Midnight Snack Won't Pack Fat

Activity, Not Meal Timing, Key to Weight Control

Reviewed by Louise Chang, MD
From the WebMD Archives

Feb. 2, 2006 - A midnight snack won't make you fatter than a midmorning munch, monkey studies show.

It's a widely accepted truism that food eaten after dark is more fattening than the same food eaten in the light of day. And there's a reason to believe this: The body does slow down at night.

Researchers have tried to look at the issue, but it's been hard to do a definitive study in humans. That's why Judy L. Cameron, PhD, senior scientist at the Oregon National Primate Research Center in Beaverton, looked at 20 female monkeys instead.

To mimic human menopause, Cameron's team removed the monkeys' ovaries. To mimic human junk food, the researchers fed the monkeys a high-fat diet. And because some of the monkeys ate about two-thirds of their calories at night, Cameron and colleagues were able to look at the effects of nighttime meals.

"The outcome was, there was no difference at all," Cameron tells WebMD. "Whether they eat by day or by night, monkeys have an equal probability of gaining weight. So weight gain depends on how many calories we eat, and not when we eat them."

When not doing monkey research, Cameron is professor of physiology and pharmacology, obstetrics and gynecology, and behavioral neuroscience at Oregon Health & Science University in Portland. She and her colleagues report their findings in the current issue of Obesity Research.

Weight Gain Knows No Clock

None of this surprises Leslie Bonci, MPH, RD, director of sports nutrition at the University of Pittsburgh Medical Center and nutritional consultant to the Pittsburgh Steelers football team.

"People just assume that after 6 p.m., calories that will become fat go straight to your butt," Bonci tells WebMD. "But the body is clueless about what time it is when you eat too much. It knows how to store calories every hour of the day."

For most of us, this is good news.

"We don't have those lifestyles where we go to bed when the sun goes down," Bonci says. "If it were true that nighttime eating made you fat, everyone in Spain would be obese -- because they don't eat dinner until 10 p.m."

The myth that midnight snacks cause fat can be harmful, Cameron says, if your weight loss plan is simply to stop eating when the sun stops shining.

"The story one wants people to take home is you should watch your calorie intake," she says. "Whenever you take in calories, they are going to matter. It is one thing not to eat at night. But if you want to lose weight, don't eat so much during the day. Just not eating at night is not an effective strategy."

The Key to Weight Control

Even though they ate a high-fat diet -- nearly quadrupling their calories -- not all of Cameron's monkeys gained weight. A few got fat. Some stayed lean.

What happened?

"It was mostly due to activity level," Cameron says. "The strongest predictor of weight gain in adult monkeys was how active they were. Very active animals did not gain weight, and very sedentary animals gained quite a bit."

But these are just monkeys -- aren't they? Actually, Cameron says that monkeys are a lot like humans in terms of how they eat, sleep, and exercise.

"We think monkeys are a very, very good model for humans," she says. "As far as we can tell, their mechanisms of weight gain are the same as humans."

A real problem with obesity research is that it's almost impossible to know for sure how much human study participants actually eat and exercise. That's not a problem in monkey studies. Cameron says we can look for more information to come from her monkeys -- including studies on the impact of menopause and hormone replacement on weight.

Show Sources

SOURCES: Sullivan, E.L. Obesity Research, December 2005; vol 13: pp 2072-2080. Judy L. Cameron, PhD, senior scientist, Oregon National Primate Research Center, Beaverton; professor of physiology and pharmacology, obstetrics and gynecology, and behavioral neuroscience, Oregon Health & Science University, Portland. Leslie Bonci, MPH, RD, director of sports nutrition, University of Pittsburgh Medical Center; nutritional consultant to the Pittsburgh Steelers and the Pittsburgh Ballet Theatre company.

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