Most diet advice focuses on calories and nutrients, but new research suggests that when you eat may be just as important.
That’s one conclusion of a new study by Dr. Satchidananda Panda, an associate professor at the Salk Institute for Biological Studies, in La Jolla, Calif. Mice that were forced to limit how many hours they ate were thinner than mice that chowed down whenever they wanted, Dr. Panda’s team found. This was true no matter what kind of unhealthy diet the mice ate.
The finding suggests scheduling may play a critical role in losing weight, according to Dr. Panda. Mice on the “time-restricted feeding” diet also had better muscle mass and lower cholesterol, even though they were allowed to “cheat” and eat normally on the weekends, Dr. Panda added.
Such findings support the rising popularity of fasting diets. The 2012 best seller “The 8-Hour Diet” was based in part on Dr. Panda’s work, while the so-called 5:2 Diet—eat how you like for five days, fast for two—draws from similar research.
“I started researching the book in 2011 because of the work done on the benefits of intermittent fasting,” says David Zinczenko, co-author of the “The 8-Hour Diet.” He says he decided to create a time-restricted plan because he found intermittent fasting, which involves fasting for days at a time, to be unrealistic. For the book, Mr. Zinczenko put about 150 volunteers on the diet. The majority self-reported that they lost weight.
Christopher Ochner, a director of research development at Mount Sinai Hospital who focuses on obesity, warns that studies of time-restricted feeding may be the latest in a long line of mice studies that don’t translate to people. He says the old idea of “calories in, calories out” matters more for health than how many hours someone eats, or what time of day they are eating.
If time-restricted feeding worked in humans, people who skip breakfast and then eat a late dinner would be much thinner than everyone else, Dr. Ochner says. Observation shows this isn’t true, and study results are “very mixed” regarding whether skipping breakfast causes weight loss, Dr. Ochner says.
In the latest paper Dr. Panda worked on, published in December in the journal Cell Metabolism, his team put obese mice on a variety of unhealthy eating regimens, including high-fat and high-fructose diets. One set of mice ate at all times, while another set ate only during periods of nine, 10, 12 or 15 hours. Both sets were given the same unhealthy food options, and both sets ate all the food available, according to the research.
Because the restricted mice could still eat as frequently as they wanted within the given timeframe, none of the research tracked the effects of large meals versus smaller, or of frequent snacking.
The benefits of restricted eating times were proportional to the amount of time fasted, said Amandine Chaix, a Salk researcher who works with Dr. Panda. The narrower the window for eating, the more weight the mice lost.
The researchers think this is partly because the restricted schedule aligns with the body’s circadian rhythm, or internal clock. Eating happens at times when the body is more efficient at breaking down foods.
“The metabolic system evolved to be able to allocate energy resources at different times of the day,” said Dr. Chaix. Cholesterol is broken down in the body by bile acids, which are controlled by an enzyme that is most present upon wakening. The mice started eating soon after they got up, which helped their bodies break down cholesterol more efficiently. In addition, when mice eat randomly, their livers cannot switch off the glucose production process. This leads to high blood sugar levels, which damages the body.
For humans, eating during the day may be beneficial because it’s been the pattern for millions of years, says Dr. Panda. The rise of eating later at night might have thrown off the circadian rhythms that evolved in humans, and might have contributed to the rise of obesity, he added.
In a previous study, his team found that mice who tended to eat high-fat diets at off times, spreading their calorie intake equally between day and night, were more likely to grow obese. But when they were forced to eat when they were most awake and active, this effect disappeared.
A second benefit comes from fasting itself, says Mark Mattson, chief of the Cellular and Molecular Neurosciences Section at the National Institutes of Health. After fasting, the body starts to use fat, instead of glucose, as a source of energy. This leads to faster weight loss, among other benefits, he explained.
Dr. Mattson believes that the three-meal-a-day diet is abnormal from an evolutionary perspective and that periodically going without food strengthens the body. His studies have found that mice forced to fast are thinner and have lower levels of insulin and glucose, meaning they are at lower risk for developing diabetes.
“We think that one of the reasons fasting is good for the body and brain is that it imposes a mild stress on cells,” he says. “The cells then adapt to improve an ability to cope with more severe stress and we think to resist diseases and aging.”
Another of his studies on mice suggests that fasting protects against stroke and slows cognitive decline. He is now starting a study in humans, focusing on whether fasting protects the brain from Alzheimer’s disease.
Krista Varady, an associate professor of nutrition at the University of Illinois in Chicago and author of “The Every-Other-Day Diet,” has found that people fasting every other day can lose up to 30 pounds in eight weeks, which is far more than those on a more typical calorie-restriction diet.
Write to Angela Chen at angela.chen@wsj.com
