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Why do some people never gain weight?

woman eating pizza
(Image: © Shutterstock)

It's one of life's great injustices: Some people must carefully pay attention to everything they put in their mouths in order to maintain their weight, while others can eat doughnuts to their heart's content and achieve the same result. So what's the secret? How do some people manage to never gain weight?

There isn't one simple answer to this question, said Kathleen Melanson, a professor of nutrition and food sciences at the University of Rhode Island. "There's genetic, nutritional, and even behavioral factors involved," Melanson told Live Science, "and the extent to which each of those factors come into play in any given individual is going to vary."

One of the most important factors has nothing to do with body type, metabolism, or performing a spell during the full moon: It's perception. Many people who appear to eat whatever they like without gaining weight aren't actually eating more than the rest of us, Melanson said. For example, your friend who eats ice cream on a daily basis might naturally compensate for those extra calories by eating less at another meal, or snacking less throughout the rest of the day. Or perhaps, when they eat pizza, they're eating slowly, getting full, then stopping after just a few slices. 

Related: Can you turn fat into muscle?

"If you measured these people's calories, they may not eat as much as you think," said Dr. Frank Greenway, the Chief Medical Officer at Pennington Biomedical Research Center. "They're just eating calorically dense foods when they do eat; things other people might have a difficult time not overeating."

Physical activity can also make a difference, but it doesn't have to be a gym workout. "Some people just move more, even if they're not necessarily athletes," Melanson said. For instance, they might fidget or pace, have an active job, or spend all day chasing their kids around. There’s even evidence that some people are genetically predisposed to want to move their body, Melanson said. That extra movement can also rev up the body's metabolism, or how much energy your body spends throughout the day, not including exercise. The more you move, the more  "the mitochondria within the cells of the muscle will increase in number and in their activity. And those are the power plants that are creating energy, using energy for movement," Melanson said. More mitochondria, means more calories burned. 

There's little evidence to suggest that — without exercise — some people are born burning significantly more calories than others, said Dr. Ines Barroso, a researcher at the University of Cambridge in England who studies the genetics of obesity. But there may be physiological differences that allow some people to naturally moderate the number of calories they consume without exercising tremendous self restraint, Melanson said. Cascades of nervous system signals and hormones that circulate in our blood interact to tell us when we're hungry or full. This is called the appetite regulatory system, and it may be more sensitive in some people than others, Melanson said. 

One important hormone involved in this system is leptin. It helps regulate how much food we want to eat over longer periods of time, not just for our next meal. So a person with a more sensitive system might go back for seconds and thirds at a party, then feel full for the next few days and eat less. "They just automatically can kind of recalibrate their energy balance because their appetite signaling system can say, 'Okay, we got enough energy,'" Melanson said. 

Genetics can play a role in a person's tendency to gain or lose weight. Researchers have identified over 250 different regions of DNA that are associated with obesity, according to a 2019 study published in PLOS Genetics. For this study, researchers compared 1,622 healthy people with low body mass index (BMI) against 1,985 people with severe obesity and 10,433 control people of normal weight. They found that the thin participants had fewer genes associated with obesity. But according to Barroso, who was a co-author on the study, genes alone don't determine your weight. "We didn't find genes that were exclusively either protecting from obesity or predisposing someone to obesity. It seemed like a continuum," Barroso said, "You also have people who have the genetic determinants for obesity yet they're not [obese]." Barroso said.

In the end, the answer is complex: our tendency to gain weight or maintain our weight isn't pre-determined, but it's also not entirely under our control. There's no genetic on-off switch that allows some people to eat all they want without gaining weight; at the same time, a tendency to gain weight isn't necessarily due to a lack of self control, Melanson said.

"It's not the same from one person to another."

Originally published on Live Science.

  • Debunk the Junk
    Genetics will play the largest factor. Scientists don't like saying this too often because it carries cultural implications but it is what it is. We are biological beings, there's no getting around it.

