At first, Miriam Merad thought she’d never be able to go without breakfast. “I was certain I wouldn’t be able to function without it,” says Merad, 50, a cancer immunologist at Icahn School of Medicine at Mount Sinai in New York City.
But as data began piling up on the benefits of intermittent fasting — an eating pattern that involves alternating between periods of eating and refraining — Merad felt compelled to give it a try. These days, she holds out for a light lunch, eats a more substantial evening meal with her family and then fasts for the subsequent 16 hours. Variations of the eating plan include fasting every other day, following a greatly restricted diet two days a week (called the 5:2 method), and, like Merad, restricting intake to an eight- or 10-hour period. Contrary to her concerns about sustainability, Merad finds that she has more energy than ever. And she believes that fasting is enhancing her health in multiple ways. “Many people are doing this, and the feedback has been phenomenal,” she says.
People have often turned to periods of eating little or no food as a way to lose weight. Studies show that intermittent fasting is as effective at helping people shed excess pounds as reducing their intake of food throughout the day. (Merad, already slim, lost a few pounds naturally even though that wasn’t her goal.) In a 2017 randomized trial involving 100 people published in JAMA, the Journal of the American Medical Association, those who fasted on alternate days lost roughly the same amount of weight as those counting calories. But because people are allowed to eat normally during the “off” days or times, fasting plans may be easier for many people to follow.
[See: The 9 Best Diets for Fast Weight Loss]
Narrowed Window
“Psychologically, it’s harder for a lot of people to keep track of their calorie intake every meal and worry about it, rather than just switching up their eating pattern,” says Mark Mattson, adjunct professor of neuroscience at the Solomon H. Snyder Department of Neuroscience at Johns Hopkins School of Medicine. Mattson, the former chief of the Laboratory of Neurosciences at the National Institute on Aging Intramural Research Program, studies the cognitive effects of intermittent fasting and himself crams three moderate-sized meals (and healthy snacks) between the hours of noon and 6:00 p.m. “When you restrict the window of time in which you eat, you’re likely to eat less,” Mattson says. While people can generally eat what they like, a plant-based diet is recommended.
In addition, research now shows that eating less at certain times of the day or week may boost health in other profound ways. “There’s good evidence that intermittent fasting can improve a variety of health indicators that are risk factors for cardiovascular disease, stroke, diabetes and cancers,” Mattson says. It’s been shown to lower cholesterol, blood pressure, insulin and glucose levels. A new study in 13 men and six women with metabolic syndrome — a constellation of symptoms including high blood pressure, high blood sugar, excess abdominal fat and abnormal cholesterol levels that raises the risk of heart disease and diabetes — revealed that eating within a 10-hour window led to lower blood pressure, improved cholesterol, and decreases in blood sugar spikes. Plus, says study author Pam Taub, a cardiologist and associate professor of medicine at the University of California–San Diego School of Medicine, “Patients have told me that they feel that it gives them more energy and their sleep quality is improved.”
Intermittent fasting seems to work by taking advantage of an evolutionary adaptation that allowed early humans to survive periods of famine. “The body knows how to handle fasting,” says Avigdor Arad, instructor of medicine at the Icahn School of Medicine and director of the Mount Sinai Physiolab. Most of the energy stored in the body is in the form of fat, he notes. “If you don’t eat anything, your body diverts to burning fat for fuel.” That leads to weight loss and the other positive metabolic changes. Although eating three meals — plus snacks — throughout the day is now the norm, “it isn’t clear whether that is optimal in physiological terms,” Merad says.
The positive health outcomes occur even when people don’t lose weight, which has an independent effect on health metrics, or change what they eat. A 2018 pilot study from the Pennington Biomedical Research Center at Louisiana State University found that intermittent fasting improved blood sugar control, blood pressure and oxidative stress even when people didn’t change what they consumed. Adds Taub: “We saw benefits in our participants that cannot be explained by decrease in caloric intake alone. For instance, in prior studies when participants lose 3% of their body weight, they get a 3% to 5% reduction in LDL, or bad cholesterol. In our study, with 3% reduction of body weight, we saw an 11% reduction in LDL.”
[See: The 10 Best Diets for Healthy Eating.]
Impact on Inflammation
And other benefits are emerging. Merad’s research has shown that intermittent fasting reduces chronic inflammation, which is behind all sorts of negative consequences — from poor cardiovascular health to cancer and immune disorders. “Fasting had a huge impact, much more than we anticipated,” she says. “We saw that there was a profound reduction of inflammatory cells circulating in the blood from fasting 16 to 18 hours a day.” Some current studies are even testing fasting’s effect on autoimmune disorders in which inflammation plays a role. In mice bred to have multiple sclerosis, for instance, intermittent fasting significantly reduced the number of disease flares, Merad says. Human trials are underway.
Other investigators are focused on brain health. Trials have shown that intermittent fasting is effective in preserving learning and memory in animals, and the National Institute on Aging is now testing the idea in people who are obese and insulin resistant. Results are due soon, says Mattson, who helped design the study in his previous role.
No wonder that many physicians are now recommending intermittent fasting. “I use it in the clinic with a lot of my patients and many do extremely well with it,” says Arad, though he admits it’s not for everyone. If you decide to give it a try, he advises, consult with your health team or perhaps a nutritionist or an endocrinologist — professionals who “understand how your body works, understand metabolism, and understand how your hormones are affecting your body.”
Then pick a protocol. Longer fasting periods and more frequent periods promote faster weight loss. But, says Mattson, “If the goal is to make long-term lifestyle changes in your eating pattern, then what seems to be the easiest and most effective for a lot of people is daily time-restricted eating. It’s pretty easy and people can gradually work on it to reach their goal.” Taub’s research suggests that a 14-hour fast may be the “sweet spot,” in terms of both the benefit and the best chance for long-term adherence.
Don’t try intermittent fasting if you are on insulin, other diabetes medications, or blood pressure medications unless you are being monitored by a doctor. Likewise, it may not be appropriate for people with a history of eating disorders, and pregnant or breastfeeding women.
[See: 15 Best Weight-Loss Diets at a Glance.]
Finally, give it time. Mattson says it takes several weeks for people to adapt to the new eating pattern so that they’re no longer hungry during fasting periods. And despite other research suggesting that eating early in the day is better for weight loss and other health indicators, he adds, the evidence in support of intermittent fasting at any time of the day is much more convincing.
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