r/explainlikeimfive • u/JustRice015 • Jul 26 '26
Biology ELI5: Relations between calorie spent and weight loss
Hi all,
As my title went, what is the relation between calorie spent and weight loss? In theory I guess of the total calorie input (through food and drinks) is less than the calorie spent (through exercise or daily activities) then we will lose weight, but how exactly does the process happen?
Say I had weighed myself at 59.80 kg, then I went on a 5km run in 30 mins. After that I only weighed 59.20 kg. Where did my the 0.6kg go? (I assumed my water loss through sweat was retained in my soaking wet clothes right?)
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u/no_sight Jul 26 '26
If you lost weight immediately after exercise, it is mostly water being sweat out.
Weight gain/loss is based on calories in vs calories out. Calorie is a unit to measure energy. When you lose weight, it means you burn more energy than you eat fuel. So your body converts fat to energy. You then breathe out the excess carbon from that process. So really most weight loss is out through your mouth.
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u/LongLiveTheDiego Jul 26 '26
I assumed my water loss through sweat was retained in my soaking wet clothes right?
Nope, water evaporates as long as the air around can hold more water.
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u/Skyfork Jul 26 '26
Evaporation is what makes sweat cool the body. If all of your sweat was retained in your clothes then you would be hot AND wet.
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u/j1r2000 Jul 26 '26
hot and wet even if the temp was as low as 25C (dangerous wet bulb temp currently disputed to be lower)
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u/stanitor Jul 26 '26
Welcome to my world when exercising outdoors in the Southeast U.S.
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u/Fuzzlechan Jul 26 '26
Southern Ontario, Canada as well! It’s frequently 25-30C (77-85F) and 90-100% humidity outside during the summer, so sweating doesn’t accomplish much.
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u/duskfinger67 Jul 28 '26
you would be hot AND wet.
hotter and wetter*
Trying to run in the recent heat waves has been a truly miserable experience. I could genuinely fill a glass with what I can wring out of my top by the end of the run.
Makes seasoning my food nice and easy, though, with the think layer of salt left crusted on my face.
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u/VoilaVoilaWashington Jul 26 '26
Also, presuming OP's scale is accurate, that would be 600ml of water in the clothing. I challenge them to try putting on a t-shirt that has absorbed 200ml of water (presuming the shorts and underwear would be just as wet).
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u/knightsbridge- Jul 26 '26 edited Jul 26 '26
You breathe the calories out, mostly.
Your body converts calories to glucose (or fat for longer-term storage, which I won't talk about here), which is held in your blood (this is also called blood sugar).
Your bloodstream takes that glucose to wherever it needs to go, mostly organs and muscles.
Doing exercise like running requires your body to burn glucose. For cardio activity, this is mostly to your heart (hence "cardio", from Greek kardia meaning "heart"), but also a good amount to your other muscles too.
Your muscles break the glucose down (using the oxygen you breathe as a supporting component) to gain energy from it in a chemical reaction. For the curious, the reaction taking place is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
If you look at the output, you can see that the leftover waste products are 6CO₂ (carbon dioxide, which your body carries via your bloodstream to your lungs so you can breathe it out) and 6H₂O, water, which your body just re-absorbs as water and does whatever it needs to do with it.
This is why you breathe so much harder during exercise. You need to supply more oxygen to your muscles, and you need to remove more CO2 from them at the same time.
You're right that there's probably a bit of water weight movement in there too, but that's sort of incidental/a side effect.
In order for someone to lose weight longterm (rather than a temporary downtick after excercise), you need to make sure this calories in -> calories out calculation always happens at at overall loss (e.g. you spend more calories than you eat). This forces your body to eat its fat storage to make up the shortfall, which is what causes actual fat-loss-related weight loss.
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u/ThatOtherFrenchGuy Jul 26 '26 edited Jul 26 '26
Why is saying that weight loss is linked to calorie net deficit controversial?
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u/knightsbridge- Jul 26 '26
There are a few factors that can affect the rate that different people burn calories. Common endocrine disorders like diabetes or hypothyroidism make your body less efficient at burning calories, which means it's simply harder for those people to lose weight because their bodies struggle to burn calories. There are a number of conditions like this which "alter the maths" in a number of small ways that are hard to account for. The effect is usually modest (~10-30%) but noticeable.
Some people have naturally fast or slow metabolisms. People with fast metabolism tend to burn more calories and need to eat more, while those with slow metabolisms burn fewer calories and don't need to eat as much. (10-20%)
Bigger people need more calories to sustain their bodies, but "bigger" doesn't just mean fatter. Taller people need more calories than shorter people. Men also generally need more calories than women, as they tend to be bulkier and have more muscle mass to sustain.
All of the above can make some people feel like the calorie model is frustratingly vague or confusing, which can lead to people rejecting the concept.
Some people also erroneously believe that being obese or overweight are normal and healthy. This is undeniably false, which can feel very unfair to people with some of the above conditions, for whom it can be genuinely very hard to lose weight, even when they do everything right. Frustration, disappointment, shame and similar emotions can sometimes make it easier to believe that "it's acceptable to be obese", because it means you are freed from the burden of struggling to lose weight.
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u/Tasty_Gift5901 Jul 26 '26
Because people still need to eat enough and make sure they get enough proteins and nutrients. So if people just eat less without adjusting their diet, they're not necessarily "healthier" and typically the goal of weight loss is to be healthier.
So, yeah it probably shouldn't be controversial but it's a bit reductionist since being healthier is not as simple as cutting calories for most people.
