Does it matter what time you eat when it comes to weight control? We used to think it didn’t matter, as long as you don’t consume more calories than you burn. That is, a calorie is a calorie, whatever time of day it is. But a recent study in the International Journal of Obesity lends support to the idea that eating earlier in the day is better, at least if you are trying to lose weight.
Researchers sorted 420 overweight and obese participants in weight-loss programs in Spain into two groups, based on their normal lunchtime tendencies: “early” eaters, who ate lunch before 3 p.m., and “late” eaters, who ate lunch after 3 p.m. (We put early and late in quotes, since Spaniards tend to eat later than Americans.) In both groups, lunch was the biggest meal of the day, providing about 40 percent of daily calories.
Unexpectedly (at least to us), the early eaters lost a little more weight than the late eaters over the 20 weeks of the study. And they lost it at a faster rate. That was despite the fact that the two groups did not differ in total calorie intake, diet composition, energy expenditure, amount of sleep or levels of appetite hormones. So perhaps not all calories are equal, after all. According to the researchers, “the timing of the main meal by itself seems to be the most determinant factor in weight-loss effectiveness, and therefore eating at the right time may be a relevant factor to consider in weight-loss therapies.”
Research: meal timing and more
In recent years, several review papers have focused on this “chronobiological” aspect of body weight. In a nutshell, the brain’s hypothalamus controls the body’s overall circadian rhythms (or “master clock”), which influence biological functions, notably the sleep-wake cycle. But scientists have discovered through lab, animal and human studies that other organs, including those of the digestive system, have their own biological clocks, which interact with the master clock.
It’s theorized that eating at an odd time (an “incorrect circadian time,” as one researcher put it) disrupts these gut clocks and that this can lead to weight gain, perhaps because of circadian variations in levels of insulin and other hormones involved in blood sugar control and appetite. In addition, so-called clock genes in adipose tissue may affect how much fat is absorbed and metabolized (burned), while clock genes in the liver may influence carbohydrate metabolism. Disturbances in clock genes have, in fact, been linked to the metabolic syndrome in people, which is characterized by obesity and other cardiovascular risk factors. Chronobiology may also help explain why shift workers, who have disrupted circadian rhythms, tend to gain weight.
Meal timing: time will tell
Though the Spanish study is intriguing, it is an observational study and so does not establish cause and effect. Much of the other chronobiology research has been done in mice, not people. There’s still much to learn about whether meal timing plays a role in weight gain, independent of calorie intake and physical activity. More study is also needed to see if meal frequency and regularity make a difference—and how much genetics plays a role. It’s very possible that overweight people have variations in clock genes that make them inclined to skip or eat little for breakfast and lunch and then crave a lot of food later in the day.
Bottom line: Many factors are involved in weight control, including physiological, environmental, psychological and societal ones. If you’ve struggled to lose weight, the timing of food intake may be another thing to consider, even if all the answers are not yet in. There’s no harm in eating an earlier lunch to see if that makes a difference. Or you might want to try eating the bulk of your calories earlier in the day and a lighter dinner. The same is true if you have metabolic risk factors, such as high blood sugar or cholesterol.
One thing is clear: Whatever time you eat your meals, watch out for late-night, high-calorie snacks, which can sabotage weight loss efforts.