What is intermittent fasting? The diet plan loved by Jennifer Aniston, Jimmy Kimmel and more

Intermittent fasting has risen as a popular diet over the last few years — stars including Jennifer Aniston, Kourtney Kardashian, Chris Pratt and Jimmy Kimmel have lauded it, though they often don't expand on what sort of health benefits they gain from doing so.

Many others online have said intermittent fasting helps them with weight loss goals. Does that mean you should try it? There's a possibility you could find some success. But health experts warn that there are caveats and exceptions you should understand first.

"Restrictive dieting is really unnecessary and usually backfires for most people," registered dietitian Jamie Nadeau tells USA TODAY. "Not only do most people gain weight back after the diet becomes unsustainable, but many end up with disordered behaviors around food. ... It often leaves you feeling like something is wrong with you or that you lack willpower, when really it’s the diet setting you up to fail."

Here's what experts want you to know about intermittent fasting before trying it.

What is intermittent fasting? 

Intermittent fasting is a diet that can be done several ways, but basically boils down to creating set periods of time when you can eat, and set periods of time when you fast. Schedules can vary from creating an eight-hour eating window daily — say, from 11 a.m. to 7 p.m. — all the way to a more extreme schedule of choosing to only eat one meal a day two days a week, according to Johns Hopkins Medicine.

"For some people, intermittent fasting helps them stay in a calorie deficit simply by allowing for less opportunity to eat," Nadeau says. "But research hasn’t proven it to be any more effective than traditional lifestyle and diet changes."

What are the negatives of intermittent fasting?

A preliminary study recently raised red flags after finding that intermittent fasting — defined by the study as following an eight-hour time-restricted eating schedule — was linked to a 91% higher chance of death by cardiovascular disease, compared to those who eat between 12 and 16 hours a day.

Johns Hopkins Medicine also recommends anyone who is under 18, pregnant or breastfeeding, has type 1 diabetes or an eating disorder steers clear of trying this diet plan.

"Because of the rigid structure of intermittent fasting and rules around when you can and can’t eat, I recommend that anyone with a history of disordered eating avoids it, as it can definitely make things worse," Nadeau says.

If you're looking to lose weight, Nadeau instead recommends focusing on small habit changes: adding more physical activity to your daily schedule, eating more fruits, vegetables and foods high in fiber and protein and drinking more water.

"New diets always sound exciting and it’s easy to get swept up in thinking they’re the magic diet you’ve been searching for," Nadeau says. "The truth, though, is that restrictive diets don’t work long-term. If it’s not something you can envision yourself doing forever, it’s not going to work. Your diet changes should be things you can fit into your life forever so that you can maintain your health and results forever."

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