eli5: How does fat gain/loss actually work?



It’s my understanding that most fat (re: loss) is essentially metabolized and exhaled as carbon dioxide. What I can’t wrap my brain around is that even after eating a calorie surplus the weight doesn’t INSTANTLY appear – maybe after a heavy meal, but if it’s something just really calorie dense this isn’t the case. But the calories have been consumed so I don’t understand why the weight doesn’t reflect that instantly. So it seems like the fat gain side of thing might be actual magic.

In: Biology

It does reflect it.

The fat/muscle gain won’t – because those calories are still sitting in your gut, they haven’t been processed and used yet – but if you weigh yourself, eat a pound of something and then jump on the scale you will be a pound heavier.

When you eat something, the weight is in your gut as you digest it. Your daily meal is maybe 2-5 pounds total. Then you pee and sweat and poo and breathe and that 2-5 pounds mostly leaves you.

But if you eat excess, your body may take a small amount of that daily food intake and store it away “for later” as fat. Maybe an eighth or quarter pound here or there or less.

Most people move up and down 2-10 pounds per day in weight depending on hydration, food intake, etc. The actual fat gain is a tiny fraction of that but say after day it can add up.


Mass is just another form of energy, and energy is just another form of mass. A calorie is a measure of energy.

For weight gain/loss to be more complicated than calorie in/calorie out would require the human body to be the only thing in the universe exempt from the laws of thermodynamics.

Your body isn’t 100% efficient at metabolizing calories. Calories are locked up in the food you eat. Some foods are easier to digest than others.

Sugar for instance is extremely easy to digest and largely is absorbed straight through the stomach lining rather than going through the digestive system. When you eat something like a chocolate bar, you get a huge energy boost followed by an immediate energy crash as all that energy is made available all at once.

Other foods, like green beans, are much more work to digest. The calories are there but they’re only gradually released as your digestive system does its work.

That chocolate bar can be processed in minutes while a healthy and diverse meal can take 36 hours or even more to fully digest. And the process to turn non-fatty calories into fat also takes time. Your liver has to do work to convert things like carbs and protein calories into fat.

So part of it is that you can eat something and simply excrete some of those calories again before your body unlocked them during the digestive process.

Part of it is that some calories are much easier to unlock and use (or store) than others.

And part of it is timing. If you have a good meal and then later enjoy some exercise, those calories are more likely to be put to use than stored. If you gorge on food and then go to sleep, those calories are likely to go straight into storage.

If you eat a chocolate bar, you basically eat an energy bomb so big that you’re unlikely to use up all that immediately available energy and it’ll get shunted into storage. And since all that energy is made available at once, you’ll likely be hungry again soon after and eat something else. Which prevents you from using that stored energy.

If you eat green beans, that energy is unlocked gradually and you get plenty of time to use it instead of storing it. And since your body is busy gradually digesting it, you won’t feel nearly as hungry.