Why Are Thin People Not Fat?

At one time or another, it seems we’ve all had that friend.

The one who seems to be able to eat whatever he or she wants without gaining weight. And maybe, if we’re close enough,

we’ve expressed how lucky they are to be so “naturally thin.”  But, does such a thing really exist?

A friend recently sent me a link to the following BBC documentary from 2009 titled, “Why Are Thin People Not Fat?,” which explored this question in some depth. In the film, a subject group of “naturally thin” folks were told to increase their calorie consumption and limit their exercise for a period of 4 weeks to see if they would gain weight as expected.

I won’t spoil the results for those of you who are interested in watching, but I will say some of them were quite interesting. It should also be noted that this experiment was done primarily for entertainment so I doubt it would hold up to rigorous scientific evaluation. Because of this, I think it’s only appropriate that we take the results with a grain of salt.  Regardless, the exploration of genetic predisposition to weight is fascinating.

Here is the complete documentary on YouTube:

Or watch the video here: http://www.youtube.com/watch?v=jPsKpnu0-X4

Do you know someone who fits the description of “naturally thin”? Are you one of these people? What do you think of the concept of a weight set point? If you did watch the documentary above, what did you think of it?

And most importantly, for all my British friends out there- what’s the deal with your fondness for eating pizza with mayonnaise?



55 thoughts on “Why Are Thin People Not Fat?

  1. Michelle

    I’ve seen about 10 minutes so far and can’t wait to see the rest. I’ve been 112lbs for 20 years, since puberty at 13 and I’ve always eaten everything and anything I wanted. I’ve always been happy about it and not really examined it past that so I’m very interested to see what they find! Thanks for sharing.

  2. Andrea

    My husband is one such naturally skinny person, as is his sister, his aunt, his cousins and his grandfather. I prepare and eat the same things my husband does for dinner, and I walk at least twice, usually 3 times as much every day (we both wear fitbits). Our main difference would be lunches, where he often goes to restaurants and I stay at home and eat a turkey sandwich or something which I would consider not extravagant, and certainly not a steak dinner, Chicago deep dish pizza or cheese risotto he orders at restaurants. I am currently doing weight watchers and am loosing the weight a little at a time, but to say that the situation is frustrating is a bit of an understatement. Is “Inability to gain weight” grounds for divorce? ;) (I feel I have to say, since this is the internet and its words and sarcasm and joking don’t come across well. I love my husband dearly and I’m completely joking about divorce. I’m one pretty lucky girl to have him.)

  3. tali

    I used to be one of those people, I didn’t even know what a calorie was. It has caught up with me & I think it does with many naturally thinner people.

  4. Routhie

    I went to school with a ‘naturally thin’ person and her life was not easy or perfect. She was in and out of hospital due to her low weight. Many whispered accusations of bulemia or, if they really didn’t know her, anorexia. The truth was she ate quite a lot and found it hard to balance athletics and diet. She was on various athletic teams and found herself on the losing end due to her weight. I see photos of her now, looking healthy after a baby, still thin, but not as drastic as when we were younger. I would dare say she has figured it out or her body equalized. She certainly taught me not to automatically be jealous or think the worst. We are all different and while I work through my own weight loss, I remind myself to be more involved of what goes into my family, while remaining conscious of eating well. It’s all we can do.

    1. midcountymomma

      I used to be the same way too. I was made fun of in school for being SO skinny. I would come home, eat, eat, eat and cry. I got over it, ate what I wanted, found a wonderful husband and had 3 kids. I just thought I would always be that way. I lost the weight with each child (not too fast), then thyroid problems hit. I yo-yo with weight (lately gaining more than I’m losing) with no end in sight. I am trying to like the bigger me that I always thought I wanted, but find myself missing the smaller me that I was. Its funny–you get what you think you want and you are not happy there either.

  5. Nuts about food

    My roomate in college is that kind of person, and so is my step mom. I have lived with them and seen them eat on a daily basis (unhealthy food and unhealthy amounts!). Neither of them exercises and the former has had three children without putting on an ounce. They exist, I promise.