    Also I find the first 3-4 paragraphs of this article not really science at all, rather anecdotal opinion based off junk science that produces inconsistent evidence. I.e. the calories in:calories out arguement. The hypothesis that you can consume a bowl of ice cream ridden in tons of sugar, high-fructose genetically modified corn syrup, and preservatives as oppsed to organic chicken breast within the same caloric intake value and the result is negligible is not reputable science you should give any mind too. As soon as you introduce the pancreas and liver into the equation this in-out argument becomes extremely silly and misinformed.

    It's important to remember that in the scientific method, hypothesis is not evidence, and evidence is not proof. When researching studies, aways look at which body is conducting the study, what is the sample size and what are the controls. Was it peer-reviewed independently by a reputable source. You can pretty much toss out studies conducted internally by manufacturers and always get a second opinion on anything FDA regulators pass. The current American nutrition pyramid is basically upside down. A real boon to those industries that rank low on the pyramid.

    No mention of thyroid conditions eh?
    Reply
  • senecarr
    Debunk the Junk said:
    Genetics will play the largest factor. Scientists don't like saying this too often because it carries cultural implications but it is what it is. We are biological beings, there's no getting around it.

    Also I find the first 3-4 paragraphs of this article not really science at all, rather anecdotal opinion based off junk science that produces inconsistent evidence. I.e. the calories in:calories out arguement. The hypothesis that you can consume a bowl of ice cream ridden in tons of sugar, high-fructose genetically modified corn syrup, and preservatives as oppsed to organic chicken breast within the same caloric intake value and the result is negligible is not reputable science you should give any mind too. As soon as you introduce the pancreas and liver into the equation this in-out argument becomes extremely silly and misinformed.

    It's important to remember that in the scientific method, hypothesis is not evidence, and evidence is not proof. When researching studies, aways look at which body is conducting the study, what is the sample size and what are the controls. Was it peer-reviewed independently by a reputable source. You can pretty much toss out studies conducted internally by manufacturers and always get a second opinion on anything FDA regulators pass. The current American nutrition pyramid is basically upside down. A real boon to those industries that rank low on the pyramid.

