Time to eat?
New research in mice suggests that restricting eating to a shorter period of the day might be a valuable adjunct to traditional diet and exercise recommendations, and might potentially confer metabolic benefits or allow weight loss even without reducing caloric intake, eating a rigid diet, or taking weight-loss medications. Several studies have established that healthy metabolism is tied closely to circadian rhythms, and that the body handles food and digestion most effectively during daytime hours. These findings may help explain observational findings that people who work night shifts or eat at night are at an increased risk for obesity and metabolic diseases such as type 2 diabetes. The observations also lead to the question: can people improve their health by eating only during specific hours of the day? Clinical research to test the effects of varying people’s mealtimes is currently lacking. However, previous research has shown that if mice are given food only during 8 hours of their active period (at night, because they are nocturnal animals), they are protected from obesity and diabetes, even if they are fed a high-fat diet that normally causes these conditions in mice with all-day access to the same food. In a new study, researchers sought to expand upon this result. For example, they sought to determine how severely feeding time had to be restricted to protect mice from the adverse metabolic effects of a high-fat diet. Working with male mice, they found through a number of experiments that mice with constant, continuous access to a high-fat diet gained significantly more weight than mice whose access was limited to 9 hours, to 12 hours, or even to 15 hours (i.e., 9 hours fasting). The shorter the eating period, the better the protection, though protection was substantial even in the 15-hour-fed group. Animals with access to the high-fat diet for 9 hours weighed no more than mice given a normal healthy diet. Interestingly, for any given diet, time-restricted feeding did not change the number of calories the mice consumed: those with 9-hour access to high fat food ate no less than those with the same diet available for 24 hours. This suggests that circadian coordination of metabolism is responsible for the effect—not a reduction in calories consumed, but simply timing that consumption to correspond to the animals’ natural active period. That is, a dieting and exercise program was not necessary to limit weight, at least in male mice. Indeed, even on a healthy diet, time-restricted feeding came with a surprise benefit: although the mice weighed the same as mice with 24-hour access to the healthy diet, they had less body fat and more lean muscle mass. Taking the experiment further, the investigators examined whether it was necessary to restrict food intake every day. When male mice fed a high-fat diet were restricted to 9-hour access for 5 days per week and then given 24-hour access on weekends, they still fared considerably better than mice given 24-hour access to the diet every day, doing about as well as mice given food for 12 hours a day, 7 days a week. Importantly, the mice with 5 days of time-restricted feeding not only weighed less than the mice with 24-hour food access, but also experienced significant metabolic benefits: better insulin sensitivity, lower blood glucose, fewer signs of inflammation, more healthy brown fat, less unhealthy white fat, and healthier blood and liver levels of fat and cholesterol.
In another series of experiments, mice that had grown obese through 24-hour access to the high-fat food were switched to time-restricted feeding. These animals lost weight, and then plateaued, while those still receiving food 24/7 continued to gain. The metabolic benefits seen after switching the obese mice to time-restricted feeding were significant. For example, their insulin levels fell and glycemic control improved, although they remained somewhat less metabolically healthy than mice that had been fed continuously in a time-restricted fashion. In contrast, when time-restricted feeders were switched to 24-hour food access, they gained substantial amounts of weight, although they retained some of the metabolic benefits they had previously earned in their period of time-restricted feeding. Further, the researchers found that the time-restricted feeding approach not only benefited animals on a high fat diet. The benefit of time-restricted feeding was also quite substantial in mice given diets that may be more representative of those contributing to obesity in humans: not quite as high in fat, but including sucrose (table sugar), for example, or a diet elevated in fructose (often found in processed foods). It is unclear to what extent this approach would be beneficial to people, whether there might be differences between women and men in metabolic benefits derived, and whether people will be more or less able to adapt to changing their mealtimes as opposed to changing their diets. However, if the effects are similar to those in mice, the discovery may be of profound importance: people who have difficulty restricting calories or curtailing fattening foods may find it more manageable to simply restrict the time when they can consume them.