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.


  1. I am unsure how I feel about genetics, especially here. For me, growing up as a scrawny ass kid with heavy parents and skinny grandparents, it is hard to be sure what is what.

    My father was thin before he married my mom. My mom was heavy and dieted all her life. She would starve herself before any major school event and I remember growing up watching her eat like a bird. She ate so very little.

    As an adult now, fat myself, I see what happened. She dieted herself into a horribly slow metabolism. I’ve done the same thing. The more I eat, as long as they are healthy choices, the more weight I lose. Just adding breakfast into my morning, within 20 minutes of waking up, has seriously changed my metabolism. I eat little bits all day, never really feeling full, but I also never feel as though I am crazy hungry. When I have a food craving, I sate it by being reasonable about it.

    When I was younger, outside of me being sick, was this the case? Did I eat small meals many times a day and was that a contributor to my fast metabolism? Did me switching to 3 then eventually to 2 meals a day when I started working and having a social life as a teen contribute to the slowing down of my metabolism? Who knows? It’s hard for me to judge, as my dad is now thin again, I am losing weight as a result of changes in my diet to ultimately alter my entire health (diabetes and heart issues) and was my mom more prone to her fatness as a result of her dieting all the time? All of my grandparents were thin. So, if I were to look at the entire family tree, I would say the above questions would be answered by a “yes.”

    I also wonder how much environment contributes to the pluses or minuses of genetics. How much did highly processed foods becoming more prevalent in the 70’s and 80’s play a part in all of this? The nutritional value of something simple we all love, like mac and cheese, alters dramatically when you compare said food coming from a box vs. being completely home made.

    So, I guess I am in conflict when it comes to genetics. I believe it can be a factor, but doesn’t have to be. Just like any other health history my family may have, I am not just going to accept I have to be that way because my family was or was not.

    I appreciate the study info on starvation and willpower. I know, for me, when I want something, there is no stopping me until that desire is sated. If I try to substitute it, then once I finally give into the craving, I binge way more than I would’ve if I had just had a small amount when I originally started wanting.

    I really enjoyed this post. I am looking forward to the next one. :)

    • It is true that dieting leads to weight gain, very consistently, so that was likely a factor for your mother (and for most women who are fat in the US today). I’ll be posting later on about how dieting leads to weight gain. I have vowed to stop calling the dieting industry the “weight-loss” industry because it’s not. It’s more accurate to call it the “weight-cycling” industry. Weight cycling is a result of dieting and is proven to be bad for your health.

      “I also wonder how much environment contributes to the pluses or minuses of genetics.”

      Environment currently contributes about 30%. That’s what the 70% heritable number means. The differences between one person’s body shape/size and another person’s is about 70% genetic and 30% environment. I’m not sure that the highly processed foods issue has changed that much. I’ve seen some reports that the heritability of weight hasn’t changed that much in the last few decades. Now, the whole population could have changed due to environmental changes, which would leave the heritability roughly the same, but environmental factors only contribute 30% to why one person is fat and another is not.

      “I guess I am in conflict when it comes to genetics. I believe it can be a factor, but doesn’t have to be. Just like any other health history my family may have, I am not just going to accept I have to be that way because my family was or was not.”

      It’s important to separate weight from health. Weight isn’t something you’re doomed to. It’s just a body type or body size. It may be a symptom of something else that is also genetically related, like PCOS or diabetes, and you can factor your family history into your medical treatment options. But don’t fall into the thinking that your weight is something to “not accept”, because that puts you in a losing battle with your own body.

      “I really enjoyed this post. I am looking forward to the next one. :)”

      Thanks! This one was research heavy and took me awhile to write. I’m not sure exactly what comes next because I’m taking a break from the research for a bit.

    • Genetics is also highly complicated and not always direct. My paternal side is thin across the board, my maternal side somewhat heavier but not what I’d really call fat. I’m the first person we know of on either side to have PCOS and I’m barely fat (BMI 31, enough to get crap from everyone but not really all that fat). I know PCOS is supposedly heritable, but I have no idea where mine came from.

      My husband’s side, however, is pretty much the genetic roulette wheel. He was thin as a child but started beefing up at puberty, and now is “morbidly obese” according to BMI. He has one sibling much heavier than him, three that are quite thin, and one that’s about average. His parents were both quite fat but have recently dieted to being significantly thinner, though 5 years hasn’t passed. Looking at his side of the family, really any body size is possible.

      Now come to our daughter, who’s only 2.5. She is VERY petite, like below the 15th percentile for weight, average for height. Thankfully my pediatrician was unconcerned about her slow weight gain, and now that she’s solidly a toddler I can see that she has a very similar build to her paternal cousins. Currently, her genes dictate that she is quite slender, but come puberty or early adulthood who knows what genes will kick in. Maybe she’ll have PCOS like me, maybe she’ll get heavy at puberty like her father, maybe she’ll stay thin like her aunts.