    No mention of thyroid conditions eh?
    All genetics are only expressible in an environment. What's more, metabolic functions are pretty nuts and bolts stuff that don't usually have huge variances. So I'm not sure what you think is so biologically determined.
    Calories in versus calories out isn't an argument. It is both an expression of one of most well observed science laws - thermodynamics, as well as a consistent observation of nutrition. You lock someone in a metabolic ward so you can truly measure all the output, and control all the dietary input, and even back into the 1800s, we don't see more than 4% disagreement between the two - smaller for any modern ones.
    I don't know what it matters about the GMO in high-fructose GMO corn syrup. Corn can be a GMO crop, but high-fructose syrup with the same fructose/glucose percentage is the same chemical, and none of it can be genetically different because saccharide molecules don't have genes. If you think the saccharide molecules of a particular type, e.g. fructose, glucose, sucrose, can be different while the same type by virtue of the plant making them, you've misunderstood somethings in chemistry.
    That said, if you keep calories the same and account for calories out (a food with more protein will have higher thermic digestion effects for output), yes, the change in weight over time between the two foods will be negligible. A step further, if you control for differences in the macros - fats, carbs, proteins - you'll have the biggest factors accounting for dietary control of body weight and body composition. That's where chicken versus ice cream can start being non-negligible.
    Your take on who conducts the research comes off as a bit much. Conflicts of interest are reasons for caution in analyzing the findings of research but they are never evidence of themselves of anything wrong with a finding or methodology.
    Thyroid conditions generally change how active someone is and perhaps a bit of their appetite. It doesn't change anything fundamental about if people can lose or gain weight.
    Reply
  • gsimonel
    I'm surprised there was no mention of the microbiota.
    Reply
  • alfie
    Have you ever heard of a skinny domestic pig? Have you ever heard of a fat wolf? How many fat lions have you seen? Genetics uber all!
    Reply
  • chrisko
    I am one of those people! Actually had my annual physical today and discussed this with my doctor. I try very hard to gain weight, but find it very difficult. I will often make month long efforts to do so, but achieve little results. I can spend an entire month, every single day, eating 6,000 to 8,000 calories per day. After 30 days, I will gain 2-3 pounds (1.5-2% gain of body weight). It is actually quite difficult for me to consume this much food in any given day! It takes a real conscious effort to eat this much, IMO. According to my Dr., my body mass index is "perfect" at 22% and he says, as such, I should be somewhat careful in such efforts as I am at the ideal point. So, I conclude that it is hereditary combined with certain habits (that are not necessarily good). There are three things that I do: I drink a fair amount of coffee (4-5 cups per day, with cream and sugar), I drink the equivalent of 4-5 cans (12 ou) of Coke per day (when I was younger I drank between 12-24 cans of Coke per day), I smoke! The coffee and Coke both speed up my metabolism (sugar & caffeine), smoking suppresses my appetite (but for me, drives my thirst or desire to consume beverages). I can't say whether there is a gene, a life style, or what. In my 30s, people told me it would change when I hit my 40s. In my 40s they told me it would change when I hit 50. I am now half way through my 50s . . . Obviously, I am thinner than I would like to be. Things that I don't do: drink much alcohol, snack, eat very many processed foods. I love red meat, bacon, BBQ, good cheeses, fatty foods, salt, but I don't like candy very much. I get a moderate amount of exercise. I am only 5'8" tall, a male. If I eat less than 2,500 calories in a day, I lose weight. Ideally, if I eat 3,000-4,500 calories per day, I neither gain or lose weight. I go to the bathroom 2-3 times per day and urinate 12-15 times per day. I try to eat every night right before I go to bed - usually this is the equivalent of two bowls of sweetened cereal with 1.5-2 cups of whole milk. I don't eat breakfast (except when I am making month long efforts to gain weight)! I sometimes worry that this sets my body up to conserve or change in a way that may contribute to my problem - but I find I'd rather have a couple of cups of coffee and smoke than take the time to eat breakfast before work. Overall, I am one of those people that eats and believes in a balanced, highly diverse diet. I eat fruit and vegetables everyday, but also eat meat every day. I don't believe in or get into taking all sorts of supplements - as I think they are a waste for people that eat diverse, well balanced diets. I also have seen how using supplements as an alternative to eating a balanced diet causes unintended harm and consequences to our bodies. I feed my body whatever it is that it is craving! I believe my body's cravings are a way for my body to tell me what it needs from a nutritional perspective. I will find if I am traveling for several days for work (which means eating at restaurants for every meal), that after a few days I will be craving a big salad, or fruit, or something (which may be less prevalent in your typical restaurant meals).

    Overall, things that I consume a lot of, a disproportionate amount of (as a % of my intake):

    Salt that I add myself to food (but I suspect it is FAR less than I would consume if eating processed foods)
    Meat, red meat in particular
    Olive Oil - we go through a LOT of olive oil
    Sweetened cereals (captain crunch, cocoa pebbles, frosted flakes, apple jacks)
    Coke (no, there is NO substitute, other than RC when I am in the mood for it)
    Butter - my wife always asks why I add butter to nearly every dish I am preparing - because it makes nearly everything in the world taste better!

    None of this is to say that I think I am good or lucky or anything of the sort. I know I have some bad habits and am intelligent enough to know I should change them (drinking too much soda and smoking) - but I simply have chosen not to - I am not a victim to anybody but my own choices in this regard. Though I did promise my Doctor that by next year's physical, I will have quit smoking. He replied that more people die every year from from being overweight and the problems it causes (especially the accumulation of fat around our organs - which roughly 20% of very overweight/obese people suffer from).

    Because I have such a tough time keeping weight on or gaining weight, I think it has given me a better understanding or appreciation for people who have the opposite problem (obesity/weight gain).

