February 24, 2020

Intermittent fasting: What does the evidence say?

Daily Briefing

    Editor's note: This popular story from the Daily Briefing's archives was republished on Jan. 21, 2021.

    Intermittent fasting was one of 2019's most-searched diets, according to Google Trends, and while it might be tempting to write this off as another unscientific diet fad, there's some evidence to suggest that intermittent fasting might actually live up to the hype.

    How exactly does 'intermittent fasting' work?

    A person who is on an intermittent fasting diet will limit the timeframe during which they eat. For instance, a person might eat only during a four-, six-, or eight-hour period each day, according to Andrew Freeman, director of cardiovascular prevention and wellness at National Jewish Health.

    According to Mark Mattson, neuroscientist at the National Institute on Aging and Johns Hopkins University School of Medicine, the idea is to get the body to switch from using glucose stored in the liver for energy to using the body's fat supplies. He explained, "It takes 10 to 12 hours to use up the calories in the liver before a metabolic shift occurs to using stored fat."

    People trying to lose weight by fasting should have at least 16 hours each day when they don't consume any calories, according to Mattson. "[T]the easiest way to do this is to stop eating by 8 p.m., skip breakfast the next morning and then eat again at noon the next day," Mattson said.

    Does the fasting live up to the hype?

    While fad diets sometimes lack scientific backing, Freeman said there is some evidence that the practice might help people lose weight and improve their health.    

    For instance, Mattson and co-author Rafael de Cabo, also of the National Institute on Aging, recently reviewed studies on intermittent fasting in both humans and animals. Their findings, which were published in the New England Journal of Medicine, showed that the practice was associated with improvements in certain disease indicators, including insulin resistance, blood fat abnormalities, high blood pressure, and inflammation, regardless of weight loss.

    That said, it's worth noting that there's been almost no long-term studies into the practice, and many studies that look at fasting's impact on certain diseases has only been conducted in animals, the New York Times' "Well" reports.

    But Freeman noted that intermittent fasting also can help people pay more attention to what and when they eat, which is the first step to improving one's diet. "The goal then is to become mindful about what you're doing and how much you're doing rather than the so-called mindless eating, which we find everywhere."

    However, intermittent fasting, like many diets, requires people to ground their diets in healthy foods and avoid bingeing, Freeman said. He warned, "[I]f you gorge yourself on super high-fat pizza and bacon and whatever while you're eating in that short period of time, it's going to end up probably doing the opposite and make people gain weight."

    Fasting might not work for these people

    While research suggests fasting holds health benefits, experts warn that it isn't right for everyone.

    "What I usually tell folks is to be careful which patients they talk to about this, because some people are definitely not the right candidates," Freeman said. "If you have a brittle diabetic or somebody who suffers from hypoglycemia or syncope, this may not be the right approach."

    In addition, according to the New York Times, intermittent fasting could be harmful to people who have had an eating disorder or have a tendency toward one.

    Further, people who are good candidates for fasting and take up the diet should bear in mind that the diet will "take the body a little getting used to," Freeman said. "If you're used to eating every couple of hours and then all of a sudden you cut down to you know 4 hours a day, the first few days are going to be rough for a lot of folks."

    Separately, in their review, Mattson and deCabo wrote, "Many people will experience hunger, irritability and a reduced ability to concentrate during periods of food restrictions." But side effects should go away without about a month, they noted (Phend, MedPage Today, 2/18; Kolata, "Well," New York Times, 2/17; Dode, Newsweek, 12/11/19).

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