We hear talk of different body types, and that it is more difficult for certain people to lose weight than others. On the other hand, when my physician recommends that I lose weight, they say that it is really just as simple as "calories in, calories out". How can we reconcile these two?

Perhaps the "calories out" portion is different for different people? I.e. some people burn more calories than others do, doing the same activities, but their hunger level does not compensate for this?

Apologies if this is off-topic


2 Answers 2


There are differences in metabolism, and the "calories in" part is the one which differs more than the "calories out" part.

People do lose different amount of calories for the same action, but for mostly mechanical reasons. If a 50 kg person walks up a mountain at 3 km/h, and a 100 kg person walks up the same mountain at the same speed, the 100 kg person will expend more energy, because they did more work. And then there are slight differences in how we do our movement - some people's gait is more efficient than others', and uses less energy. But this is not really interesting for the explanation of having it easy/hard to lose weight.

The point is that the amount of food we intake is determined by our hunger, and our hunger is determined by hormones. Then, the amount of fat we store is also determined by hormones. If your body "thinks" you should be putting on weight, and you go on a calorie-reduced diet, it won't just start complementing the missing energy from the fat stored in your adipose tissue. Rather, it will continue diverting some of your food intake to make more fat storage, all the while rationing the energy available for other activities: making you feel tired so you don't move much, shutting down some luxurious, energy-hungry processes in the brain, mostly the ones associated with higher mental functioning, and making you feel hungry and cold so you will be more motivated to seek out food and eat. The hormones which regulate these processes are many, most of them are poorly understood, some are probably not yet discovered (orexin, one of the main hunger regulating compounds, was only discovered sometime in the last 5 years I think), and we really don't have a complete picture of everything that is involved. It is a complex mesh of finely balanced homeostatic systems which interact with each other and the environment.

The people who say that it is as simple as "calories in, calories out" are right. They are right in the sense that, if you just observe a human being and measure their energy household, you will be able to document the amounts of energy consumed, expended and stored, and the energy consumed will equal the energy consumed plus the energy stored.

But to derive from this that we are talking about a passive overflow system and, if we reduce intake, storage is also reduced, is a big leap of faith and has been proven wrong again and again. Living systems are actively reacting to any influences from outside, by mechanisms much more intricate than simple mechanic rules. Mechanics tells us that if I push you, you will fall over; experience tells us that if I push you, you will brace yourself so you don't fall over, and then try to stop me from pushing you by using words, force, or whatever else you have at your disposal. Our physiological systems are the same way. They like the situation they have chosen to be in as much as you like standing upright. If somebody comes from outside and tries to forcibly change their status quo, they do whatever they can to 1) resist and retain the status quo, and 2) stop the outside force if possible.

As for the difference in metabolism: the hormone systems of different people are differently "tuned". For example, eating pure glucose causes insulin production to spike, and excess insulin causes glucose to be removed from the bloodstream, plus a handful of other effects. But the size of the insulin spike is different in different people. Also, the extent to which cells react to elevated insulin is different. When the reaction becomes dangerously weak at even high insulin levels, we say that it's a pathological variation, and call it "metabolic syndrome". If goes really towards zero, it's called diabetes type two. And this is an example about just one of the hormones connected to metabolism and the distribution and use of energy in the body.

On a simpler level, there is no difference in metabolism where physical energy calculation is concerned. Expending one molecule of ATP will let the same number of Na+ ions to move the same distance along a nerve dendrite, across humans. But this level is not really interesting when it comes to gaining or losing weight.


Some people burn more calories than others, in the default mode.

Just sitting in a chair, I warm the seat much more than some other members of my family. That heat has to come from somewhere.

That's a different metabolism.

But, that's not something you can order and install. It's not a pill. Your doctor is right to focus on what you can control, and that's intake - along with your activity level; or outgo. (But most people are unwilling to actually do the work to change their activity level to a higher one, which is why doctors don't even try anymore - patients just lie, and then doctors have to try to catch them at it, and nobody's got time for that).

What you eat may also impact the number of calories that stick. Not everything you eat gets digested, some passes through. And digestion itself burns calories. Things that are harder to digest take more energy, and leave less surplus behind. Easy things are... easier to digest and get all the calories from.

In addition, thinner people lose weight easier. Not fair, right? That's life.

It may have something to do with your microfauna, but that's not well studied. And, we're not even sure if you can permanently change those populations.


The warming of the seat is anecdotal speculation.

So you're arguing that heat happens magically, and doesn't require caloric input? Do tell us more, I can't wait to hear about it.

And why should thinner people loose weight easier?

If I had a handy cite, I'd give it to you. Yeah, yeah, Abraham Lincoln said don't believe everything you read on the internutz

You say "Not well studied" - It is important to cite some material that is studied in your answer.


And what are 'those populations'?

Populations of microfauna

It is more of a comment as it stands. Earn 9 more reputation and you can comment anywhere.

Thanks for the upvote, to help me comment - oh wait.

  • $\begingroup$ The warming of the seat is anecdotal speculation. And why should thinner people loose weight easier? You say "Not well studied" - It is important to cite some material that is studied in your answer. And what are 'those populations'? This answer may need some work. It is more of a comment as it stands. Earn 9 more reputation and you can comment anywhere. $\endgroup$
    – AliceD
    Commented Feb 20, 2015 at 13:17

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