Is Intermittent Fasting a Safe Way to Lose Weight?

Sure, fasting will get you into a smaller size pair of jeans. But is it smart?

We've long heard that not eating sends your body into starvation mode, leaving your body clinging to every precious calorie in order to make it through the day. For weight loss, that sounds no bueno.

So suddenly, the intermittent fasting bandwagon everyone's climbing on seems to fly in the face of all our conventional nutritional wisdom. Think breakfast is the most important meal of the day? Think again! Who should we listen to? And what will get us into a new size pair of jeans next month? Read on.

What is Intermittent Fasting?

Intermittent fasting includes various eating schedules that incorporate regular periods of little or no food consumption. That could mean 16:8, fasting for sixteen hours a day and restricting food consumption to eight hours. Or you could try the 5:2 method, eating a regular diet five days out of the week and eating 500 calories on the other two days. There's also Eat Stop Eat, where you fast for 24 hours once or twice a week.

Is Intermittent Fasting Safe?

In a word, yes. (Though like any diet, you should check with your doctor first.)

What intermittent fasting is not, for many, is easy. For some, the headaches and "brain fog," of fasting are one of the biggest deterrents. How are you supposed to crush it at the sales conference if your mind is running on empty? These symptoms will abate, say proponents of the diet.

"It does get easier, honestly," writes StephB on the 5:2 Fast Diet Forums. "! In the first few weeks I felt incredibly sleepy by mid-afternoon and also found on occasion I could barely string a sentence together (not great when you're trying to teach a class of 30 teenagers!). Keep persevering, the 'brain fog' (brilliant term!) experienced on fast days does subside when you've been following the plan for a while."

It also takes serious discipline to forgo food for long stretches of time, and family life and social engagements can make this even more difficult. On the other hand, some find the appeal in the diet lays in knowing you won't have to go without cheesecake or martinis forever. The glory of the fast day is the feast day that follows.

"Eating big meals, not counting calories or carbs (or cookies!), and having no restrictions on the foods I ate during my feeding windows was so freeing," writes Jenny Sugar at PopSugar. "I felt satisfied physically and emotionally, and even though I had specific times to eat and fast, it didn't feel strict or hard to follow. Fasting for 24 hours also let me know what true hunger felt like, which helped prevent mindless eating on my nonfasting days."

How Does Intermittent Fasting Work, Like, Scientifically?

When your body doesn't have a constant energy source from the food you eat, it allows your body to utilize the fat stores it already has, helping you decrease your overall body fat percentage.

"The food we eat is broken down by enzymes in our gut and eventually ends up as molecules in our bloodstream," writes Monique Tello, MD, MPH on Harvard Medical School's blog. "Carbohydrates, particularly sugars and refined grains (think white flours and rice), are quickly broken down into sugar, which our cells use for energy. If our cells don't use it all, we store it in our fat cells as, well, fat. But sugar can only enter our cells with insulin, a hormone made in the pancreas. Insulin brings sugar into the fat cells and keeps it there."

"Between meals, as long as we don't snack, our insulin levels will go down and our fat cells can then release their stored sugar, to be used as energy. We lose weight if we let our insulin levels go down. The entire idea of IF is to allow the insulin levels to go down far enough and for long enough that we burn off our fat."

And while we usually think of "stress" as a bad thing, this kind of stress is good for the body in the same way the "stress" of weight training is. Mark Mattson, chief of the Laboratory of Neurosciences at the National Institute on Aging and a professor of neuroscience at the Johns Hopkins School of Medicine, says intermittent fasting can combat inflammation at the cellular level.

"People undergo a metabolic switch in which the liver's energy stores are depleted, and so the body's cells start using fat and ketones for energy," he told Time. "These cycles of challenge, recovery, challenge, recovery seem to optimize both function and durability of most cell sites."

Will Intermittent Fasting Help Weight-Loss?

Chances are, yes. In one study reported on WebMD, overweight adults who cut calories by 20% every other day dropped 8% of their body weight within 8 weeks.

Fasting may also usher in a host of other health benefits. "There continues to be good evidence that intermittent fasting is producing weight-loss benefits," Benjamin Horne, director of cardiovascular and genetic epidemiology at Utah's non-profit Intermountain Healthcare system told Time. "We also have some evidence that these diets can reduce inflammation, they can reduce blood pressure and resting heart rate, and they seem to have beneficial effects on the cardiovascular system."

The Takeaway

If you're kind of an all-or-nothing person, intermittent fasting could help you break through a weight-loss plateau, and it could benefit your long-term health.

"The best medicines are rest and fasting," said Benjamin Franklin. And if old Ben grants permission to take a nap — and look and feel our best — who are we to argue?