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Why do we gain weight?

Harry Mills, Ph.D. Updated: Mar 10th 2016

The 'calorie' is a unit of measurement of the energy contained within foods. Living bodies require a certain number of calories each day as fuel. Food calories are metabolized (burned up) by the body to create energy necessary to keep it going. Calories in excess of what the body needs as fuel get stored in the form of fat reserves which buffer the body against the possibility that calories might be hard to get a hold of in the future. Stored fat calories get burned and used up when no food is available and the body must look for alternative sources of fuel.

magnifying glass on question markPeople gain weight and get fat when they consistently eat more calories than their bodies require to meet daily demands. That excess calories get stored as fat is an adaptive evolutionary response inherited from times not that long ago when food was less abundantly available and people had to work far harder to get a hold of what food was available. People who were able to store food in the form of fat when food was readily available were more likely to survive and reproduce through times when food was difficult to get than were their skinny peers. Because of this evolutionary advantage, our bodies have developed so that it is rewarding and natural for us to eat a lot of food when it is available. Despite being efficient storers of body fat, ancestral humans were not often obese as they had to work hard to eat and in the process burned up what calories they ate. However, the dramatic agricultural and technological changes of the past two thousand years have made food extremely easy to obtain and evolution has not been able to keep pace in so short a time span. At this time we are required to use our intellect to understand our bodies' instincts and to develop a more twenty-first century appropriate relationship with food.


Reader Comments
Discuss this issue below or in our forums.

RE can't lose weight - learn about metabolism? - - Aug 28th 2009

Thank you for sharing your stories.  I wanted to encourage you to read about what you are doing to your metabolism in regards to only eating a few carrots a day and still gaining weight.  Your metabolism may have slowed down too much to make up for so few calories coming in.  Just a suggestion...


Not fat gain but water? - J. Nathan - Jul 20th 2009

Deborah, gaining weight at the rate you say you do -- five pounds in one day -- that may not be fat, that sounds like water retention. If you gain several pounds after eating a small salad, I would count on it. That sounds just like my mother. She was near-anorexic for years to avoid the severe bloating she had when she ate anything at all. Go to a different doctor.

Also, stress can be a big problem -- it is much more of a factor than medical authorities will admit.

Weight - Harry - Feb 12th 2009

I feel so much like Deborah it's not even funny, at one point I weighed about 335lbs and went all of the way down to 230, however, now no matter how I eat I can't lose any additional weight. The author of the article goes back to same old conclusion that you gain weight because you eat more calories than you should but in reality there are so many other factors involved in gaining and losing weight. Your stress level, the amount of sleep, genetics, and when and what you eat. For example, you could consume foods that are high in sodium and stay the same weight or gain because of water retention. Or not sleep enough and stunt your metabolism and that would contibute to weight gain as well. No one ever speaks about these items and always focus on diet and exercise. The one ridiculous thing I ever heard of is that scientists are trying to figure out why tibetian monks live long lives and have almost no health problems. News flash: they pray all day, don't work, and get to rest. Everyone else does not have that opporotuniy and that could be why they are gaining weight.  

weightloss! - RAJ - Dec 22nd 2008


First of all Stress is the biggest asset to FAT!

the more people stress the better it is for the body to convert the food to fat so please don't stress. I will be glad to offer you some help and some diet tips since you have pretty much tried everything and still it didnot work, but first you have to promise me that you are NOT GOING TO GIVE UP AND WILL NOT LET STRESS TAKE OVER YOU, let me know when you are ready,



Me too! - Hamana - May 18th 2008

We must be related and not know it. I am 61 years old and have exercised more than anyone I know for years. Every single day, 7 days a week, with no exceptions unless I am away visiting someone, which is rare, I exercise two hours in the morning. For two days in a row, I do 700 calories each day on the rowing machine and stair climber and on the third day, I do two or more DVDs of Leslie Sansone, averaging 7 miles total, then start the cycle over the next day. I occasionally overeat, but keep that to only once a week, eat my supper as a lighter meal than breakfast and at 4:00 if possible, then no more food at all until the next morning. And in spite of this, I weigh 140 on a 5'2" frame. Can you hear me screaming from here?

- sciano7 - Feb 25th 2008


 Does diabetes run in your family at all? It sounds to me like you may have insulin resistance. It's not diabetes... YET. I too suffer from this and it is absolutely impossible to lose weight without starving yourself (and keep it off). BUT, there is a med called Metformin that helps regulate your insulin levels. You can have a totally normal glucose level (as I did) and still have high insulin. Your dr needs to order a 3 hour glucose tolerance test with insulin and c-peptide levels.

Another thing it could be is your thyroid (have your tsh, free t3 and free t4 levels checked).

