“We know from animal models,” says Mark Mattson at the National Institute on Aging, “that if we start an intermittent fasting diet at what would be the equivalent of middle age in people, we can delay the onset of Alzheimer’s and Parkinson’s.”
Until recently, most studies linking diet with health and longevity focused on calorie restriction. They have had some impressive results, with the life span of various lab animals lengthened by up to 50 percent after their caloric intake was cut in half. But these effects do not seem to extend to primates. A 23-year study of macaques found that although calorie restriction delayed the onset of age-related diseases, it had no impact on life span. So other factors, such as genetics, may be more important for human longevity.
That’s bad news for anyone who has gone hungry for decades in the hope of living longer, but the finding has not deterred researchers who study fasting. They point out that although fasting obviously involves cutting calories — at least on specific days — it brings about biochemical and physiological changes that daily dieting does not. Besides, calorie restriction may leave people susceptible to infections and biological stress, whereas fasting, done properly, should not.
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.