What happens to your body without food

Your quick reserve lasts about a day. It is glycogen - sugar bundled up in your liver and muscles. Liver glycogen goes first, within a day without food, and faster if you walked a lot and felt cold that day. Muscle glycogen does not enter the shared pool: a muscle can use its own glycogen only for itself and cannot release it into the blood.

Water goes along with the sugar: every gram of glycogen holds about three grams of water. That is why the scale shows a pleasing number in the first few days that has nothing to do with fat. This water will return with your first meal.

Then comes the real challenge: how to fuel the brain. It uses about a fifth of all resting energy, runs almost entirely on glucose, and cannot switch to fat - fatty acids barely cross the blood-brain barrier.

For the first two or three days, the liver makes glucose from whatever it can, including amino acids - meaning from muscle. If this continued, a person would break herself down in just weeks. So the liver starts up a second workshop: it turns fat into ketone bodies, a fuel that can enter the brain. After about a couple of weeks, ketones cover about two-thirds of its needs, and muscle breakdown slows noticeably.

What happens during a long lack of food is known from Ancel Keys's Minnesota experiment: in 1944-1945, thirty-six volunteers lived for six months on roughly half their usual rations. Their bodies did not just lose weight - they turned down the volume on everything that could be muted.

They felt cold in a warm room, moved slowly, lost interest in people, and talked mostly about food: collecting recipes and studying menus for hours. Their personalities had not changed: on starvation rations, the brain narrowed its focus to one task. They recovered slowly: even after months of normal eating, many still could not get enough to eat.