Intermittent fasting—periods of voluntary abstinence from food and drink—is a broad term that can be applied to many different practices. This type of dieting has spurred many books and received a lot of attention in the last few years, since studies (mostly in animals) have shown that it may reduce the risk for several diseases as well as promote weight loss.
Additional research, including a small study of four fasting people published in Scientific Reports in 2019, suggests that intermittent fasting may also help boost metabolism.
The most popular approach to intermittent fasting is the 16/8, which requires fasting for 16 hours a day. Another version, alternate day fasting (ADF), alternates 24-hour periods of fasting (which are actually very restricted 500-calorie diets) with days of eating freely.
The 5:2 approach limits fasting to just two days a week, while the Warrior Diet follows a 20-hour fast with one large meal consumed at night. “Part of the confusion with intermittent fasting lies in the lack of a definition,” says Robin Foroutan, RDN, a New York City–based registered dietitian nutritionist and spokesperson for the Academy of Nutrition and Dietetics. “To some people, intermittent fasting means that they fast every day, while to others it means they only eat between 11am and 6pm.”
1. Intermittent fasting helps you lose weight without following a traditional, calorie-restricted diet
Research suggests that counting calories and limiting your food options can cause stress and increase cortisol production, which can subsequently lead to abandonment of the diet, feelings of deprivation, uncontrolled cravings, and weight regain. Adapting to intermittent fasting, a method of scheduled eating and fasting, relies strictly on time.
Some people want more flexibility when it comes to losing weight, says William Yancy Jr., MD, program director for the Duke Diet and Fitness Center in Durham, North Carolina. “They don’t want to think about dieting every day of the week, [and] they lose motivation after a certain period of time of restricting calories.”
Intermittent fasting works for people who like to follow rules, explains Elisabetta Politi, RD, nutrition director for the Duke Diet and Fitness Center. “Rather than saying, ‘Just eat less,’ we tell them not to eat after 6 p.m.,” she says, “and for those who have the discipline, it works.”
2. Intermittent fasting helps you keep the weight off over the long term
Following an intermittent-fasting diet may make it easier to maintain the weight you lost over the long term. A two-part study of 40 obese adults, published in Frontiers in Physiology in 2016, compared the combined effects of a high-protein, low-calorie, intermittent-fasting diet plan with a traditional heart-healthy diet plan.
The results showed that while both diets proved to be equally successful in reductions in body mass index (BMI) and blood lipids (fatty acids and cholesterol), those on the intermittent-fasting diet showed an advantage in minimizing weight regain after one year.
3. Intermittent fasting may help those at risk for developing diabetes
Losing weight, moving more, and eating a healthy diet can help fight off developing type 2 diabetes. “When you lose weight, you become more insulin-sensitive,” says Politi. “It drives the blood sugar down.”
When we eat, our body releases insulin into the bloodstream to supply the cells with energy, but those who are pre-diabetic are insulin-resistant, which means their blood-sugar levels remain elevated. Intermittent fasting may help those who are pre-diabetic because it requires the body to produce insulin less often, explains Foroutan.
“If you are pre-diabetic or have a history of diabetes in the family, this type of diet can be helpful.” Research has shown promise in backing up these claims: A study published in the journal Cell in 2017 found that a diet mimicking fasting cycles could restore insulin secretion and promote the generation of new insulin-producing pancreatic beta cells in mice with type 1 and 2 diabetes. While further research still needs to be done, early studies on human cell samples suggest similar potential.
4. Intermittent fasting helps sync circadian rhythms and fight off metabolic diseases
Your circadian rhythm, or your internal body clock, is a natural system that regulates feelings of sleepiness and wakefulness over a 24-hour period. Research published in the Annual Review of Nutrition in 2017 suggests that intermittent fasting may help us to stick with our body’s circadian rhythm, and that can help with our metabolism.
Eating certain foods before bed has also been linked to weight gain and sleep disturbances, especially when they cause acid reflux. “We know that insulin sensitivity is increased during the day, and we are less sensitive to insulin at night—the same goes for digestion,” Foroutan says.
“It makes you wonder if eating at night is working against our body clock.” If you want to honour your circadian rhythm, she adds, you need to go to bed earlier and sleep so the body can repair itself.
5. Intermittent fasting may lower your risk for cardiovascular disease
You can reduce your risk for heart disease by following a healthy lifestyle: eating right, exercising, not smoking, and limiting alcohol intake. Research also shows that intermittent fasting may help.
“If you restrict calories every day, it improves cardiovascular risk, glycemic control, and insulin resistance,” explains Dr. Yancy. In one small study of 32 adults, published in the Nutrition Journal in 2013, an alternate-day fasting regimen resulted in weight loss as well as cardiovascular benefits, including improved LDL cholesterol and triacylglycerol concentration.
“The studies use alternate-day fasting, but keep in mind that fasting does not mean not eating—it means eating less,” adds Dr. Yancy. This type of diet represents a different way of doing things, and it might appeal to some because they can restrict a couple of days of the week rather than every day. “As an obesity physician, I like to have options for people, because people are different,” Dr. Yancy says. “If it appeals to you, let’s try it!”