Study: People Who Eat After 8 p.m. Have Higher BMIs
By Brenda Goodman
WebMD Health News
Reviewed by Louise Chang, MD
June 14, 2011 (Minneapolis) -- Checking a clock may be as helpful as counting calories when it comes to controlling body weight, a new study suggests.
The study shows that people who snack after 8 p.m. have higher body mass indexes (BMIs) than people who don't nosh at night, even though they don't eat significantly more total daily calories.
Previous studies in animals have found that even when calories are held steady, the timing of meals and sleep and exposure to light can impact metabolism and BMI.
The new study is one of the first to explore those relationships in humans.
Tracking Mealtimes, Sleep, and BMI
Researchers at Northwestern University in Chicago recruited 52 young and middle-aged adults.
Study participants wore sensors around their wrist that recorded movement and sleep times throughout the day for seven days.
They kept food diaries to track what, when, and how much they were eating.
Night owls tended to be late sleepers, with a midpoint of sleep that was after 5:30 a.m.
Late sleepers typically logged less sleep than normal sleepers. They also started their days later, a pattern that pushed back mealtimes throughout the day.
Additionally, they had higher BMIs than normal sleepers, ate more calories after 8 p.m., and ate fewer fruits and vegetables.
Researchers took into account factors that are known to increase the risk of weight gain, like age, sleep duration, and sleep timing.
"After we adjusted for all of those things, the one major thing that remained positive, that remained correlated, was eating after 8 p.m.," says study researcher Phyllis Zee, MD, associate director of the Center for Sleep and Circadian Biology and a professor at Northwestern University's Institute for Neuroscience in Chicago.
"Although those late sleepers were eating more calories, the number of calories between the normal sleepers and the late sleepers was not significantly different," Zee tells WebMD.
The study was presented at the 2011 annual meeting of the American Academy of Sleep Medicine and the Sleep Research Society in Minneapolis.
Advice for Dieters
"Many of our patients struggle with night eating," says Elisabetta Politi, RD, nutrition director at the Duke Diet and Fitness Center in Durham, N.C.
People who have trouble sleeping go for longer stretches of time between their dinnertime and their bedtime than people who sleep well. That often means they get hungry late at night. When they eat late, they're not as hungry for breakfast in the morning.
"It creates a vicious cycle that I really feel promotes weight gain," says Politi, who was not involved in the research.
Still, she says, there are many unanswered questions about why late eating may lead to weight gain.
In some European countries, for example, it is customary to eat dinner at later hours, which doesn't seem to contribute to higher rates of obesity in those countries.
Still, she says, for people trying to lose weight, it probably wouldn't hurt to curb nighttime eating. Though she says it needn't be a rule that is militantly observed.
"It makes perfect sense to eat more when you are more physically active. You burn off the calories you eat," Politi says, "But at the same time, we don't want people to feel that if they eat something healthy at 10 p.m., it is going to lead to weight gain."
This study was presented at a medical conference. The findings should be considered preliminary because they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.