Where does energy come from during ultra-endurance events?
Ultra-endurance athletes burn a small amount (about 5-10%) of the required kcal in proteinsKnechtle2018. (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 kcalboron1171
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 energyboron1211
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 TAGsboron1182.
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 Pi --> 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.
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 + Pi + ∆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 reactionsboron1204
Thus, the energy liberated from the fatty acid creates kinetic energy and more heat.
So, to summarize, this is what fat turns into during exercise:
- It is chemically converted into CO2.
- It is chemically converted into H2O.
- It is converted into thermal energy.
- It is converted into kinetic energy.
[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