Intermittent fasting — a diet that reportedly helped stars including Jennifer Aniston, Kourtney Kardashian, and Chris Pratt shed unwanted pounds, according to People — has been transcending its status as a celebrity fad as research about its weight loss and other benefits continues to emerge. The latest, a review published in October 2021 in Annual Review of Nutrition found that intermittent fasting leads to roughly the same amount of weight loss as traditional calorie-restrictive diets, and may improve other markers of cardiometabolic health as well.
“One of our main findings was that people who do intermittent fasting lose about the same amount of weight as people on a regular calorie-restriction diet that cuts out 500 calories a day,” says lead author Krista Varady, PhD, researcher and professor of nutrition at the University of Illinois, Chicago.
Additionally, Dr. Varady notes that intermittent fasting appears to help with metabolic health. “It did help lower blood pressure,” she says. “Some studies reported decreases in LDL [bad] cholesterol and triglycerides, and insulin resistance.”
What Is an Intermittent Fasting Diet?
In general, intermittent fasting (IF) refers to any eating schedule that alternates periods of going without food (fasting) with meals. There are many different types of plans, including those that restrict calories for only certain hours of each day or certain days of the week. The main difference between IF and traditional calorie-restriction diets is that IF doesn’t limit portions or foods, only when you eat them.
The review, which included more than 25 studies that looked at intermittent fasting in humans, found that IF consistently resulted in people consuming fewer total calories — between 10 and 30 percent fewer than they had been eating at the onset of the studies — which led to weight loss.
The review also included safety considerations and practical advice for how these diets should be implemented in everyday life.
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What Types of Intermittent Fasting Plans Lead to Weight Loss?
The review included studies on three types of intermittent fasting:
- Alternate day fasting (ADF), which typically involves a day of unrestricted eating alternated with a fasting day where 500 calories consumed in one meal account for the total daily calorie intake
- The 5:2 diet, a modified version of alternate day fasting that involves five “feasting days” and two fasting days per week.
- Time-restricted eating (TRE), which limits eating to a specified number of hours per day (most often 8, but plans range from 4- to 10-hour “eating windows,” as they are called), followed by 16 hours of fasting (In time-restricted eating, there are no calorie restrictions during the eating period.)
It’s worth noting that the review wasn’t designed to compare the effectiveness of different types of intermittent fasting plans because the studies were done in different groups of people and using different parameters. But the results did indicate that individuals who followed either the alternate-day diet or the 5:2 plan both averaged a loss of around 7 percent of their body weight (that’s roughly a 14-pound loss for a 200-pound person), and were able to maintain that loss for a year. Despite fasting much less frequently than alternate-day fasters, subjects who participated in the 5:2 diet lost an equivalent amount of weight, a finding that surprised researchers.
While all plans led to some weight loss, following a time-restricted eating plan was the least effective plan for shedding pounds. In total, participants with obesity who followed a time-restricted eating plan lost an average of 3 percent of their body weight.
“Because time-restricted intermittent fasting is the most popular form of fasting right now, we were hoping it would produce the same amount of weight loss as the other types,” Varady says. The disparity may be due to the fact that people following a TRE plan aren’t restricting their calories as much as those following other plans. That said, the length of the “eating window” in a TRE did not appear to impact weight loss. People who only ate for only four hours a day, for example, lost the same amount of weight as those who ate for eight hours.
Although time-restricted fasting didn’t result in as much weight loss as the other methods, it appears to be a more sustainable way of eating that a person could stick with over a lifetime, even compared with calorie-restriction diets. That means that it could have more long-lasting weight loss benefits for some people, says Varady.
“Many people drop out of alternate day fasting in trials because they can’t hack just eating 500 calories every other day,” she says. “People tend to drop out far less frequently with time restricted eating — maybe just 1 out of 20 people.”
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Does Intermittent Fasting Work for Everyone?
Varady and her colleagues hoped the review would help debunk some myths regarding intermittent fasting. Two such myths — that intermittent fasting negatively affects metabolism, and that it causes disordered eating — are not true, according to the studies reviewed. People are often concerned that fasting will make them feel lethargic and unable to concentrate, says Varady. Yet many people in these studies reported experiencing a boost of energy on fasting days, she says.
“Although it’s different for each person, I would say that it’s a very small subset of patients who really notices improved mental clarity or energy with intermittent fasting,” says Fatima Cody Stanford, MD, MPH, associate professor and obesity medicine physician scientist at Massachusetts General Hospital and Harvard Medical School in Boston, who was not involved in the review. “Many people don’t feel great while they are fasting for a variety of reasons. In some cases, their blood sugar might get too low during the fasting period,” she says.
There are several groups who should not fast, according to the studies. Those individuals include:
- Those who are pregnant or lactating
- Children under 12
- Those with a history of disordered eating
- Those with a body mass index, or BMI, less than 18.5
- Shift workers, who studies have shown have difficulty with fasting regimens because of the changes in work schedules
- People who need to take medication with food at regimented times
People with diabetes, particularly type 1 diabetes, should approach intermittent fasting with caution, says Dr. Stanford. “It can be done, but I think it would be important to have medical supervision,” she says. This would allow for oversight and possible adjustments in medications during periods of fasting, according to a review published in February 2021 in Clinical Diabetes and Endocrinology.
“Intermittent fasting isn’t for everyone,” says Varady. “It will probably work well for people who are good at meal skipping; for example, people who don’t eat breakfast, or for people who consume most of their calories during meals and don’t snack a lot.” And, she adds, “People who successfully use this eating pattern to lose weight are those who can do it over the long-term. If people only do intermittent fasting for a short period of time, and they resume the diet they had prior to that, the weight typically comes back.”
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How to Make Intermittent Fasting Most Effective
If you’re interested in trying intermittent fasting, it’s a good idea to start slowly, suggests Julia Zumpano, RD, a registered dietitian at the Cleveland Clinic in Ohio. “I often tell people to try to fast after they finish their dinner for a few nights a week and see how that goes,” she says. “If you’re doing well, then you can gradually increase your fasting window.”
The authors of the review offered insights on what to expect when trying intermittent fasting, as well as some best practices:
Give yourself time to adjust. Side effects such as headaches, dizziness, and constipation subside after one to two weeks of fasting. Drinking more water can help alleviate headaches caused by dehydration while fasting. “Headaches are common with fasting,” agrees Zumpano. You may want to add a drink with electrolytes, she adds.
Make your diet quality count. There are no specific recommendations for food consumption during intermittent fasting, but the reviewed studies suggest that eating fruits, vegetables, and whole grains can help boost fiber intake, which can help alleviate the hunger associated with fasting.
Don’t avoid exercise. Moderate- to high-intensity endurance or resistance training during food abstention can be done, and some study participants reported having more energy on fast days, according to the authors. If you follow an alternate day fasting plan, studies recommend eating your fasting day meal after exercise.
Do avoid alcohol. For people on an alternate day or 5:2 fasting plan, alcohol is not recommended on fast days; the limited calories should be used on healthy foods that provide nutrition.