During starvation, the body begins to atrophy and it is said the body is digesting itself for sustenance. What is actually happening cellularly to digest your muscles for energy?
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The body will start by burning body fat. Most of the body tissues can burn fat directly. There are two exceptions. The red blood cells only burn glucose, and that is made by the liver from the glycerol that is left over when fat is burned. The brain burns some of that glucose as well but there’s not enough available, so the liver starts burning fat, converts the byproduct into ketones, and releases those into the blood to power the brain. That’s what being ketosis is.
If the body gets low body fat, it will start tearing down muscles. The protein is broken down into amino acid, some of which are converted to glucose, some of which are converted to ketones, and some of which are utilized by being burned directly.
The protein in your body is constantly being broken down and reformed – the vast majority of proteins in your body last <7 days before they’re broken down. Most only last a few hours.
Proteins are broken down into their component amino acids. If energy is available, those amino acids will almost immediately be remade into a different protein. If energy is not available, the amino acids are converted into glucose and ammonia. The glucose is used for energy, while the ammonia is converted to urea, which you pee out.
The reason that you pee out urea is because this isn’t a perfectly efficient process and your body will still convert about 20-30 grams of protein into urea each day, even if you’re eating at a calorie surplus.
The rate at which your body breaks down protein is basically constant, but the rate at which is creates new protein is dependent upon available energy. During starvation, protein breakdown into amino acids continues but synthesis of amino acids to new protein slows. Since there is only two things your body can do with amino acids – turn them into protein or glucose/urea – if your body isn’t turning amino acids into protein it turns them into glucose and urea instead.
The process of cell death is called **apoptosis** – this can be triggered by a number of situations, including cellular age or cellular stress. Once it starts, there are a sequence of cellular processes that lead to the collapse of the cell. These changes include blebbing (cell membrane deformation and internal structure collapse), cell shrinkage, nuclear fragmentation, chromatin condensation, DNA fragmentation, and mRNA decay. The fragments than get carried in the blood to organs such as the liver where the fragments can be broken down and reused, or to the kidneys where they are filtered out. However, larger protein fragments can clog the kidneys, causing reduced kidney function and possibly renal failure if a large number of cells are undergoing apoptosis.
Note that most adults lose between 50 and 70 billion cells each day due to apoptosis in normal circumstances. During starvation, it is not so much that more cells die, but that there are insufficient resources to replace those that undergo apoptosis.
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