We need protein for building muscles. We get calories from protein. Is the protein not burned to produce the calories or does the burning leave the amino acids we need intact?

Question

We cannot just have carbohydrates to burn for calories. We need protein and fat. Which presumably means we do not just burn them for calories but need some part of them intact.

So do we not burn them for calories? Do we burn part of them for calories and use other parts intact? Does the calorie count include just the part we burn or do we count the other parts we use for calories?

In: Biology

Answer 1

We can have just carbohydrates to burn for energy, and in a healthy body, carbohydrates are burned first. Brain and other nerve cells can only process glucose; if you don’t ingest it; your liver has to break down muscle tissues to extract glycogen and supply glucose to the brain.

Fats are both used as energy store and as building blocks; most cell membranes are based on triglycerides and many metabolic processes require fats as solvents for other things.

Protein does not need to be burned for energy; proteins are used as energy source only if there are no others; among other things because cells can’t process them as easily as carbohydrate, because there’s some nasty nitrogen residue left overs etc. Normally proteins get broken down in amino acids and cellular mechanisms use these to repair and build cells, build enzymes, build transport molecules for key chemical reactions etc.