Asked by: Annie Martoni, by email
Within one or two days of your last meal, your body will have exhausted all the glycogen stored in the liver and muscles. Most of your energy requirements will be met by breaking down stored fat into ketones, but brain cells and red blood cells can't metabolise ketones, so their glucose requirements must be met by breaking down muscle.
Even if you have lots of body fat left to burn, you can still starve to death if you don't have enough muscle because vital muscles like the heart will have been weakened to the point where they stop working. For this reason, doctors normally consider 40 to 50 per cent weight loss as life-threatening, regardless of your initial body weight.
Total starvation is normally fatal in eight to 12 weeks. In less extreme cases, however, where you are still receiving some calories - particularly if these calories include a high proportion of protein - being fat will help you survive much longer, because your body will be able to meet the majority of your calorie requirements from stored fat.
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.