  6. Miss M

    Have you tried mayo on pizza? Its fab. Garlic mayo is of course, much much better.
    Also coleslaw, for creamyness and crunch.
    Try it – i dare you – you wont be sorry!

  7. Pam

    I got my degree as a nutritionist at a London university and one of my teachers showed us this video when we were learning about our bodies natural weight regulation theories, including set point which I think is a pretty accurate theory, interesting stuff! I’ve lived in London for 4 years now and have never seen people eat pizza with mayo, maybe it was a 90’s thing?! :)

  8. Alice

    I have a naturally thin friend who can eat buckets and non gain a pound. In fact, he struggles to keep weight on. We used to live together and he would eat easily double what I ate and he was still incredibly slender.
    I, on the other hand, have struggled with my weight since puberty hit, but then my eating has been disordered for most of my life. I often wonder what my body would look like had I not had eating disorders. My mother was very slim at my age, only after having children did she become heavier. Maybe that would have been me.

    However, another slim friend I have used to be heavy. It looks, and sounds, like she eats whatever she wants, but I think she is actually very carefully controlling her intake to maintain her lower weight. She will split a bag of Minstrels, relies heavily on vegetables in her meals, eats very healthy food and doesn’t drink much. I think this is what is called “flexible restraint”, and this is what I aspire to.

    1. Alice

      Having said all that, a colleague has just rocked up to his desk after raiding the snack machine and is cramming in crisps, chocolates, soda… He in thin, I am green with envy. Even though I don’t like too much junk. It would just be nice to be so unconscious about food, like “yeah, I feel a bit peckish, I’ll have a Twix”. For me it’s “yeah, I feel a bit peckish, I’ll drink a litre of water and wait the two hours til lunch”.

      On the subject of a set weight, I hovered around 145 with minimal effort and attention for around three years. I’ve now put weight on, but am quite confident that when I lose it I will be able to maintain 145 with the same lack of care. However, it’s not all that helpful when I want to be 125. I did read somewhere that after a few years at a certain weight your body resets your set point to that, so maybe mine will reset a bit lower than 145. Otherwise, I will have to learn to accept 145 because I am not signing up for a life of struggle to maintain a lower weight.

  9. Anele @ Success Along the Weigh

    I’ll have to watch it later but yeah the person I know is my cousin. The man could put away 3 plates of food and 3 full plates of dessert. It was like watching a train wreck you couldn’t look away from. He never gained an ounce back then and I wonder if part of that played into his sister becoming bulimic as she always struggled with her weight. But once he hit 40, all of that changed and he started to put on weight, maybe 30ish pounds. I’m very glad to say that he’s gotten the weight off and is helping keep my aunt and uncle healthier. I think ‘naturally thin’ people only get away with it for so long. ;)

  10. Susan

    I have a friend that was always super skinny. Until a few years ago. She kept eating the same and not exercising, which is what she has always done. She is now very heavy. I tell my daughter to eat healthy and be active. She sees the kids in the neighborhood and they used to be thin but now that they are getting older they are starting to put on weight. It’s because they eat junk food and are sedentary. She gets mad at me because I only have healthy stuff to snack on. We always have a salad or veggie with dinner. I am trying to show her healthy eating habits.

  11. Stephanie

    I know a girl that struggles to gain weight. She eats a lot of to no avail she can’t gain. The only thing that put on 5 pounds was chugging protein shakes. So that is what she does now to try and maintain body fat.

    I wonder if it is the same for some people that are overweight? I know my family is Italian and they are all short and plump. All of us eat super well and stay without our caloric intakes but all of us struggle with gaining weight no matter what we do.