 You  might want to have your hormones checked as well, especially testosterone (which can be high if you have insulin resistance also).

hmm - zoom - Jan 6th 2008

Has your doctor actually tried testing you for anything? many doctors simply wave problems like these away without any testing. For you to be exercising that much and not losing (and even gaining) is not normal, and dont let anyone tell you otherwise. There are 3 obscure conditons that i suggest you research and ask your doctor about a) thyroid problems- you can be tested for this, doc may roll his/her eyes but that is their job!. b) PCOS- no official test that I know of but google it and see if the symptoms match up. VERY often missed. c) Cushings Syndrome- rare, but still worth researching and asking your doctor about. Dear friend of mine was just like you with the diet and exercise and still gained 30 lbs in a month, went through 4 doctors and found out that she had PCOS.

hope this helped 

reply to Deborah - rebecca - Jun 26th 2007

Everything you said, ditto, except I will not work out that much(I used to....4 classes a day..advanced aerobics, kick boxing. etc)

 I did lose 58 lbs about 5 years ago...i was emaciated, and honestly did look very ill.   weight just started coming back....i did not change my diet at all.  i looked great for about a year, but it has kept coming....i am not fat, i wear a 10, but my stomach and arms and thighs are so yukky.

Did anyone say anything that helped you?  I was on low carb, and did not really start eating very many carbs, tho occasionally i do eat some.

i dont know if i have messed up my metabolism....i want to go have everything SUCKED OUT!!

e me if you have any ideas, or have gotten any helpful remarks.

best to you


weight gain - Deborah - May 4th 2007



I have had a weird time with my weight. you wouldn' think I'm too overwweight when you look at me, but you'd have no IDEA what I go through,  I struggle to keep my weight down. I MEAN STRUGGLE. To look at me, you'd say, I'm average size, maybe need to loose 10-15 pounds. If anyone else on the planet did as much dieting and excersize as me, they'd die a toothpick. My boyfriend decided to copy my diet style, and I'm trying to force feed him more because he has become really skinny. If I put any other person in the world, on my diet and excersize plan, they'd all lose weight except for me. The fact is, to lose those pounds is almost a miracle for me. I work like a DOG at it, and I'm lucky if I lose anything at all . What I do, would make most people into models in a month.


Do you know, I really don't overeat, and in fact, I have a rediculously healthy diet, I don't take in excessive calories at all, the amount they say you can have, the minimum, always makes me fat, I usually can eat half that, and I "feel" like I'm starving, but I STILL gain weight! I don't eat fatty foods, in fact, now I'm on a detox plan and eatng mostly vegtables. It made me a loose a couple of pounds, but they came back. I also do a LOT of excersize. I take advanced classes at a gym, I lift weights, I run a lot, I do aerobics, and I push myself harder than anyone else. I'm even inspirational to others, and make them feel like I challenge them. People tell me I'm so energetic I should be the instructor, I put most of them to shame!  I'm pretty well, athletic. Whenever they measure me, they find that my bodyfat is slightly high, but everything else, like an athlete! Last time it was 33%. But why is it, I can't keep the fat off, even though I take in few calories, very nutritious calories, and I excersize like mad 3-7 days per week like there's no tomorrow? I have always been like this, and no matter WHAT I do, I always struggle to keep fat off.


I even got liposuction once, and guess what? the cat came back the very next day.  Honestly, it doesn't matter WAHT I do, I gain weight anyway. you coul chop my entire body in half, and I'd gain that weight back shortly.

To be honest, it must be genetic. Both my parents are obese, but I still cannt understand why the fat won't melt off, why it sticks to me like CEMENT! I don't even eat gluton anymore, or dairy or red meat, all the foods that are harder to digest. But it doens't matter WHAT I do, what I eat or how much excersize...I can eat all protein if I want to, or all juices or all just water, or 1 celery stick per day....I'll STILL gain weight! It's true and no exaggeration.


WEll actually one time, I excersized for an hour class, and did three of those 1 hour classes every single day for two years. I did SO MUCH excersize (and by the way I pushed myself as hard as an athlete in each of them, harder than most people) and when I did that, I did lose a lot of weight and kept it off. But, it killed me. What happenwed was I OVERTRAINED, and my body get overtired...and told me to STOP. I was so exhausted by it, it was unbeleivable. And when I did finally listen to my body and stopped pushing so hard, bringing it back down to 1 class about 3-4 times a week,  the weight came back. i took a long time before I was able toworkout harder again, because my body was so tired for such a long period of time. But that was the only time I lost the weight and kept it off ever in my whole life.


I just can't explain it. Its as if my body loves to be fat, and won't give in. If I did no excersize, I'm SURE I'd be obese in a short amount of time. Even if I ate only a few carrots per day and a few glasses of water, my body would hang on to it, and I'd get fat anyway!


I am more muscular now, but...I can't get rid of that 33%. It sticks to me like glue. If I lose a few pounds, and I eat small amounts of food, I gain weight. If I eat more, I gain too. I  remember starving for a few days, just to see what would happen, and here I was feeling slightly dizzy, and I stepped on the scale, and...I gained 5 pounds!

 Does anyone else have this problem? I asked my doctor and she had no explanation. She was happy when I got down to 130 pounds, but I had to do 3 excersize classes per day and eat almost like a bird to do it. I can't go out and eat a  tiny salad, with lo cal dressing,  and come home not gaining a few pounds.


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