system
Given the following post, respond as if you're commenting on Reddit:
human
Why does starvation burn muscle and not fat?

When someone starves, the thing I’ve heard is that the body will consume muscles instead of fat for energy. Is this true? Why would the body consume muscles that could be useful for getting food by climbing trees or hunting instead of consuming body fat, which is meant to be an energy source?
gpt
It mostly burns fat - the fat is burned directly by the tissues that can, but that's not true of all tissues...

The brain needs some glucose (it can also be partly powered using ketones made from fat), and red blood cells can only be powered by glucose. 

The body can make glucose out of the glycerol that is left over when triglycerides are broken down into fatty acids, and it can also make it out of amino acids.  If there's not enough glycerol to meet the needs of the brain and red blood cells, that energy can only come from amino acids. 

\*Generally speaking\*, I'd expect that there's enough energy in the glycerol to make enough glucose. 

Humans unfortunately don't have the machinery to make glucose directly from fat.