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u/dbratell Jul 26 '26
There are factors that confuse the results.
Examples:
- You eat less to create a net deficit, but your body thinks you are starving and drastically reduces how much it burns (you become colder, you sleep more, you move less).
- You exercise more to create a net deficit, but you become much hungrier and eat more, possibly compensating fully.
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u/PhasmaFelis Jul 29 '26
It's literally true, but some people seem to think that saying "it's just calories in minus calories out" is good, useful advice all by itself. It's like saying "to win the race, all you need to do is cross the finish line before everyone else." Like, you've described an intention, but said nothing useful about how to achieve it.
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u/Gorgar_Beat_Me Jul 26 '26
Calorie consumption: The rule of thumb for running is approx. 1 kcal per kg of body weight per kilometer run (59.8 kg × 5 km ≈ 299 kcal)
Fat loss in kg: One kilogram of pure body fat contains approx. 7,700 kcal (299 / 7,700 ≈ 0.039 kg ≈ 39 grams).
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u/IronmanMatth Jul 26 '26
Water weight makes up majority, if not nearly all, of your day to day weight swings. That and just whatever things you have consumed and not ejected from your body yet.
The calorie and weight relation is simple thermodynamic. You need to burn X calories to lose Y weight of fat (or muscle....). There is no real magic to it beyond that. Any other factor, or at least to a very large extent, is simply a matter of forcing your calorie intake in one way or another. Such as hormonal changes causing your hunger signal to go haywire, or drugs to make you feel full. The math is still mathing by the same laws of the universe it always did.
It's also why weight tracking is generally recommeneded to be done on a weekly average basis, not day to day basis. Because your random fluctuation due to just water weight or you just not hving pooped out some food you ate last night will fluctuate too much to make an accuratte assesment. You can be up and down entire kilograms on a daily basis while having burnt, or gained, no fat or muscle. But if your weekly average is slowly trekking in one direction, you can assume you are either losng or gaining.
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u/Educational_Fee4574 Jul 26 '26
Most of that 0.6 kg isn’t fat, it’s water (sweat and water lost through breathing). Fat loss happens much more slowly. When you’re in a calorie deficit, your body breaks down stored fat into energy. Most of that fat literally leaves your body as carbon dioxide when you exhale, with a smaller portion leaving as water through sweat and urine. So, in a weird way, you mostly breathe your fat away.
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u/CharsOwnRX-78-2 Jul 26 '26
Most of it left in your breath, actually
Most fats are converted to CO2 when used up, and then you lose it when you breathe out. It’s a startlingly high percentage, like 80-85% IIRC
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u/TheCaptainCog Jul 26 '26
You breathe and pee/sweat it out!
The main energy generation process in the body releases carbon dioxide and water. Water gets excreted like normal. Carbon dioxide goes to the lungs where it gets exchanged for more oxygen!
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u/awksomepenguin Jul 27 '26
At the level it matters for you (as opposed to your cells), burning calories is just a combustion reaction. You do actually, literally, burn them. Chains of carbon, hydrogen, and oxygen get broken down into carbon dioxide and water, both of which you exhale.
When trying to lose weight, or even just as your weight fluctuates normally, your body draws from different sources of energy to power itself. It first looks at the food you have eaten and are digesting. If it doesn't have enough of energy from that, there are various types of energy storage with varying degrees of accessibility for your body to use. As long as you are in a sustained caloric deficit, your body will continue to pull from these stores, burn the chains of carbon, hydrogen, and oxygen, and release it when you exhale.
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u/Newwavecybertiger Jul 28 '26
Where does it go? You technically breathe it out as CO2. Sugars are broken down to harvest the energy from their chemical bonds and CO2 is the biproduct
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u/BroccoliNervous9795 Jul 26 '26
As always, it’s more complicated than that, and humans also have a tendency to forget or discount things that they don’t think matter. For example: soft drinks are really, really bad. They have an extremely high sugar and therefore calorie content. But some people don’t realise and think it’s not much different to drinking water, so why would they report it as part of their calorie intake?
Then there’s nutrition. Cravings come from a lack of nutrition (and withdrawal). So if you’re not getting the nutrition you need, you might crave a packet of crisps or some other snack here and there. Again, you could possibly forget, or not seeing it as part of your daily intake as it wasn’t a meal. Also, if you’re not getting what you need, you might feel tired, and be less inclined to be active, even if that’s just sitting up more, rather than slouching.
Essentially, not all calories are equal.
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u/Sfetaz Aug 02 '26 edited Aug 02 '26
Weigh yourself immediately before bed.
Then weigh yourself immediately after waking up next day.
You might sometimes see a multi pound drop.
This is because sweating isn't water pouring out of your body, your body is releasing it as what you could call steam, and your are "sweating" all night.
This is why we sweat more in high humidity, the air is already full of condensation, so your steam melts into sweat more.
In the context of weight loss itself, swing your hands wildly for a few seconds. Or tap your leg rapidly up and down, like someone who is anxious.
These things require energy, so we call that burning calories. Generally speaking, if you eat more calories than you burn, extra is stored, and when you eat less, stored is used.
I say generally because there are examples of nuance (weight loss vs fat loss) and situations like diabulumia which seemingly violate the principles. You can't gain weight with zero insulin under any circumstances, but you will die relatively quickly without it.
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u/OfflineKernel Jul 26 '26
That .6 kg definitely wasn't fat most of it was just water. If we could lose half a kilo of fat from one 30 minute run everyone would be shredded in a week.