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
Hey, it helped Kimmel and Jackman shape up. But Mills warns that while a fasting program may aid weight loss, it's not a plan that is practical or sustainable for everyone.
"It's not something that I personally recommend in my practice because I think there are lots of ways to get a jumpstart on weight loss without going cold turkey with food," she explains. "You can instead focus on eating more vegetables and fruits. That way you're focusing on picking healthy calories and adding nutrients. It's a positive change as opposed to an all-or-nothing mindset."
You also don't know how you will react physically and mentally to calorie restriction, she adds. "You may not know how your body will respond to, say, low blood sugar," Mills says. "Or, some people find that fasting seems like a piece of cake until around 3 o'clock, and then suddenly cravings come on and you end up eating all sorts of things you normally wouldn't."
The bottom line? "You have to consider how you personally are affected by restriction," she says.
You should talk to a doctor before trying a fasting diet
Your personality is just one factor to consider before you try IF; your overall health is another.
"Whether you're thinking about trying a fasting system for preventive reasons or as a treatment, the doctor should be involved," Longo says. "There are many factors that must be considered, like your current diet, or whether you have diabetes or a metabolic disorder."
Also, it's important to determine with a health or nutrition professional what sort of system makes sense for your lifestyle.
"An athlete with a perfect pescatarian diet may benefit from only fasting twice a year," Longo explains. "But someone with high cholesterol and excess abdominal fat may see more improvement by doing it more regularly."
The long-term effects of fasting diets aren't well understood. Much of the research on the topic has been done across short time frames. And while experts have done some studies in humans and are doing more, a lot of the current info is from animal samples.
A lot more research needs to be done, Mills says. But if you are curious about incorporating fasting into your eating plan, you should ask a health professional to help you design a plan that ensures you are eating the right foods on both fasting and non-fasting days to guarantee you stay in good health.