  12. Marie

    Seems like all of my friends from high school were “naturally thin”. I have yet to watch the video, but give or take 90% of my closest, could eat whatever the hell they wanted and never show any sign of it. Me.. on the other hand, I could just look at food and gain weight. The ones that I keep in touch with, are still struggling to gain weight and at 30 years old, still only weigh 100lbs. I do believe that genetics has something to do with our ability to “control” our metabolism, but it is ultimately up to us, to control the end result

  13. Mary Norton

    I would have been more interested in this study done on an older age group. These people all looked mid-20’s to me. (which is when most people’s bodies are at their prime or finding their prime.) I wonder what the results would be with those who are 30-40, etc. I did watch the entire documentary and found it very interesting as it is…a fat virus though? Really? ;) Thanks for sharing, I had never seen this before, made for a good learning experience!

  14. Trish

    My sister! She is 40 years old and has never reached 100 pounds. Usually right around 98 pounds. The thing is, she looks good. She eats EVERYTHING.

    We all know what a struggle it is to lose weight and maintain that weight loss well my sister fights a different battle of wishing she could gain weight.

  15. Cija

    Thank you for sharing this documentary, absolutely fascinating and enlightening. Gives me a better idea what I am up against and it’s most likely not just my will power.

  16. Hayley

    Pizza with mayo? I’ve lived in the UK all my life and never heard of that!

    However, Domino’s and Pizza Hut often serve little thingies of BBQ sauce or garlic and herb dip, I’m guessing to dip the crusts in. That sounds pretty good.

  17. Norma

    Maybe we need to not care about what others can “get away with” and just learn to take care of our own body, the only one we we’ll ever have, and the one for which we are 100% responsible. Envying someone slim who eats junk food and doesn’t gain weight is like envying a lifelong cigarette smoker who never gets lung cancer…pointless and petty. Aspire to be fit and healthy and to treat your body with respect. Expect — no, DEMAND — good things from yourself and for yourself. What others do and don’t do/have and don’t have is not of importance.

    1. Lisa DeGrave

      I live in Green Bay, Wisconsin and we have a pizza place that sells a BLT pizza. It is a regular pizza with bacon and cheese, but when it is done it is spread with mayo and then topped with lettuce and tomatoes. It IS SO GOOD! I thought gross until I tried it. But….you wonder why so many people are fat, including myself. Another big thing around here is that people including my children like to dip their pizza in ranch dressing.

  18. Aimee

    I also have these ‘thin friends’ and have lived with them too. My brother is also a naturally thin person… and what I have noticed is that although it LOOKS like they are eating more.. on average they are usually eating less. For example, sometimes we would sit down to a dinner, in which a thin friend would eat TWICE as much as all of us.. only to learn that she barely ate lunch or breakfast. I also noticed that I would snack a lot more- on small things here and there which can really add up.. while my thin friends would ONLY eat at meal times.

    OR sometimes my brother would eat two burgers, fries and a milk shake.. but then the day before he would have not really eaten that much..

    So while it may seem like many thin people eat more than we do, I honestly do not believe that is true. I feel that from experience, on average a thin person probably eats less (even just a little less can make a huge difference in weight over the years.. and days! ) for example if they dont eat as big of a lunch.. or miss breakfast one day.. or dont consume a snack you maybe ate.. this can all make a difference!

    just my two cents! :)

    1. Radhika

      I couldn’t agree more :) Also, these ‘naturally thin’ people tend to move a lot more than we might imagine.

    2. Jessica

      I want to second this! My husband, his mom, and his sisters are naturally thin, and even when they gain they can always quickly drop back down to their usual slim selves. At gatherings they eat the same as others – and my husband is totally the guy who has 2 burgers and a hot dog! plus all the sides! – and are always talking about all the junk food they eat BUT really they will frequently go long periods without eating. This is not a conscious or disordered eating on their part (for sure not my husband) they just plain aren’t hungry. Other friends I have who are naturally thin are like that too – they are the people who do not wake up hungry, those that say – eh I don’t like breakfast. (How?! My stomach growls first thing every day!)

