Does Intermittent Fasting Work?

For years, we’ve been told it’s important to eat breakfast within an hour of waking up, to rev up our metabolism and get our day started right. But a more recent trend, intermittent fasting, throws that wisdom out the window. The technique relies on restricting your eating to set times and alternating between feasting and fasting.

“Intermittent fasting is allowing the body to have a prolonged period of rest without calorie intake,” explained Dr. Adam Perlman, an internist at the Duke Center for Integrated Medicine in Durham, North Carolina.

Proponents of intermittent fasting say it’s less of a diet than a lifestyle. “What makes intermittent fasting different from dieting is that you are regulating when you eat, not what you eat,” said Dr. Luiza Petre, a board-certified cardiologist and weight management specialist who follows the principles herself. However, she said, limiting your dining window “does not give you an excuse to binge eat, especially on unhealthy foods, when you are not fasting because you will not see the benefits.”

So, what are the benefits of this practice? And should you be doing it? We got the breakdown from experts. Here’s what you need to know:

How it works

There are various ways a person can incorporate intermittent fasting into their daily routine. Below are some of the most popular techniques.

The 16:8 Method: This structure involves consuming your meals within an eight-hour window and giving your body a break from food for the next 16 hours. “You are asleep for the majority of the fasting period, which makes this a medium difficulty compared to other variations,” Petre told HuffPost in an email. “16:8 also is the most common way of intermittent fasting and has gained the most media attention.”

The 5:2 Diet: Also known as the “Fast Diet,” this type of intermittent fasting includes two nonconsecutive days of a strict 500-calorie diet and five days of normal, healthy food. “This method fits the profile of people who have busy family lives or social commitments that would make it difficult for them to stick to a daily regimen,” Petre said. One caveat, per Perlman, is that this eating schedule “can have an impact on a person’s sleep, mood and energy level.”

Alternate Day Fasting: This, according to Petre, is becoming a popular way to kick-start weight loss. The practice involves fasting every other day but eating whatever you want on the non-fasting days. People who follow this trend typically eat 500 calories during their fasting days and then don’t count calories on the other days.

A study published last year in JAMA Internal Medicine concluded that this fasting method hasn’t proven to be more effective than a restricted-calorie diet. “It is least sustainable of all fasting methods in [the] long term and is associated with more over eating in the non-fasting days,” Petre said.

Eat Stop Eat: This method involves consuming only non-caloric beverages for a full 24 hours, one to two times per week. So, for example, if your fasting day was going to be Tuesday, you’d stop eating after you finished a meal on Monday and wouldn’t eat anything else until that same meal the next day.

The potential outcomes can be beneficial for some...

According to Perlman, the potential benefits of intermittent fasting include weight loss and increased muscle mass. “During the fasting state, the body burns more stored fat for energy,” he said. Intermittent fasting can also enhance metabolism, he said, allowing you to more efficiently utilize food for energy.

“Research has also shown the intermittent fasting has effects on the gut flora, which might also explain some of the effects on metabolism,” Perlman said. He added there may be positive effects on insulin sensitivity and various hormones in the body, which can lead to effects such as decreased appetite and improved energy levels.

“My whole body seems more efficient,” said Jazmine Giovanni, a Los Angeles-based writer, of her experience with intermittent fasting. “With digestion limited to those few hours, my body’s energy reserves are focused elsewhere the rest of the time, making healing and recovery faster. I’m more focused and have more pep.”

Studies have shown that intermittent fasting can decrease inflammation in the body and improve blood pressure and heart rate. It’s also been claimed that intermittent fasting can lead to improved brain function and decreased risk of cancer and Alzheimer’s, but these claims have less evidence backing them up, Perlman said.

Erin Wathen, a food addiction counselor and author of Why Can’t I Stick To My Diet, says that in general, the practice is a great way to give your digestive system a break. “It reduces the number of opportunities we eat so our body is not constantly having to process food,” she said. “This is a huge benefit for our GI track, appetite control, sleeping and even our teeth, which will benefit from reducing how often we ask our bodies to metabolize food.”

...But intermittent fasting is definitely not for everyone

At the most basic level, intermittent fasting can be difficult for your schedule. Fitness and nutrition coach Ivana Chapman warns that people may have a hard time fitting intermittent fasting into their social life, for example. If your friends want to get dinner at 7 p.m. but your last meal has to be done by 6 p.m., that could be an issue.

And when people break the fast, they often overdo the portions. “Larger meals may be harder on the digestive system and can trigger acid reflux in susceptible individuals,” Chapman said.

Lisa Cooper, a dietitian with Orlando Health, noted that fasting can come with some initial side effects, such as “increased hunger, low blood sugar, headache, irritability, hypoglycemia/low blood sugar, dizziness, lightheadedness, tiredness, nausea and fatigue.”

And the practice can be particularly dangerous for people with certain health conditions.

“Some women, especially those who are already lean and who are active, may encounter hormonal issues if they reduce calorie intake and intermittent fast for days on end, or simply too often,” said Josh Axe, a clinical nutritionist and author of The Real Food Diet Cookbook. “These women may benefit from intermittent fasting only a few days a week, rather than every day, and by paying close attention to how their body responds.” He also emphasized that pregnant and nursing women should avoid intermittent fasting.

Cooper said fasting would not be appropriate in circumstances where people need extra calories or nutrients for growth and development, such as during childhood or adolescence, or when breastfeeding. “People with certain diagnoses who require calories or set meal timing, such as those with diabetes, hypoglycemia or underweight, should avoid fasting, along with people who require food to be taken with medications,” she said.

And anyone who has a history of eating disorders should forgo the practice. Perlman said that for people with such a history, any type of dieting or food restriction program “runs the risk of increasing the focus on food and exacerbating an already challenging if not unhealthy relationship with one’s diet.”

Axe noted that eating disorders, including anorexia and bulimia, “can lead to malnutrition, which has a variety of negative effects on hormonal balance and someone’s metabolism.” Intermittent fasting can make those problems worse if someone is already undereating and struggling to produce sufficient hormones.

“Fasting may basically make all types of symptoms associated with eating disorders more severe, especially if someone is already underweight or very active,” Axe said.

What you need to know before you try it

Many experts endorse the practice of intermittent fasting, as long as it is properly and safely carried out. Part of this means continuing to adhere to a healthy eating plan, regardless of when you are consuming your calories.

“I’ve seen some people go overboard with the food during their feeding window,” said Sunny Brigham, a board-certified clinical and integrative nutritionist in Texas. She emphasized that the goal is to maintain a healthy amount of calories for your body, not to “rebound eat.”

The bottom line, Cooper said, is that “if intermittent fasting is incorporated, it should be included as part of a healthy, balanced diet and lifestyle change and under the supervision of a health care provider.”

For those starting out, Petre suggested implementing a 12-hour fasting/12-hour eating window and building up from there, eventually finding the schedule that works best for you. She also stressed the importance of breaking your fast with fresh, unprocessed, nutrient-dense whole foods, prioritizing healthy sources of protein and not going crazy with junk food, as that would negate the benefits.

If you’re struggling with an eating disorder, call the National Eating Disorder Association hotline at 1-800-931-2237.

CORRECTION: A previous version of this post misidentified Brigham as male.