New research suggests there may be an ideal window of time to eat during the day.
Eating relatively early may be beneficial for weight loss, and keeping meals within a 10-hour period could improve blood sugar and cholesterol levels, according to two small studies published Tuesday in the journal Cell Metabolism.
The first study found that eating on a later schedule made people hungrier over a 24-hour period than when they consumed the same meals earlier in the day. Late eating also led the study participants to burn calories at a slower rate, and their fat tissue seemed to store more calories on a later eating schedule than an early one. Overall, the study suggests that eating later can increase a person's obesity risk.
The second study, done among a group of firefighters, found that consuming meals within a 10-hour window shrunk "bad cholesterol" particles — suggesting a potential reduction in risk factors for heart disease. That eating window also improved blood pressure and blood sugar levels among firefighters with underlying health conditions such as diabetes, high blood pressure and high cholesterol.
The two studies add to existing evidence that there may be optimal times to start and stop eating, according to Courtney Peterson, an associate professor of nutrition sciences at the University of Alabama at Birmingham who wasn’t involved in either study.
"You have this internal biological clock that makes you better at doing different things at different times of the day. It seems like the best time for your metabolism in most people is the mid- to late morning," Peterson said
Satchidananda Panda, a co-author of the firefighter study and professor at the Salk Institute, said a 10-hour window seems to be a "sweet spot" because the more severe restriction that characterizes many intermittent fasting diets is hard to maintain.
"When we think about six or eight hours, you might see a benefit, but people might not stick to it for a long time," Panda said.
Late eating could 'tip the scale' toward weight gain
The first of the two new studies involved 16 people who were overweight or obese. They tried two different eating regimens for one day each. First, some of the participants started eating an hour after their natural wake-up time, while the rest waited to start eating until about five hours after waking up. Then the two groups switched schedules on a later date.
The meals they all consumed were identical and the quantity of calories and nutrients was consistent across both schedules, according to Frank Scheer, the study’s senior author and director of the Medical Chronobiology Program at Brigham and Women’s Hospital.
The researchers measured participants' hormone levels and found that late eating decreased levels of leptin — a hormone that helps people feel full — by 16% on average. Late eating also doubled the odds that people felt hungry (people self-reported their appetite level at 18 times throughout the day).
Furthermore, the researchers found that late eaters had an increased desire for starchy and salty foods, as well as meat, dairy and vegetables. Scheer said that might be because people crave more energy-dense foods when they're hungrier.
The study also found consistent changes in fat tissue associated with the late-eating regimen, suggesting an increased likelihood of building up new fat cells and a decreased chance of burning fat.
Finally, the results showed that late eaters burned about 60 fewer calories than early eaters per day, though Peterson said that was "equivalent to eating an extra half apple a day, so it’s not that big of a change.”
Although a study published last month in the same journal found that people did not burn more calories by eating a big breakfast and light dinner, Peterson said the two studies measured a different set of outcomes.
"Your body processes calories differently when you eat late in the day. It tips the scale in favor of weight gain and fat gain," Peterson said, adding, "from this study, we can get pretty clear recommendations that people shouldn’t skip breakfast."
But Scheer said more research is needed before he's comfortable making any recommendations.
A 10-hour eating window could reduce risk factors for heart disease
In the second study, 137 firefighters in San Diego, California, followed a Mediterranean diet rich in fruit, vegetables, fish and olive oil for 12 weeks. Seventy firefighters ate their meals within a 10-hour window, while the rest generally ate over 13 hours.
The firefighters logged their meals in an app and wore wearable devices to help researchers track their blood sugar levels. Most participants in the 10-hour group ate between the hours of 8 or 9 a.m. and 6 or 7 p.m. (though they occasionally strayed outside the window, extending to an 11- or 12-hour period).
Among healthy firefighters, time-restricted eating showed "favorable effects that should translate into less built-up plaque in the arteries and less cardiovascular disease," Peterson said. The firefighters in that group also reported an improved quality of life.
Among firefighters with pre-existing risk factors for heart disease, time-restricted eating decreased blood pressure and blood sugar levels.
"There have been lots of hints that time-restricted eating improves blood sugar control and blood pressure, but this is the first study to really test this in a large scale in people who do shift work," Peterson said.
Panda said past research in animals has shown that during periods of fasting, "organs get some rest from digesting food so they can divert their energy towards repairing cells."
A fasting period also seems to allow for the break down of built-up toxins, Panda said. And Peterson added that during fasts, the body can get rid of sodium, which in turn lowers blood pressure.
She said she wouldn't be surprised if we eventually see national recommendations about eating windows or meal times in the next five to 10 years in the U.S.