Some even argue that we are evolutionarily adapted to going without food intermittently. "The evidence is pretty strong that our ancestors did not eat three meals a day plus snacks," Mattson says. "Our genes are geared to being able to cope with periods of no food."
Trying out a fast
Fasting will leave you feeling crummy in the short term because it takes time for your body to break psychological and biological habits, researchers say. There isn't really agreement, though, on what fasting entails. To research this article, I am trying out the "5:2" diet, which allows me 600 calories in a single meal on each of two weekly "fast" days. (The normal recommended daily intake is about 2,000 calories for a woman and 2,500 for a man.) Proving that fasting is not necessarily about losing weight, I am allowed to eat whatever I want on the five non-fast days.
A more draconian regimen than the 5:2 plan has restricted-calorie fasts every other day. Then there's total fasting, in which participants go without food for one to five days. (Fasting for more than about a week is considered dangerous.) This might be a one-off experience, or repeated weekly or monthly.
Different regimens have different effects on the body. A fast is considered to start about 10 to 12 hours after a meal, when you have used up all the available glucose in your blood and start converting glycogen stored in liver and muscle cells into glucose to use for energy. If the fast continues, there is a gradual move toward breaking down stored body fat, and the liver produces "ketone bodies," short molecules that are byproducts of the breakdown of fatty acids. These can be used by the brain as fuel. This process is in full swing three to four days into a fast.