In very basic terms, IF is occasional starvation done in a strategic way. The idea is to cycle between periods of regular eating and fasting, during which you severely restrict your calorie intake or don't consume any food at all. Some people fast for hours, while some may go for a full day or longer.
IF may mean something different depending on who you talk to. One of the more commonly known fasting systems is the 5:2 diet, which involves restricting calories for two non-consecutive days a week and eating without calorie restraints on the other five days. (Jimmy Kimmel credited the 5:2 diet with his weight loss in a Men's Journal story last year.)
Others may fast on a day-to-day basis by eating only during a specific time window. Mark Mattson, a neuroscientist at the National Institute on Aging in Maryland who has researched the subject extensively, shared with the New York Times that most days he skips breakfast and lunch and eats all of his calories within a six-hour window starting in the afternoon. And, Hugh Jackman revealed that he fasted for 16 hours and ate within an 8-hour window to get in shape for his role as Wolverine in 2013.
The health benefits of fasting go beyond weight loss
Fasting may improve your overall health and extend your life, likely due to the ways that it affects cell and hormone function, according to several studies. In one recent study in Cell Metabolism, for example, periodic fasting was linked to lower risks of heart disease, diabetes, cancer, and aging.
So why does fasting have such a positive health impact? During the fasting phase, many cells die and stem cells turn on, which starts a regeneration process and gives rise to new, younger cells, study author Valter Longo, PhD, recently explained to Health. "It sounds too good to be true, but it's not," he said.
Other studies have shown that intermittent fasting may decrease low-density lipoprotein (LDL), or "bad" cholesterol, as well as inflammation. Additionally, IF may improve insulin resistance, which, in turn, helps stabilize blood sugar levels.
It typically focuses on when to eat, not necessarily what to eat
There's no one-size-fits-all fasting diet; plans can be highly individualized. Some folks allow themselves to drink black coffee and green juice during the no-food period, while others may give themselves a cap of 500 calories on fasting days.
For instance, Kimmel told Men's Journal that on fasting days his "meals" might consist of peanut butter and an apple, the whites of hard-boiled eggs, or possibly a bowl of oatmeal. "The rest of the week I'm a glutton—pizza and pasta and steak," he told the mag.
But here, Kimmel reveals one of the issues with fasting diets, says Libby Mills, RD, a spokesperson for the Academy of Nutrition and Dietetics. "The focus isn't always on nutrition," she says. "A lot of the time it's just about calories."
"Some people may also interpret the normal eating time as free rein to go calorie crazy," Mills adds, "which can backfire."
It can help with weight loss, but it may not work for everyone