I have come across some online fitness site which claims that one would start burning fat, not just carbonhydrates, after 20 minutes of cardio exercise.
Is there any truth in this claim?
Cecil Adams says it’s true, and he’s omniscient. So I guess that settles it.
This site lists the various fuels burned by the body at various stages of exertion
The basic idea is sound, but as 11811’s cite indicates, the details are a bit off. The short version is that burning stored fat requires oxygen and takes a while to get started. If you’re capable of intaking large amounts of oxygen, or you’ve already been exercising for some time before you start cardiovascular activity, you’ll get to burning fat much more quickly.