When you eat — early or late in the day and over the course of how many hours — may have a big impact on your metabolism, your cardiovascular health and how many calories you store as fat, two new studies find.
The studies, published in Cell Metabolism, suggest that eating late in the day may slow metabolism and promote fat storage, while constraining food consumption to fit into a 10-hour window may lead to improvements in blood pressure and cholesterol in people with heart disease risk factors.
In one study, researchers closely monitored 16 overweight or obese volunteers in a lab setting for four days of eating three meals early — 9 a.m., 1 p.m. and 6:20 p.m. — and then four days with the three meals shifted to four hours later — 1 p.m., 5 p.m. and 10:20 p.m. The calories consumed were the same for the early and late meals.
Half the volunteers were randomly assigned to the early eating regimen first and the other half to the late eating regimen. After a multi-week break, the groups were switched.
Each day the volunteers were in the lab, they documented their levels of hunger and food cravings. Environmental factors, such as lighting, room temperature and humidity, were tightly controlled by the researchers.
Meanwhile, the researchers collected hourly data on hunger hormones, body temperature and energy expenditure. They also collected information on whether the volunteers’ bodies were leaning toward storing calories consumed as fat.
The data suggested that compared to early eating, late food consumption left people hungrier, while also increasing levels of ghrelin, the hunger hormone, and decreasing levels of leptin, the satiety hormone. Moreover, eating late left people with lower body temperatures and slower metabolisms, which resulted in fewer burned calories. There were also signs that late eating could increase fat storage.
The new findings suggest that eating late “would promote weight gain,” said study co-author Frank Scheer, Ph.D., director of the medical chronobiology program at the Brigham and Women’s Hospital and a professor of medicine at the Harvard Medical School.
“We have one of the most controlled and comprehensive studies to look at this,” Scheer said. “So we were able to isolate the influence of meal timing.”
Basically, by eating late in the day, you’re fighting your biological clock, Scheer said.
The other study explored the impact of a 12-week program of time-restricted eating — meaning people are allowed to consume food for a set number of hours during the day — in 137 San Diego firefighters who worked in shifts of 24 hours.
While all 137 were asked to maintain a Mediterranean-type diet that was rich in fruits, vegetables, whole grains and healthy fats, 70 were asked to limit their food consumption to a 10-hour time window.
The firefighters agreed to wear devices that kept track of activity, sleep and blood glucose levels and to log when foods were consumed and hours spent sleeping. Everyone in the group that practiced time-restricted eating was allowed to choose the timing of the window during which they would consume food; most started eating between 8 and 9 a.m. and finished between 6 and 7 p.m.
Time-restricted eating plans often have shorter windows than 10 hours, but the researchers used a 10-hour window “because it’s easier to implement than eight hours, especially since this is our first time looking at firefighters,” said study co-author Dr. Pam Taub, director of the Step Family Foundation Cardiovascular Rehabilitation and Wellness Center and a professor of medicine at the University of California, San Diego.
Overall the two groups were fairly similar, except that the size of LDL (bad cholesterol) particles was larger in those who did not engage in time-restricted eating. While there hasn’t been a lot of research on cholesterol particle size, there is some evidence that larger sizes may be implicated in heart disease.
The biggest difference was seen in firefighters who had risk factors for cardiovascular disease, such as high blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol levels, at the outset. Among these participants the time-restricted regimen led to improvements in blood pressure and blood sugar levels.
“The bigger concept is that we don’t need to be eating all the time,” Taub said, adding that the human body needs to take a break from dealing with the food we consume. So, with the restricted eating schedule, the firefighters’ metabolisms got a “14 hour metabolic rest,” Taub added.
During that rest time, cells get to recharge and tidy up, Taub said. “It’s kind of a cellular cleaning and repair process,” she explained.
The two new studies follow on the heels of another study published online in September, which looked at the timing of the biggest meal of the day among people consuming a diet designed to help them lose weight. The study found it didn’t make any difference whether it was breakfast, lunch or dinner.
While that might seem to be a bit contradictory to the other papers, that one, like the lab study, did find that some people who ate their big meal at the end of the day were hungrier early on. In the end, both groups lost about the same amount of weight over the 12-week period.
The biggest message people should take from these studies is that eating your last meal close to bedtime isn’t a good idea, said Marie-Pierre St-Onge, Ph.D., an associate professor of nutritional medicine at the Columbia University Irving Medical Center in New York. “This means you should really end eating earlier in the day as opposed to later in the day,” she added.
“We have a paper coming out next week that looks at the timing of eating, which showed eating close to bedtime results in lower fat oxidation,” St-Onge said. In other words, people who ate late burned less fat, and over the long term, these late eaters would have a tendency to accumulate more body fat than muscle.
Ultimately, it’s still best to personalize your eating plan and to listen to your body, said Dr. Megan Jenkins, a specialist in weight loss surgery at NYU Langone Health in New York City.
So, while eating breakfast early in the day may quell appetites in the morning hours for many, some people don’t get hungry until later in the day, Jenkins said, adding that she's one of those people. Eating early if you’re not hungry could result in more hunger later on, she added.
Still, if you eat your first meal closer to the typical lunchtime, you shouldn’t shift your entire eating schedule later, Jenkins said. “I think most would advise people not to eat within three hours of bedtime,” she added.