intermittent fasting eating clock
© Illustration: Gwenda Kaczor
New research is showing the profound benefits — for weight, longevity and fighting disease — of eating only during limited hours

Fasting is one of the biggest weight-loss trends to arise in recent years. Endorsed by A-list celebrities and the subject of a spate of best-selling books, it was the eighth most-Googled diet in America in 2018.

But fasting shouldn't be dismissed as just another fad. At the Charité University Hospital in Berlin, I've employed what's called intermittent fasting, or time-restricted eating, to help patients with an array of chronic conditions. These include diabetes, high blood pressure, rheumatism and bowel diseases, as well as pain syndromes such as migraines and osteoarthritis.

There are different ways to go about it, but I advise patients to omit either dinner or breakfast, so that they don't ingest any food for at least 14 hours at a stretch. That makes lunch the most important meal of the day. It also reduces the time spent each day processing food and lengthens the period devoted to cleansing and restoring the body's cells, both of which have positive health effects.

Adopting this technique is not as difficult as it may seem. If you sleep from 11 p.m. to 7 a.m., you've already fasted for eight hours. Now you only need another six. It's healthy to avoid eating late in the evening to let your body burn energy from food rather than store it, so if you eat dinner by 7 p.m., that's another four hours. For breakfast, you can limit yourself to coffee or tea (maybe with a small piece of fruit) and make lunch your first proper meal. By that time, you're clearly beyond the 14 hours and don't need to restrain yourself: You can eat until you are full.

Comment: The inclusion of coffee and a piece of fruit outside the eating window is considered by most fasting gurus to be a no-no. While there is some evidence that beneficial effects can still be achieved even when having non-caloric things like coffee or tea, fruit would certainly knock one out of their fasting window. In general it is recommended to have nothing but water during the fast.

The biologist Satchidananda Panda at California's Salk Institute showed the possibilities of this approach in a 2012 report in the journal Cell Metabolism. He fed a group of mice a high-fat diet around the clock for 18 weeks; they developed fatty livers, pancreatic disease and diabetes. Another group was fed the exact same number of calories a day, but all during an eight-hour span. Surprisingly, the second group stayed slimmer and healthier for much longer.

There is a logic to it. When we eat, our body releases insulin. That disrupts the process of autophagy (from the Greek, meaning "self-devouring"), by which cells deconstruct old, damaged components in order to release energy and build new molecules. Autophagy helps to counteract the aging of cells and builds immunity. Fasts stimulate autophagy and allow the full molecular process to take place, as a team led by Frank Madeo at the University of Graz in Austria found in 2017.

Fasting also can contribute to brain health and happiness. The neurobiologist Mark Mattson, who retired this year from the National Institutes of Health, has demonstrated in experiments for two decades that nerve growth factors contribute significantly to brain health and positive mood. He also found that fasting, restricting calories and exercising spur distinct increases in the best-known nerve growth factor, BDNF.

Test animals in Dr. Mattson's laboratory that fasted intermittently even showed a significantly lower risk of developing Parkinson's disease, multiple sclerosis and Alzheimer's, though those results would have to be clearly confirmed in large human studies to reach any firm conclusion.

All of this presents a question: If we should generally eat only two meals a day, which meal is it best to skip? Many of us have heard the saying: "Eat breakfast like a king, lunch like a prince and supper like a pauper." Scientific evidence for the glory of breakfast is scarce, however, and realistically, it's easier to sustain skipping breakfast than skipping dinner.

Instead of breakfast, we should eat lunch like kings. A rich lunch beats a robust dinner. A U.K.-led study published in the American Journal of Clinical Nutrition in 2016 showed that among 69 women, those who consumed most of their calories at lunch shed 3.3 pounds more in 12 weeks than those who ate a bigger dinner. After all, it's around lunchtime that the body requires the greatest amount of energy for keeping its body temperature up. Less energy thus passes into our fat reserves.

Comment: Something to keep in mind is that eating dinner earlier in the evening has been found to improve sleep quality. There is also evidence that those whose eating window is earlier in the day see better improvements overall than those whose window is later. There may be more to this than simply restricting the number of hours during which one eats. Meal timing may be equally important, or at least a close second. See: Chrono-nutrition, circadian clocks and the importance of meal timing

Researchers are increasingly probing the optimal timing of meals, duration of fasting and the various potential health effects. Scientists at the University of Padua have found, for instance, that young, healthy athletes fasting for 16 hours benefited from metabolic changes over eight weeks compared with their peers. The regimen lowered the levels of inflammatory factors in their blood and factors accelerating the aging process, including insulin.

Fasting might even be effective in preventing the recurrence of cancer, as suggested by initial results of an epidemiological study conducted by researchers at the University of California at San Diego, published in 2016 in the journal JAMA Oncology. Among 2,400 women with early-stage breast cancer who provided information on their eating rhythm, roughly 400 suffered from new tumors within seven years. But women who fasted for 13 hours nightly had 26% less risk of recurrence than the control group. One possible reason was suggested in data summarized last year from a decade of animal experiments by Valter Longo and a team at the University of Southern California: Cancer cells are less able than normal cells to survive a lack of sugar.

As a practice, fasting is more than simply restricting calories or nutrients. For many people, it is also a spiritual experience. Over the course of our lives, we encounter many kinds of deficiency, whether of money, success or affection. Fasting is a conscious renunciation, a controlled exercise in deprivation. That's why successful fasting increases self-efficacy — we overcome an instinctive need in a way that gives us physical and mental strength. In his novel "Siddhartha," Hermann Hesse describes this wonderfully: "Nothing is performed by demons; there are no demons. Anyone can perform magic. Anyone can reach his goals if he can think, if he can wait, if he can fast."

Dr. Michalsen is a professor at Berlin's Charité University Medical Center. This essay is adapted from his new book, "The Nature Cure: A Doctor's Guide to the Science of Natural Medicine," which Viking will publish on Aug. 6.