    I sometimes wonder if I should have my intestines and/or stomach tested for enzymes/bacteria types and the such to determine if this contributes to my scenario.
    Reply
  • Aleph Null
    Hormones. Ask most ‘always skinny‘ females what happened after menopause. Unwanted weight gain with no other changes in life.
    Reply
  • Minamii
    Debunk the Junk said:

    No mention of thyroid conditions eh?
    I spent forty years wondering why I could only maintain my weight if I ate no more than 800 calories per day whilst my friends could eat whatever they liked without a problem. Fast forward to when I reached the age of 45 and finally got a diagnosis that explained the weight gain, the extreme fatigue, the brain fog and the joint aches and pains I'd always suffered from - seems my immune system, back when I was 9 years old, decided I didn't need a thyroid gland and destroyed it. Since I got my diagnosis I've had access to a tiny pill once a day which provides the thryoxine hormone I lacked, I've found my weight has been dropping, plus all my other health problems have resolved. So yeah, I agree, everybody is different, and fat people are not necessarily "lazy" or unmotivated. Some of us have a medical condition that means we really can't control our weight. I wish the doctors hadn't just brushed off my various complaints over the years, had taken a blood test and found out that I had no thryoid. It would have saved me such a lot of grief.
    Reply
  • Nytowl223
    I
    chrisko said:
    I am one of those people! Actually had my annual physical today and discussed this with my doctor. I try very hard to gain weight, but find it very difficult. I will often make month long efforts to do so, but achieve little results. I can spend an entire month, every single day, eating 6,000 to 8,000 calories per day. After 30 days, I will gain 2-3 pounds (1.5-2% gain of body weight). It is actually quite difficult for me to consume this much food in any given day! It takes a real conscious effort to eat this much, IMO. According to my Dr., my body mass index is "perfect" at 22% and he says, as such, I should be somewhat careful in such efforts as I am at the ideal point. So, I conclude that it is hereditary combined with certain habits (that are not necessarily good). There are three things that I do: I drink a fair amount of coffee (4-5 cups per day, with cream and sugar), I drink the equivalent of 4-5 cans (12 ou) of Coke per day (when I was younger I drank between 12-24 cans of Coke per day), I smoke! The coffee and Coke both speed up my metabolism (sugar & caffeine), smoking suppresses my appetite (but for me, drives my thirst or desire to consume beverages). I can't say whether there is a gene, a life style, or what. In my 30s, people told me it would change when I hit my 40s. In my 40s they told me it would change when I hit 50. I am now half way through my 50s . . . Obviously, I am thinner than I would like to be. Things that I don't do: drink much alcohol, snack, eat very many processed foods. I love red meat, bacon, BBQ, good cheeses, fatty foods, salt, but I don't like candy very much. I get a moderate amount of exercise. I am only 5'8" tall, a male. If I eat less than 2,500 calories in a day, I lose weight. Ideally, if I eat 3,000-4,500 calories per day, I neither gain or lose weight. I go to the bathroom 2-3 times per day and urinate 12-15 times per day. I try to eat every night right before I go to bed - usually this is the equivalent of two bowls of sweetened cereal with 1.5-2 cups of whole milk. I don't eat breakfast (except when I am making month long efforts to gain weight)! I sometimes worry that this sets my body up to conserve or change in a way that may contribute to my problem - but I find I'd rather have a couple of cups of coffee and smoke than take the time to eat breakfast before work. Overall, I am one of those people that eats and believes in a balanced, highly diverse diet. I eat fruit and vegetables everyday, but also eat meat every day. I don't believe in or get into taking all sorts of supplements - as I think they are a waste for people that eat diverse, well balanced diets. I also have seen how using supplements as an alternative to eating a balanced diet causes unintended harm and consequences to our bodies. I feed my body whatever it is that it is craving! I believe my body's cravings are a way for my body to tell me what it needs from a nutritional perspective. I will find if I am traveling for several days for work (which means eating at restaurants for every meal), that after a few days I will be craving a big salad, or fruit, or something (which may be less prevalent in your typical restaurant meals).

