Genetics, Starvation, and Willpower

This photo comes from reader Jo, who says, "I come from a loooong line of ‘husky’ waist-less people, and I come by it VERY honestly. I love these pictures of my grandparents. They remind me of who I am."

I’ve written about how diets don’t work, which means it’s pretty difficult to make a fat person thin. It’s popular to blame fat people for that, but it turns out the story is a little more complicated. I’ve found a few other interesting places to dive into, including tales of thin people who try to get fat and the research on the genetics of fatness. After that, let’s tackle the big, bad willpower thing that everyone’s always going on about!

Making Thin People Fat

Dr Ethan Sims is famous for experimenting with trying to make thin people fat. He used prison inmates who would earn early release from their sentences if they could gain 20-25% of their weight. The inmates ate and ate, some eating as much as 10,000 calories a day while also reducing their activity. As the experiment went on, they had a worse and worse time of it, some developing an aversion to meals. Some of the men completed the task. Some of the men dropped out. Some were unable to gain the required weight, even though they were eating more than the men who were successful. For the men who had gained weight, their metabolisms increased by as much as 50% which means their bodies began burning way more calories than they normally did. In order to sustain their new size they had to aggressively eat 10 times more than “calories in/calories out” suggests should have been necessary for their new weights. As soon as the study was over, the men who had gained weight effortlessly dropped back to their previous weights and stayed there.

While not as scientific, similar results were obtained during a UK documentary (headless fattie alert). 10 thin people spent a month trying to eat enough to reach excessive calorie targets while not exercising and trying not to walk very much. One participant put on 8 pounds. Another put on 12 pounds. One put on just 1 pound. Another put on 12 pounds but saw a decrease in body fat percentage. Another put on 10 pounds, but his appearance didn’t seem to change. Despite his lack of activity, the weight had gone on as muscle instead of fat as his metabolism rose 30%. What happened after the experiment? Those who had gained dropped back to their previous weights without dieting or exercising.

It seems that it’s as difficult to make a thin person fat as it is to make a fat person thin.


When asking whether you’re in control of how fat you are, we have to start with your genes. It turns out that genetics plays a huge role.

In one experiment, pairs of twins were fed 1000 extra calories a day, 6 days a week, for over 5 months. If the calories in/calories out people were right, the subjects should have all gained the same amount. Instead, their weight gains ranged from 9.5 pounds to 29 pounds. Furthermore, each set of twins gained the same amount as each other, and they put that extra weight on the same body area.

Another study looked at 673 pairs of twins, including identical twins, fraternal twins, twins reared together, and those reared apart. Here, again, it turns out that genetics are the key. Identical twins had the same BMIs, whether they were raised together or not. Fraternal twins varied more, even when they were raised together. The study concluded that weight is about 70% heritable, and that “childhood environment has little or no influence”. This didn’t really surprise the researchers, since this finding agreed with previous research.

When looking at 540 adult adoptees who had been adopted in their infancy, the adoptees were as fat or as thin as their biological parents, and their weight had no relation to that of their adoptive parents. The researchers concluded that “genetic influences have an important role in determining human fatness in adults, whereas the family environment alone has no apparent effect.”

Variation in weight is genetic. To put that 70% figure into perspective, think of other things you think of as genetic. Breast cancer? Only 30% heritable. Weight is also more heritable than heart disease, hypertension, or schizophrenia.


In 1959, Dr Jules Hirsch performed experiments with fat people so that he could learn what happened to their fat cells when they lost weight. Through a rigorous, scientifically monitored diet, the fat people spent 8 months turning into thin people. The participants lost 100 pounds on average, but after the diet program was over they put the weight right back on, and Dr Hirsch wanted to know why. He and Dr Rudolph L Leibel repeated the diet with more fat people, to the same results. During the experiments, the doctors measured the participants’ metabolism, vital statistics, and psychiatric conditions, leading them to discover some interesting things. After they began to lose weight, the metabolisms of the fat people nose-dived. Whereas before the diet they burned the same number of calories per square meter of body surface as did thin people, after losing weight they burned as much as 24% fewer calories per square meter of surface area. The participants also developed issues with food: they dreamed about food, fantasized about food, fantasized about cheating on the diet, hid foods in their rooms, and binged. They became anxious, depressed, and even suicidal. This collection of symptoms even had a name, because it had been seen before.