  2. I meant to comment on this earlier, but have been pressed for time. Kudos for writing it! It is obvious it took a lot of time and effort, and is well written!
    I’m with you on most of your argument, and appreciate the science to back it up. It makes sense that our body shapes would be mostly genetic. However, I’m not convinced about how large a role genetics plays in *amount* of weight we gain. I haven’t had time to look at that study, but I don’t see how 70% is possible.
    How do these studies explain our obesity rates skyrocketing since the mid-1970’s? It’s increased from about 15% to 33% in only a generation. That 30% environmental factor seems too low to account for that. Also, most of our ancestors came from other countries (some more recently than others), so why are Americans by far the fattest?
    Personally, the only person in my family history (that I know of) to be obese is my maternal grandfather, and that only happened in middle age. Both my parents are average and have been most of their lives. My mom has gained/lost modest amounts of weight for as long as I can remember, but has never been fat. None of my extended family is obese, so I guess I must have eaten enough to overcome my average-size genes?

    • The 70% heritable number is showing how much of the difference between members of a population is explained by genetics. It doesn’t necessarily explain why a single person weighs a certain amount. For example, if person A weighs 150 pounds and person B weighs 250 pounds, the reason for the difference between them is 70% genetic and 30% environmental. Does that make sense?

      “How do these studies explain our obesity rates skyrocketing since the mid-1970′s? … so why are Americans by far the fattest?”

      Those are good questions, but as long as we vilify fat people we have trouble finding good answers. Did you know that in 1998 BMI categories were changed so that 29 million Americans suddenly became overweight without actually gaining any weight? Did you know that there has been no change in our rates of obesity since about 2004? Why is that? Why do we still hear about obesity rates rising instead? Why does fat correlate with poverty? What have three decades of the diet industry done to our collective weight? I can think of all kinds of good questions. But if we want to know the answers, we have to drop our biases and look for actual information.

      “None of my extended family is obese, so I guess I must have eaten enough to overcome my average-size genes?”

      As Ashley points out above, genetics is complicated. I cannot answer the question about why you look differently from some relatives. But there’s no evidence that it has to do with what you eat. In Dr Sims experiment with thin prisoners who were offered early release, one third of them could not eat enough to gain enough to qualify. Put another way, one third of them could not get fat to save their own lives. Other research shows that, in general, fat people do not eat more per pound or eat differently than thin people. Fat people don’t have different metabolisms (unless they’ve dieted). There are medical reasons for weight-gain (stress, PCOS, thyroid, medications, etc) but that can be hard to sort out given all the weight bias that exists even in the medical field.

    • How do these studies explain our obesity rates skyrocketing since the mid-1970′s?

      I don’t have any data at hand to back this up (although I’m sure that the data exists), but my sense is that the 70’s is roughly when the diet industry started to get really big in America. Most people would say that, if obesity tracks the diet industry, it is because fat people are trying to lose weight. I would posit the reverse: that attempting to diet causes increased obesity.

  3. By the way, I didn’t mean to get hung up on the 70%. There were lots of really fascinating things you talked about, like fat people who are starving and trying to make thin people fat. I also really liked how you said our bodies are working to keep us a certain way. Very nice work.

  4. Very informative post! Definitely sharing this! I’ve watched so many women I admire struggle with their weight and body shape despite them having healthy lifestyles. Having learned that genetics take a crucial role in this saddens me somewhat because it really solidifies the idea that for some people, trying harder will never work. I think a change in perspective about the many beautiful shapes of the female body is in order. Now this does not mean that people should not be mindful of what they consume or their fitness level but that adopting a healthy lifestyle is the priority above attaining a certain weight or body image.

  5. I really enjoyed reading all of this. It’s some interesting shit! Every time I read something like this, I’m always curious about any change in the last 50 years? 100? I completely buy that we all have different body types and will naturally revert to that, but it’s easy for me to believe that our food sources have become less natural and healthy and that’s not good for us.

  6. If that’s the case, then what exactly is a healthy weight? The “set weight” that the body programs for the person? (Either way that is something interesting to show to my parents)

    • I don’t like to think about things in terms of “healthy weight”. People can be healthy at all weights and unhealthy at all weights. Health and weight are separate concepts. “Set weight” might be thought of as synonymous with “natural weight”, except that your set weight can be changed. Dieting, for example, is thought to raise one’s set point. So it isn’t as straightforward as thinking that there’s a certain weight that is “right”, and our desire to find that number and shoehorn our experience into it isn’t very helpful. Instead, it may be more helpful to set weight aside entirely and focus on other things, like healthy habits, if you want to.

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