When I burn off a very large number of calories, where does the weight go?

Question

Let's say I ride my bike for 20 hours non-stop with no (significant) breaks, burning 650 kcal/hour. My basal metabolic rate is perhaps 3,000 kcal/day, making a total of 16,000 kcal expended over the course of the day.

Over the course of such a ride I'd normally consume around 3,000 kcal of food (more than this on slower-paced, multi-day rides), and my muscles would maybe have another 2,000 kcal or so of glycogen stored in them to begin with. That leaves around 11,000 kcal.

1. Does this 11,000 kcal come from body fat?

If so, then it would amount to a bit more than 1 kg of fat.

2. What happens to that 1 kg of fat? Do I come home 1 kg lighter (ignoring other factors like water loss)? Or does it leave my body later on? I typically find that my appetite is elevated for a couple of days after such a ride, but not during or immediately after.

3. Given that I can't replenish all the calories I'm consuming in one day, is my multi-day endurance ultimately limited by the amount of body fat I have?

Answer 1

Yes, the other 8kcal will come from the body. I am not sure if it will only purely be fat, the body can break down other tissue such as muscle tissue if it needs to. But yes it will come from burning tissue such as fat.

You could (I would even say you should) in fact consume a lot more calories than 3000 during a 20 hour bike ride, though you need to train your body to be able to do so. It shouldn't be too hard to eat about 400kcal an hour, which results in 8000 over the entire 20 hours. The important part here is that you simply have to eat, whether you're hungry or not. You will need to practice this during shorter training rides. Pro cyclists actually train their body to consume up to 100 grams of carbs an hour during a 6-7 hour race.

When your body burns fat storage the majority of this is exhaled as carbon dioxide. About 85% of burned calories is exhaled, the last 15% leaves the body in liquid form (through your skin as you sweat, through urinating, etc).

Your multi-day endurance is limited by a number of factors which includes the amount of glycogen and bodyfat you have stored. This is not the only limiting factor though. Things like you VO2max and aerobic threshold are factors also you need to take into account. (on top of replenishing stores outside of the workout)

EDIT: A great video that explains how the burning of calories works can be found here

Other sources, When somebody loses weight, where does the fat go? and Where fat goes when you lose weight.

Answer 2

Regarding this bit:

I typically find that my appetite is elevated for a couple of days after such a ride, but not during or immediately after.

It’s not just your appetite or calorie deficit from the ride itself. Resting Energy Expenditure is increased in the hours after exercise. See for example this study: https://onlinelibrary.wiley.com/doi/full/10.1038/oby.2006.236

It’s unclear which mechanism is exactly causing it. The Discussion in the study is interesting, for example they mention:

The increased REE in the untrained subjects was accompanied by significantly higher serum creatine kinase levels at 24, 48, and 72 hours post-exercise, suggesting that significant muscle damage had occurred as a consequence of the training.

Interestingly enough after injuries or surgery REE is also increased. Kind of shows that inflammation and repairs in general seem to required food and energy.

Answer 3

Where does energy come from during ultra-endurance events?

Ultra-endurance athletes burn a small amount (about 5-10%) of the required kcal in proteins_Knechtle2018. (If you are interested, you can look into glucose-alanine cycle). Furthermore, over such long distances, they'll deplete all their available glycogen. This can be approximately 3000 kcal worth.

A typical 70-kg human has up to ~700g of glycogen. Thus, the total energy stored in the body in the form of glycogen can be nearly 3000 kcal_boron1171

The remaining required energy will come from fats.

Most stored energy is in the form of triacylglycerols. In the prototypical 70-kg person, adipocytes store ~132,000 kcal of potential energy_boron1211

What happens to a burned kilogram of fat?

releasing fatty acids from adipocytes

Adipocytes (fat cells) store triacylglycerols (TAGs), which are the main constituent of body fat. TAGs consist of glycerol and three fatty acids. Fatty acids make up around 95% of the available energy in TAGs_boron1182.
Adipocytes release fatty acids into the bloodstream such that they can be used by muscles.