In 1944, the Minnesota Starvation Experiment took 36 men of “normal” weight who lived in a dormitory for the duration of the experiment, and provided a thorough and fascinating look into what’s really going on when people reduce their caloric intake. The men walked about 3 miles a day and had their calories adjusted each week to help them achieve a weight-loss goal of 2.5 pounds a week. Their average calories for the “semi-starvation” period was 1600 a day. Want to guess what happened? They experienced dizziness, soreness, hair loss, hysteria, hypochondria, loss of sex drive, social withdrawal, severe emotional distress, and almost all of them became depressed. Their body temperatures, respiration, and heart rates declined. Some had swelling in their extremities. Some had ringing in their ears. One participant cut off three of his fingers. One became suicidal. They became obsessively preoccupied with food. They were irritable and anxious when calorie-adjusting time came. They couldn’t leave the dorm alone to ensure that they wouldn’t sneak food. They developed elaborate eating rituals and ways to make their food last longer. The semi-starvation period lasted almost 6 months, followed by a 3 month controlled re-feeding period. Some symptoms like dizziness and lethargy went away quickly, but others such as hunger and loss of sex drive lasted for many months.

The physical and emotional state of these men was called “semi-starvation neurosis”. Now go back to Dr Hirsch’s fat subjects who lost weight. They had the same emotional issues and metabolic measurements, leading the researchers to an interesting new conclusion: fat people who lose lots of weight might look like thin people, but they are actually fat people who are starving. All the people who lived at the hospital for these experiments developed the physical and psychological markers of starvation.

The Mythical Willpower

Thin people have trouble gaining weight, which means that fat people aren’t just thin-people-who-got-fat. Fat people who lose weight become fat-people-who-are-starving, and genetics help tell the tale of which is which. Even with that information, the anti-fat people always swing back around to willpower. They’ll concede that some people will have a harder time of it than others, but that if you just knuckle-down and try really hard you, too, can be thin. In Dr Hirsch’s study, a handful of people remained thin. They did so by essentially making being thin their life’s work, such as by becoming Weight Watchers leaders, and maintaining themselves in a permanent state of starvation. Setting aside the casual cruelty of people wanting you to starve, let’s look at what that actually means inside your body.

Some of the research with twins I mentioned earlier provided evidence that people have a “natural” weight to which they will gravitate, sometimes called a “set point”. This set point might span 10-20 pounds or so. Losing or gaining more than that will be very difficult. Whenever you lose or gain much more than that, your metabolism shifts to compensate and nudge you back to your set point.

Efforts to lose weight kick in other powerful biological responses. Increasing research is showing that reducing weight is countered by various neurochemical changes that increase the urge to eat and decrease energy expenditure. For example, food restriction and weight loss decreases leptin, a protein hormone. This decrease initiates aggressive food-seeking behavior. A gastrointestinal hormone called ghrelin is responsible for feelings of hunger and this hormone increases when food is restricted. There are also cortico-limbic controls that regulate eating and that kick into overdrive when deprived. Peptide YY is a hormone that helps determine your appetite by communicating with multiple parts of your brain to regulate how much you eat and the pleasure involved in eating.

These things cannot be controlled by willpower. These are powerful biological controls as basic as those related to breathing, blinking, sleeping, and pissing and shitting. Your so called “willpower” is simply not in charge. You can’t just “decide” to eat less. You can’t just “try harder”. We’re talking about how your body works here. We’re talking about your body working. When you try to eat less, your body compensates to help you fix the problem. You can call it a lack of willpower and discipline but the evidence shows that the resistance is biological.


The conclusions are the same, study after study. Diets don’t work. You cannot generally make fat people thin or thin people fat. Genetics play a huge role in deciding which is which. Your bodily processes take care of the rest. It’s as simple as that.