It is pretty well known that when lifting weights you are not building muscle as you lift but as you rest afterwards… And I read that when running you are not doing any fat burning in the first 20 minutes but only after 20 minutes does you body start to use or burn fat… (true?)
Which led me to the following…
I was running for exactly 35 minutes today and wondered;
Am I actually burning any fat right now as I run past 20 minutes, or does the burning fat happen at any other point afterwards? -Say 5 minutes after? And how do I know if it is really working at the moment? Is there a callculation of how much fat one can expect to burn based on said exorcise for said amount time period at said intensity? I guess this isn’t really all that specific and could open a gateway to all sorts of misinformation… maybe I should delete this post…
Your muscles and your liver keep a store of glycogen, which is related to glucose. During activity, your muscles will try to use that glycogen to power them rather than stored fat (it’s more easily converted to energy than fat).
Once that reserve of glycogen is drained, then your body will begin to use stored fat to generate energy. That’s why there’s a delay for when the actual fat burning kicks in during excercise.
As for the actual times involved, I don’t know. I’d imagine it varies based on genetics, diet, type of excercise, and other factors.
The real limiting factor in when your body can start using fat is how much oxygen you can take in. Stored fat has to be released before it can be used for fuel, and releasing it requires oxygen.
How long does it take? That depends, and probably on a lot of things. The big issue, of course, is how fast you’re taking in oxygen–the more you get, the more fat you can release. The other big factor is how long you’ve been moving in the current session. If you just start jogging from a cold start, it could take a little while for you to start burning fat. If you lift or do something else for a little while beforehand, you’ve already started releasing fat and it’s available right away.
Finally, even once you start using fat as a fuel source, it’s still not your primary fuel source. I want to say the best you can do during aerobic exercise is roughly 50% fat/50% glycogen and other sources.
Years ago I read a book by Covert Bailey called Fit or Fat. In it he explained that you burn no significant amount of fat while exercising. Some, yes, but not much. The point of aerobic exercise is to build muscle tone to make the muscle leaner, which in turn uses subcutaneous fat for fuel. When this happens, the muscles burn fat all the time, even while sitting or sleeping.
Unexercised muscles don’t burn fat, they store it.
Of course this is my own memory of a book that I no longer own, and I have seen no confirmation or refutation of Bailey’s claims, so take this with a grain of salt.
I should add that Bailey claims that in order to get any benefit from exercise, you need to stay in your target zone for 12 minutes. This is not 12 minutes from when you start to work out, but counted from the time when your heart rate reaches 60-85% of its maximum rate. Anything beyond the 12 minutes is extra beneficial.
