If you stop eating food, within 6 hours your body will break down glycogen into glucose and fat into acetyl-CoA so that your biological processes remain uninterrupted. This is pretty normal. But after 72 hours, your body will need to start to break down protein in order to synthesize glucose from amino acids otherwise your brain will starve. This is because the brain can’t use fatty acids for energy directly as they can’t cross the blood-brain barrier.
Well, after about a week of starvation, fat stores will become depleted, and now all that is left for energy is – proteins. At this point, essential proteins – like antibodies that fight off invaders that cause harm or infections, or muscle proteins that give you strength – are now being used to make proteins.
With no immune system-essential vitamins and minerals being consumed and total loss of all the essential proteins, you will start to get sick, and death can come as early as 3 weeks to 70 days “commonly from a heart attack due to the tissue degradation of the heart, diaphragm, and body.” Your survival time without food, however, depends primarily on how hydrated you stay, your activity levels and also how many fat stores you had to begin with.
Watch this video by AsapSCIENCE.