Just How Healthy Is Intermittent Fasting?

Intermittent fasting, which is when you go without eating for a certain length of time every day or sprinkle in some days every week where you eat very few calories, has been put on a bit of a health pedestal lately. Not only is it relatively easy to do—eat whatever you want, just cram it into an eight-hour window!—but it allegedly has loads of health benefits. Everything from weight loss to longevity (at least in mice!) has been linked to the trendy diet. But how much of that is true, and how much is just wishful thinking? As it turns out, that question isn’t all that easy to answer.

“The studies that currently exist on intermittent fasting are very small in size—usually less than 50 people,” says Dorothy Sears, PhD, a professor at Arizona State University College of Health Solutions. “They just don’t have enough participants in them, so the results are hard to generalize.” 

With larger randomized controlled trials missing, most of the existing intermittent fasting research has been done on mice. But those mouse studies, and the ones done on smaller groups of people, are pretty promising. First, there’s the weight loss that can come from fasting for a portion of the day (0.4 to 1.8 pounds a week, on average). On top of that, intermittent fasting might be able to help improve insulin resistance and decrease something known as fasting insulin (if that number gets too high, according to Dr. Sears, it raises your risk for type 2 diabetes and even certain cancers). 

In new research from Matt McAllister, PhD, director of the Metabolic & Applied Physiology Lab at Texas State University, intermittent fasting has been linked with improved blood pressure and cholesterol as well as lower body fat—participants lost about two pounds of fat over a month of intermittent fasting.

The most surprising part: All of this happens without calorie restriction. “We thought that people fasting for 16 hours a day would consume fewer calories, but in our study the participants ate the same amount of food as they normally did, just in a shorter period of time,” says McAllister.

What’s going on? There are a few things to factor in: “I believe the benefits are primarily due to circadian rhythms,” says Sears. “There are mechanisms in your body that tend to align with day and night—they are optimized to do certain actions at certain times.” That includes the body’s production of melatonin, which is released later in the day to help you feel sleepy, as well as insulin, which moves glucose out of your blood when you eat. 

“Insulin secretion is very efficient in the morning and less efficient later in the day,” says Sears. “If you eat at night, when your insulin doesn’t work as well, you end up with extended periods of high insulin and glucose levels. This can drive insulin resistance.” The 16-hour fasting window often starts in the evening and continues overnight, which means your insulin and glucose levels can go down and stay that way.

But that’s not all. “When you avoid taking in calories for a long stretch of hours or even just reduce the calories you’re eating for a few days a week, it takes some of the load off your liver,” says McAllister. “Your liver processes everything you consume, and without a break, it can get overwhelmed. When that happens, it increases enzymes that can lead to weight gain, high cholesterol, and other negative changes in your body.” Let your liver have time to chill and your body reaps the rewards.

Curious to give it a shot? Make sure you consult with a registered dietitian and your doctor before beginning any diet plan. You’ll also want to consider which style of intermittent fasting is right for you.

Two of the most popular methods are either picking a window to avoid food every day or eating your normal way five days a week and choosing two days a week to eat significantly less food—typically around 500 calories. The latter one can be tough, which is why a lot of people try the first option. “It can be a 16-hour fast, but it can also be 12 or 14,” says Sears. Ideally, it would be great to eat mostly healthy foods during your eating window—for an added health boost.  

Note: Fasting isn’t for everyone, particularly if you’re still growing (under 18 years old), pregnant, have a history of disordered eating, a chronic disease like diabetes, or are taking certain medications. Again, always consult with a health professional first.

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