      1. Rocio

        I also agree with this theory, although there might be people that have really fasts metabolisms, most thin people do eat less then us. I used to be naturally thin, I would not get hungry or anxious like Jessica said. Everything changed when I started hanging out with heavier people. I would eat a lot more. Now I get hungry every two hours or so and gained about 50 pounds :(. Now I’m anxious and hungry all day.

  19. Katy

    Mayonnaise is nothing, my Scottish flatmate was 18 before he realised that pizzas were meant to be round…before that the only pizzas he’d knows were folded in two and deep fried!

  20. Hanna Wenger

    You are truely such an inspiration to me. I was never overweight but lost 15 lbs my senior year of high school and gained it all back in college. Now I struggle with my weight in that I feel like I need to go to the gym all the time and eat super healthy which quite frankly is not at all possible in college. You have set me free in my own body and I thank you a million times over for helping me love my body for what it is without running and eating salads everyday. Thankyou thankyou thankyou. <3

  21. Jeannttaf

    When we discuss naturally thin people, we discuss it with awe and often envy. But in many instances, if people attempt to talk about being naturally fat, no one believes it. “no, no. They just eat too much and need to exercise” is often what is said. Angering.

  22. jennali

    If you watch the documentary, it’s not necessarily that these people can eat whatever they want. There is a certain gene (or don’t have the gene, I forget which group was which) that has been located where people with the gene eat and then they are full and have no desire to keep eating. Others who do not have the gene eat and still do not feel full so they keep eating. So I think that a large part of it is that they just don’t have the desire to eat as much, although it may seem like they eat a lot at one time, they don’t have that constant craving feeling that makes losing weight so hard.

  23. Jennifer

    For most of my life I was what is now being called “skinny fat”. My diet as a kid was pretty much made up of hot dogs, pizza, Doritos, and Little Debbies. Fruits and vegetables were the enemy. I was thin as a rail – so thin, in fact, my pediatrician encouraged my mom to give me a milkshake (!) every day to get my weight up. The funny thing is I always failed the skin fold fat test they gave us in school (this was the ’80’s, do they even do that anymore?). I always argued “how am I supposed to lose any weight? Look how skinny I am!” Then went home and ate an entire bag of chips for my snack.

    Now that I’m in my 30’s, my body is finally telling me what I was doing wrong. I’m still not overweight, but I have insulin resistance and an autoimmune disorder. I MUST work out or I start to gain bit by bit even with a healthy diet. And the junk food sets off my autoimmune issues every time. I may have been skinny, but I was making myself sick, even then. If anyone had ever bothered to do blood work, I bet it would have shown that I was severely malnourished. I feel so much better now at a healthy weight than I ever did when I was model thin.

  24. Abby

    i kind of hated the documentary. yeah i get it that what you eat affects what you do but as soon as they started eating normal… maybe not even THAT healthy they went right back to being skinny.

    i feel like i can track what i eat for the rest of my life and not lose any weight. so it just made me more depressed to think i am biologically bent towards being unhappy in this body.


  25. Laura

    I’m interested in watching the video. My husband is “naturally thin” but after being married 5 years I’ve learned that even though he seems like he can eat whatever he wants, he actually stops when he is full and turns down food if he isn’t hungry. Even though he *seems* to be eating a lot, he mostly is just enjoying his food. FWIW I think height makes quite a bit of difference as well. In college I had a “naturally skinny” friend. She was 5’9”. I was 5’3”. We consumed what I would consider similar amounts at meals but I was overweight. In actuality she weighed only 10lbs less than me (mostly bc she skipped meals regularly when she wasn’t hungry). I wasn’t skipping any meals and was 6 inches shorter than her, so I just couldn’t carry as much weight as her!

  26. Anna

    Interesting documentary, but I wish they’d said something about quality of food, not just quantity. However, I know that one can stuff oneself silly on healthy food e.g. wholegrains and veges and find it hard to come close to those high calorie totals – one of the great things about a plant-based diet (something I strive for).

