If you’re plumbing the causes of the great American weight gain over the past few decades, you could do worse than starting with the fact that food is available all the time, and very often we’re putting it in our mouths. According to a 2015 study that tracked eating patterns using smartphones, at least some of us are chowing down at almost every hour of the day (except in the wee hours of the morning). The study found the least frequent eaters had about 3.3 eating occasions per day, defined as food or drink consisting of at least five calories, while the most frequent eaters had about 10.5 such occasions a day.
As a scientific statement on meal timing from the American Heart Association noted earlier this year, “This study clearly demonstrated that adults in the United States eat around the clock.”
Nutrition research often looks at what people are eating. But researchers are also investigating whether when we eat has implications for weight or other health markers.1 Guidance on meal timing has made its way into dietary advice, from continued admonishments to eat breakfast that you probably heard in grade school to, more recently, popular interest in intermittent fasting.
The evidence on meal timing as a weight loss tool, especially as a sustainable, long-term approach, is preliminary and inconsistent. That said, there’s a scientific rationale behind it. Just as the brain has an internal clock that controls sleep and wakefulness, almost every organ has one, too, said Satchidananda Panda, a professor in the regulatory biology laboratory at the Salk Institute and a co-author of the study that showed we basically never stop eating. For example, there are better and worse times for complex tasks depending on your brain’s schedule. “Similarly, almost every organ has a peak performance time,” he said. And the liver and pancreas seem to be early birds, not night owls. So there might be metabolic benefits to consuming calories earlier in the day and giving the digestive system a longer stretch without food in the stomach, he said.
Drawing granular conclusions on such advice with any certainty is harder, given the existing research. “We haven’t seen enough trials in humans, especially of long duration,” said Carolina Guizar, a registered dietitian nutritionist in New York City. But here’s what we know so far. (And please, if you have any health conditions, talk to your doctor before you start messing around with meal timing or skipping; this is definitely not a good idea for people who take insulin for diabetes, who take certain other medications or who have a history of an eating disorder, among others.)
Let’s begin with the evidence on breakfast, which we all know starts any good day! Or maybe not. The authors of that American Heart Association statement noted that skipping breakfast is associated with habits such as high calorie intake and infrequent exercise. But after reviewing the published research, the authors said that from the limited available evidence, using breakfast as a lever for losing weight appears to have “a minimal impact,” though it could improve other measures of health, such as insulin sensitivity. “There are some really well-controlled studies showing that people should do what works for them. Some people get nauseous if they don’t eat breakfast, and others feel sick if they do,” said Krista Varady, one of the authors of the statement and a kinesiology and nutrition professor at the University of Illinois, Chicago. If you’re in the breakfast-makes-me-boot club, no need to force yourself to change in an attempt to lose weight.
There is some evidence that avoiding late meals, however, can improve health and possibly help in losing weight. A study led by neuroscientist Frank Scheer of Brigham and Women’s Hospital and Marta Garaulet of the University of Murcia in Spain, for example, showed that eating the main meal of the day earlier rather than later produced about 4 pounds more weight loss and lowered diabetes risk for obese and overweight people. The authors of the American Heart Association review said that while this idea deserves further study, there are no firm conclusions yet.
Varady is particularly interested in intermittent fasting as a potential tool for weight loss. It comes in a few flavors (like the 5:2 diet espoused by Jimmy Kimmel), but Varady has studied the type called alternate-day fasting — alternate-day energy restriction, really, because dieters do consume some calories during their fasts. The method involves switching between days of feast, targeting 125 percent of your usual caloric intake, and days of fast, cutting back calories by 75 percent, leaving you with about 500 calories (if your usual diet is about 2,000).
In May, Varady and her colleagues published a randomized trial of this approach in 100 obese adults, with some doing intermittent fasting, some cutting daily calories by 25 percent and some in a control group with no intervention. After six months of dieting and another six of maintenance, the alternate-day fasting group lost about 6 percent of body weight, while the calorie restrictors lost about 5.3 percent relative to the control group. So, not a big difference. And more people dropped out of the fasting group (38 percent vs. 29 percent for the calorie restrictors). But a subset of the participants really liked and had success with the fasting structure, Varady said.
So there doesn’t appear to be anything magical about intermittent fasting. To lose weight, you have to, on average, take in fewer calories than you burn off, said Mark Mattson, chief of the Laboratory of Neurosciences at the National Institute on Aging. For some people, alternate-day fasting might be a more palatable way to do that. And it might also have other health benefits, such as lowering triglycerides and blood pressure, that accrue regardless of whether you’re losing weight, Mattson said.
Another way to derive some of those benefits could be confining your eating to a smaller window of the day — say, within an eight- to 12-hour stretch. Panda’s research in rodents has shown that so-called time-restricted feeding can ward off the typical effects of diets high in fat, sugar or both. And obese mice lost body weight when their eating was limited to an eight-hour window. There aren’t a lot of prospective controlled trials in humans, however, and the evidence that does exist shows mixed results, according to this 2014 review. And results might differ depending on diet and body composition, the authors said. But behaviorally, narrowing your food window could reduce how much you take in without requiring you to count calories — which, as Panda pointed out to me, we are generally bad at. Once again, no one is saying this is a magic train to weight loss, but it could be effective for some. (If you want to participate in this line of research, check out this website.)
Diets are often framed as something that will work for everyone.2 But given how much our tastes and diet preferences vary — with vegans and Whole 30-ers and intermittent fasters and pescatarians all seemingly happy and healthy among us — the real question is whether an approach works not for everyone, but for you. And until there’s more science behind these questions of timing, the easiest way to find out is to go ahead and try, if it’s appealing.3 Does it work to meet your weight or other health goals? Great! If not, try alternatives until you find something that does and that you can keep up for the long haul.