Let’s get the fasting and weightless issue out of the way first. "Fasting" refers to an extended period of time in which you consume no food, typically for 3 to 7 days, while "intermittent" fasting refers to not eating, or eating less, for much shorter periods – typically 8 to 24 hours. Unsupervised fasting is a terrible strategy for weight loss. (I say “unsupervised” for special medical or spiritual cases, where a fast supervised by a health professional is in order.) The majority of people doing fasts and cleanses are doing them for the wrong reason: weight loss.
See also To Detox or Not To Detox?
Even though you may lose weight quickly, it’s temporary. Fasting can induce metabolic changes and hormonal dysregulation that can make losing body fat more difficult. Weight loss aside, there may be some real benefits for “intermittent fasting,” especially in the form of "alternate-day fasting." Researchers at the University of California, Berkeley, fasted mice on a reduced-calorie diet (25% of their usual dietary intake) on alternate days, but allowed them to eat whatever they wanted on non-fast ("feast") days. “We found that fasting can reduce cell proliferation rates in skin and breast,” said lead researcher Krista Varady. “That’s equivalent to a decrease in both breast and skin cancer risk.”
Varady’s mice studies have been followed up with nearly 20 human trials, and the results have been impressive. On "fast" days, participants ate up to 500 calories and on "feast" days they ate whatever and however much they wanted. Alternate-day fasting appears to lower measures of cardiovascular risk and help with weight loss, regardless of whether one's regular diet is high or low fat. Importantly, research has shown that dietary compliance with the program is high (86% in one study). And the consistently promising results seem to be amplified when you throw exercise into the mix – one study showed that while either alternate-day fasting or exercise reduced body weight and had favorable effects on cholesterol, subjects who did both had superior results when compared to the subjects who only exercised or only fasted.
We’ve known for a while that eating less calories can produce a bunch of health benefits, largely by turning on a group of genes (known as the SIRT genes), which are involved with longevity, and also by producing less metabolic wear and tear. I suspect that intermittent fasting – in one form or another – is going to turn out to have a lot of advocates. But it’s still a little too early to recommend it without reservation. We’ll keep you posted!