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.

The thought experiment: How fat do you have to be to stop a bullet?

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luis villazon
Luis VillazonQ&A expert

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.