Research Story Tip: Eating a Late Meal May Be Harmful To Your Metabolic Health, Particularly For Early Birds


People have anecdotally claimed that the timing of a meal may affect metabolism, but there isn’t much solid evidence in support. To see how timing of eating influences metabolism, Johns Hopkins Medicine researchers studied 20 healthy volunteers by giving them a meal at a traditional hour (6 p.m.) or a meal at a later time of the day (10 p.m.). They found that when people ate later, they had higher spikes in blood sugar, slower fat breakdown and even increases in the stress hormone cortisol, believed to be a factor in promoting weight gain. When they looked more closely at different responses to the late meal, they found that people who normally went to bed early — so-called "early birds" — experienced an even bigger impact from eating late.

In their findings published on June 11, 2020, in The Journal of Clinical Endocrinology & Metabolism, the researchers say eating late could promote obesity if these conditions happen often enough.

“What time you eat could be just as important as what you eat when it comes to metabolic health,” says Jonathan Jun, M.D., associate professor of medicine at the Johns Hopkins University School of Medicine. “When people eat identical meals at two different times, their bodies apparently process those calories differently. How an individual responds depends on their particular biorhythms and sleep behaviors.”

For the study, each participant was given a “routine-time” dinner at 6 p.m. or a late dinner at 10 p.m., and drank a beverage containing a non-radioactive tracer that allowed the researchers to follow fat burning in the body. All participants went to bed at 11 p.m. Each participant had an intravenous line, which allowed researchers to take blood samples during the day and overnight without interrupting or waking them.

Just after a late dinner, participants had on average an 18% higher blood sugar spike and a 5% increase in cortisol levels. Furthermore, the fat in the late meal burned slower, resulting in a 10% reduction in fat breakdown by the next morning.

The researchers then looked at how each participant’s body responded to the late meal based on their normal habits or daily rhythms. Among the “early birds,” who were accustomed to an 11 p.m. bedtime, blood sugar levels rose 30% higher while the fat in their meals burned 20% less by the next morning.

The people who typically went to bed between 2 a.m. and 3 a.m. — the so-called “night owls” — were barely affected by the late meal.

Because they only looked at what happens after eating one late meal, the researchers say they aren’t sure if continually eating late will eventually allow the body to adapt. They also say it’s unclear if this slower metabolism after a late meal is caused by how soon a person goes to sleep after they eat, or if it results from each person’s natural variations in metabolism over a 24-hour period (their circadian rhythm).

The researchers plan to investigate these concepts in future studies.