    I couldn’t skip a meal to save myself, so yeah, I get jealous of those who can and do regularly. Or those thin people who do eat tons of junk – there’s a young guy at work who eats McDonalds for brunch nearly every day. But when I get annoyed about it, I remind myself that skinny does not necessarily mean healthy – one doesn’t have to be fat to have heart disease, diabetes, high blood pressure, etc. (Not that I would wish those things upon anyone; but it’s just a self-reminder not to be too envious of such people.)

  27. Abby A

    This was a great video! It really gets to the bottom of that question any overweight girl has ever asked: “why can she eat that and stay skinny, and I have to count every piece of salad that goes in my mouth?”
    I think it opens up so much more for research! I’ve always been slightly on the normal/heavy side- and no matter what, since I turned 18 years old, have been hoovering around 150lbs. Sure, I’ve lost weight for my wedding, and when I was exercising obsessively and only eating cottage cheese- but I always end up back at 150. My body LIKES to be there. It doesn’t mean that I won’t continue to workout and eat well, but it’s a fact I have to face- and if I don’t lose any weight after all that effort (and if I do, but don’t keep it off) then I’ll just have to be okay with that too. Hard truth.
    Thanks again for the video and discussion!

  28. Danielle

    I found this video very discouraging :( I’ve been overweight since middle school hitting my all time high of 185 lbs in 8th grade. I ended up staying around 150 in high school, just to go back up to 170 my sophomore year of college. I changed all my eating habits and love to exercise now, and I’m maintaining at 115-120. I’ve noticed I gain it back very easily and I have to be super careful about calories in and calories burned to maintain. It bothers me to think that after all my hard work, my body chemistry is fighting to get me back to 150.

    1. Amber

      I totally agree with you – this video has seriously made me sad/frustrated/discouraged…the list goes on. It seems so unfair. I’ve tried to lose weight time and time again and my body consistently keeps me at around 170. Pretty frustrating to hear that no matter how much I exercise or watch what I eat it will always be a losing (not in a good way) battle. suck

        1. jeneralities

          I’ve heard of the set point before… but I’ve also heard that if you maintain weight a new weight for a while, whether higher or lower, your body will adapt, and your set point will change.

        2. Rebecca

          My body is consistently happy at a higher number than I’d wish for (I’m 5’6, and I can easily maintain about 155-160). Other factors definitely include body type and overall health. Are you curvy, or straight-figured? Can you run a couple miles or lift some weights without killing yourself? I think we set too much store by a number, and not enough by health. I’ll never be “skinny,” but I think I look good and as long as I’m healthy, I’m content.

  29. Catherine

    Very fascinating documentary, thanks for sharing! I happen to be one of those girls whose friends have categorized her as “naturally thin”. The truth is, however, I work very hard to maintain a healthy physique. I tell everyone this, but no one seems to believes me. They see me eating butter, cheese, ice cream, and pizza. But what they ignore is that I only have a few bites here and there of the fatty foods and fill most of my meals with salads, vegetables, and tofu. I also work out 5-7 days a week. It’s not easy. I am not naturally thin. It takes an enormous amount of effort and self control for me to appear the way that I do.

  30. Kandyce

    My brother is one of those naturally thin people, and he eats somewhere in the neighborhood of 6,000 calories a day. He was drinking 10-12 cans of Mountain Dew every day and the doctor told him to cut back because he wasn’t getting enough calories because the soda was filling him up. The thing is, he has Gilbert’s Syndrome, which is a genetic syndrome that essentially gives him a super fast metabolism. So, I might be a little jealous of him, but then I remember that his liver doesn’t function properly.

  31. MIchael

    I am a 39 year old male, 5’5″, and I weigh in @ 122 lbs. I try to use the least amount of energy as possible. I can eat whatever I want. But, this is my point. I do not like being so skinny.
    The grass is not always greener on the otherside.