Turning fatty acids into ATP

Now let's see what happens to one of these fatty acids.For example, let's completely oxidate the fatty acid palmitic acid (CH₃(CH₂)₁₄COOH):

CH₃(CH₂)₁₄COOH + 23 O₂ + 106 ADP + 106 P_i --> 16 CO₂ + 16 H₂O + 106 ATP + heat

So during the lipolysis of palmitic acid you'll create CO₂, which you will expire. The H₂O you might lose through sweating, urination, or your body might retain it, especially if you are not drinking loads. Part of the chemical energy of the fatty acid will be turned into thermal energy (i.e. heat).

Finally, after 'burning' our fatty acids, we are left over with the cool stuff, ATP.

Using ATP

ATP is the reason we're burning all these fatty acids (and other forms of chemical energy):

The cross-bridge cycle that underlies contraction of skeletal muscle requires energy in the form of ATP.

By turning ATP back into ADP, we get energy:

ATP -> ADP + P_i + ∆G _boron1209

ΔG is the ~11.5 kcal per mole of ATP of free energy released under physiological conditions. The body uses this energy as follows:

Skeletal muscle converts only ~25% of the energy stored ... into mechanical work. The rest appears as heat, due to the inefficiencies of the biochemical reactions_boron1204

Thus, the energy liberated from the fatty acid creates kinetic energy and more heat.

TLDR

So, to summarize, this is what fat turns into during exercise:

1. It is chemically converted into CO₂.
2. It is chemically converted into H₂O.
3. It is converted into thermal energy.
4. It is converted into kinetic energy.

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Citations

[Knechtle2018] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5992463/
[boron1171] Boron and Boulpaep, Medical Physiology - 3rd edition - page 1171
[boron1182] Boron and Boulpaep, Medical Physiology - 3rd edition - page 1182
[boron1204] Boron and Boulpaep, Medical Physiology - 3rd edition - page 1204
[boron1209] Boron and Boulpaep, Medical Physiology - 3rd edition - page 1209
[boron1211] Boron and Boulpaep, Medical Physiology - 3rd edition - page 1211

Answer 4

Yes, the kcal that do not come from the food you consume or the glycogen stored in your muscles comes from "your body".

The most of it will come from fat; I do not know how large of a percentage (if at all) would come from muscle mass (i.e. protein) if you have plenty of fat in storage.

What happens to that 1 kg of fat?

Within your fat storage cells, which are always there, fatty acids are stored whenever you have enough (or rather too much) nutrients floating around in your bloodstream.

The cells are always there and do not get created or destroyed; think of them like fuel tanks where the fatty acids can be or - if depleted - not be in store.

When the conditions are right (i.e., low amounts of fuel in the form of carbs, fatty acids or proteins floating in the blood stream), fatty acids get released from storage and float around... to be consumed in whatever places they are needed; in this case your legs pumping your bicycle around.

In the muscle cells, the fatty acids are used up, i.e. split into their constituents, the process of which releases energy. You'll have to check that article or other sources for the slightly complex process, but basically everything gets converted into much simpler products like CO2 (which you breathe out), and probably water and other side products which get filtered out in the liver or kidneys and eventually excreted in some manner. Other bits and pieces go into further biochemical cycles, and all of it is directed by a not completely trivial smorgasboard of hormones and complicated subsystems...

Answer 5

As a deleted answer stated (albeit with a falsehood and in a slightly puerile manner), one of the explanations for where the weight goes is indeed in excrement. As your body breaks down food, fat, and other bodily tissues, it produces waste products that must be removed from the body, and one of the ways in which that is done is through fecal matter and urine (other aspects of the excretory system include sweating, which is one of the ways your body gets rid of salts, and exhalation, which removes carbon dioxide).

So unless you've been taking bathroom breaks, some of the weight of the calories that you have burned off in that marathon 20 hour cycling session is in your lower intestine, and bladder, waiting to be removed from the body. This is, of course, why the general advice is to use the bathroom before weighing yourself, to get a more accurate measure.