    Overall, things that I consume a lot of, a disproportionate amount of (as a % of my intake):

    Salt that I add myself to food (but I suspect it is FAR less than I would consume if eating processed foods)
    Meat, red meat in particular
    Olive Oil - we go through a LOT of olive oil
    Sweetened cereals (captain crunch, cocoa pebbles, frosted flakes, apple jacks)
    Coke (no, there is NO substitute, other than RC when I am in the mood for it)
    Butter - my wife always asks why I add butter to nearly every dish I am preparing - because it makes nearly everything in the world taste better!

    None of this is to say that I think I am good or lucky or anything of the sort. I know I have some bad habits and am intelligent enough to know I should change them (drinking too much soda and smoking) - but I simply have chosen not to - I am not a victim to anybody but my own choices in this regard. Though I did promise my Doctor that by next year's physical, I will have quit smoking. He replied that more people die every year from from being overweight and the problems it causes (especially the accumulation of fat around our organs - which roughly 20% of very overweight/obese people suffer from).

    Because I have such a tough time keeping weight on or gaining weight, I think it has given me a better understanding or appreciation for people who have the opposite problem (obesity/weight gain).

    I sometimes wonder if I should have my intestines and/or stomach tested for enzymes/bacteria types and the such to determine if this contributes to my scenario.
    I was just like you for decades, and even after I became disabled, I didn't gain weight, simply lost muscle mass. The only difference was that I don't add salt to much of anything. The rest, though, could have made us twins.

    When I reached 53, I was put on a medication for chronic pain that changed all that. In a week less than 5 months' time, I gained 10 sizes. I kept pointing it out at every appointment, but was told my body would get used to it. At the 5 month mark, I begged my doctor to take me off off it and put me on something - anything - else.

    Well, I then stopped gaining, but have not been able to lose ANY of the excess weight I gained due to that medication. At all. So, now I am stuck not only disabled, but horribly fat, too. And the depression that causes follows me everywhere. Going from a size 8/10 to a size 18/20 in 5 months is a nightmare, and one from which I can't seem to wake. Feel privileged you can still complain about being thin.

    run. It ruined what little life I had left.]
    Reply
  • Minamii
    senecarr said:
    All genetics are only expressible in an environment. What's more, metabolic functions are pretty nuts and bolts stuff that don't usually have huge variances. So I'm not sure what you think is so biologically determined.
    Calories in versus calories out isn't an argument. It is both an expression of one of most well observed science laws - thermodynamics, as well as a consistent observation of nutrition. You lock someone in a metabolic ward so you can truly measure all the output, and control all the dietary input, and even back into the 1800s, we don't see more than 4% disagreement between the two - smaller for any modern ones.

    Thyroid conditions generally change how active someone is and perhaps a bit of their appetite. It doesn't change anything fundamental about if people can lose or gain weight.

    I'm afraid I have to take issue with this - when I was unaware that I had no thyroid, my resting heart rate was at 30 bpm. Compare that to a "normal" heart rate of around 60 bpm. Now, after taking replacement hormone for my lack of a thyroid, my bpm has gone up to 45. Still not that much, and still not so-called "normal". But enough for me to start losing weight on my maintenance calories of 800 per day. Also, my body temperature is low. Before I got my diagnosis of Hashimoto's Thyroiditis (the medical term for no working thyroid) my body temperature was around 34.5 degrees C. I spent almost my entire life from the age of nine years in a state that is normally described as "severe hypothermia". Which is miserable and saw me feeling cold all the time, even in the middle of an Australian summer in the bush, where temperatures can quite easily rise to 46 degrees C.
    Since taking the replacement hormone, I have seen my temperature rise one degree C to 35.5. Still not the "norm" of around 36-37 C, but better than it was. My weight has been gradually coming off due to this rise in temperature and heart rate since, and apart from taking the hormone, I have done nothing differently. I still eat no more than 800 calories a day.
    Reply