, such high intakes should have led to a dramatic weight gain, especially since exercise was forbidden during the experiment. So how did some of the inmates stay thin?

This is the question that the BBC experiment tries to answer. I recommend watching the whole documentary, but here's a summary of the results:

• All participants had trouble reaching their energy intake goals
• Energy-dense foods such as chocolate made reaching the goals easier
• Some of the subjects gained more weight than others
• One of the subjects gained almost no weight but increased his muscle mass
• All subjects returned to their normal weights after the experiment

These results confirm the observations from the Vermont prison study: despite very high calorie intakes, some people have a harder time gaining weight than others. The documentary also explains how naturally thin people are able to 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

There's a lot of debate these days over the importance of basal metabolic rate (BMR) in the calories in, calories out model. It's interesting to note that nobody eats the exact same amount of calories per day, and yet weight remains in a very narrow range (at least in healthy, thin subjects). The one subject who stuck to his 5,000 kcal intake but gained almost no weight supports the idea that there is a kind of setpoint that the body tries to maintain regardless of calorie intake.

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 (Michal Eades comes to mind) 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. I think this theory makes sense.

The last two points of the list are especially interesting. There are two key attributes to fat tissue: the size and number of fat cells. The number of fat cells in your body is typically pretty much determined during adolescence. Thus, eating affects first and foremost the size of your fat cells. As you store and burn energy, the fat cells in your body grow and shrink accordingly.

That's not all there is to it, however. If you keep eating even after the fat cells have grown to their maximum size, at some point the body will begin to produce new fat cells to store all that extra energy. The tendency to produce more fat cells probably depends on the individual.

The problem is that according to our current understanding, the number of fat cells can only be increased, never decreased. This means that any new fat cells produced as a result of (prolonged) overeating will always stay with you. What's worse, as the purpose of fat cells is precisely to store energy, the body will now send more signals of hunger to your brain to keep those fat cells filled up. Obviously this makes following diets that rely only on cutting back on calories very difficult.

The overall message of the documentary is that being naturally thin is a combination of many factors, some of which are genetically determined and some a result of the environment. Of course, individual choice also plays a role, but the studies on small children given unlimited candy show that even before we have the capability to think rationally about our food choices (kids will eat as much candy as they desire), there are differences among people.

For those who have to struggle to maintain or lose weight, things are more difficult – though not impossible by any means. It just means paying attention to your diet, venturing beyond governmental recommendations, and trying on yourself what works. I've had many overweight people tell me how difficult it is to lose weight, and then when I ask them if they've tried for example a basic low-carb diet, they've either tried it for a few weeks and quit, or they've deemed it "unhealthy", because all they can picture is Atkins on his deathbed and slices of bacon clogging their arteries.

Are you a naturally thin person who can eat and eat without gaining weight? Are you the exact opposite? Share your experiences in the comment section!

For more information on diet and weight loss, see these posts:

Alternate-Day Feeding and Weight Loss: Is It the Calories Or the Fasting?
Green Tea and Capsaicin Reduce Hunger and Calorie Intake
A High-Protein Diet Is Better than a High-Carbohydrate Diet for Weight Loss
Low-Carb vs. Low-Fat: Effects on Weight Loss and Cholesterol in Overweight Men