# How efficiently does the human body convert the energy stored in fat into kinetic energy?

I read that one gram fat contains 9 kcal energy. There are many tables around claiming that x type of exercise for t duration burns roughly E kcal energy. I'd like to estimate how much weight one will lose by following a certain training plan based on these two values. I don't know whether I need to calculate with the heat loss too, or not. Most of these tables don't tell anything about the method they used to estimate the E values. I could workaround this problem by ignoring efficiency, but I can do that only if it is relative close to 100%. As far as I know fat goes through beta-oxidation and the citric acid cycle. After that ATP is used to move the muscles. It is hard to find anything about how efficient this process is.

this depends on largely how you define it, what we normally measure is efficiency including how much energy is used on metabolic upkeep and heating the body in addition to muscle output. we use oxygen consumption to know exactly how many calories we have "burned" when doing this. In this study we get between 18-26% efficiency, thats calories ingested to work done. Most of the loss is in converting to ATP (~60% loss) and how much ATP we "waste" heating the body. Note efficiency drops quite a lot when you switch to anaerobic exercise.

keep in mind however the body will not start out burning fat, it has glycogen reserves it will start using first.

• "keep in mind however the body will not start out burning fat, it has glycogen reserves it will start using first. " - Actually that is not true. It starts with glycogen and intramuscular fat... Commented Nov 29, 2018 at 20:55
• @inf3rno do you have a source for that, its not that I don't believe you but it is the first I have heard of that but it does make sense physiologically.
– John
Commented Nov 29, 2018 at 22:47
• You can use google to find some. For example this is a good starting point: journals.lww.com/acsm-essr/Fulltext/2005/10000/… Commented Nov 30, 2018 at 3:56
• There was this fat burning myth, that you start "burning fat" only after 20-30 mins of aerobic exercise. That is partially true. You start with IMTG until the triglicerides arrive from the fat cells through the circulation. Without that people on ketogenic diet would not be able to do exercise... Commented Nov 30, 2018 at 4:07
• the paper you linked states thew fat in question does not start getting utilized until 60min of exercise.
– John
Commented Nov 30, 2018 at 5:19