  32. Lenis

    Im what u can call naturally thin im 27, 5’7 and have never weighed more then 127 pounds and in order to get there i had to get on a crazy “diet” i couldnt sleep in on days off because i had to have a protein shake and breakfast with the works about 30 mins later. then i had to make sure i had something to snack on about 1 hr 30 mins after that meal and on to lunch it was another big meal then more in between snacking. A giant dinner and another shake in the night. And still at 127 my mother was like u could use another 10 pounds(really mom thanks for the encouragement). And i loved looking a little thicker and my face more filled in but its insane all of that. I read a comment from someone else on here about being able to stop eating when full and not being anxious about eating well thats so true because when i went on that diet about a month into it would wake up starving and no matter how often i ate i was always thinking about the next meal and what i would do for dinner that night. I just felt like it was getting out of my hands and it was too much work, so i just slowly cut the protein shakes and just let my body do its natural thing. My husband is always struggling with his weight so i get a lot of recipes here so we are both satisfied and i can make him feel good when i say babe you can have another its only….calories!! And the desserts here are perfect for me too because i like desserts i just think most are way too heavy and sweet and all the desserts ive made from andies recipes seem to be made just for me in that sense and my husband has a major sweet tooth. In the end ive always felt inadequate being so thin my mom use to tale me to the same dr i still go to now as a child, my dr and i still laugh at how my mom would take my older brother and i to her and tell her is there anything we can do they’re to skinny any vitamins. But this is how i know the human condition can be quite crazy my mother had 6 of us and after having my youngest brother at 38 still stayed at around110 pounds now shes almost 58 and weighs about 128 to 135 she yo yos in between there. My father is also on the thin side so i dont know why it was unusual that we were thin…P.S I LOVE U GUYS I HOPE U GET WHERE YOU WANT WITH UR BODIES AND AS LONG AS ITS NOT CAUSING HEALTH ISSUES AND U KNOW U R EATING HEALTHY DONT BEAT URSELFS DOWN SO HARD UR BEAUTIFUL AND WE ARE ALL ALWAYS WORKS IN PROGRESS THIS IS SPECIALLY FOR THOSE WHO DIDNT FEEL TOO GOOD AFTER WATCHING THE DOCUMENTARY.

  33. jeneralities

    I’ve heard of the set point before… but I’ve also heard that if you maintain weight a new weight for a while, whether higher or lower, your set point will change. So people whose bodies want to be heavier, don’t be too discouraged! Your body can get used to your new weight and be happy there!

  34. Alexia

    My boyfriend and I weight the same 122 lbs,
    it kinda pisses me off because he eats so much and so much junk in front of me,
    I do have a good metabolism, it’s weird because I only gained 20 lbs in a year eating around 3000-4000 calories a day, but it still pisses me off, he never gains and sometimes even loses so much weight.
    It’s not fun to weight the same weight as my boyfriend, I feel fat haha.

    Well, I guess everybody has to know how to nourish themselves and do their meals/portions etc in order to stay fit.

    1. Simon

      How on Earth do you eat 3000-4000 calories a day? Just the idea of doing that seems almost unreal to me no matter how much exercise you would do.

      Personally, I’m one of those who might be categorised as naturally thin. However, as a point in the documentary shows, oftentimes people who are thin seem to be eating more but are actually eating a lot less. Of course, thin people often eat ‘whatever they want’ but in my personal experience this is exactly where the difference lies; most of the time, I couldn’t care less about eating burgers or ice creams or chocolate gateaux because I’d much rather eat something nutritious and flavorful. Of course there are times when I’ll eat a whole bar of chocolate in one day but these events are rare because over the years I’ve learned about the depressing feelings that come from over eating junk food and besides, go for a small Greek salad over a dominoes pizza any day (despite how much I like the better kinds of pizza).

      At it’s core, I think this is mostly habitual. I mostly eat what I want, but because of a lifetime of habit, and because my skin crawls when I read high figures for saturated fat etc. junk food and sweets are massively overrated in my opinion. Fresh fruit is a much better snack than all the candy bars in the world, and the only reason people think otherwise is because of addiction.

      Of course, it should be noted that women gain weight easier than men, and require fewer calories.


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