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Harvard Commentaries
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Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School


Medical Myths Medical Myths
 

Does Nighttime Noshing Make You Fat?


February 27, 2013

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center


Last reviewed and revised February 27, 2013

Perhaps you've heard this advice before: Don't eat before going to bed.

People who are trying to lose weight hear this warning all the time. And it's often recommended even for people who aren't trying to lose weight.

The logic behind this advice sounds reasonable: If you eat and then go to sleep, your body will convert the food you ate into fat rather than using it right away as fuel. Ultimately, you'll gain weight.

But is this true? That is, assuming that total calories consumed and physical activity are otherwise similar, do people who eat at night gain more weight than those who avoid eating late? Or is this a myth?

Does the Timing of Meals Matter?

I was taught in medical school that the human digestive system was remarkably efficient at absorbing nutrients, perhaps in response to evolutionary pressure. The idea was that our evolutionary ancestors were hunters and gatherers who had to adapt to long periods of time between meals. As a result, they evolved to waste very little of any meal they were fortunate enough to find. Physiological studies support the notion that what humans consume is quickly broken down into its basic components (including carbohydrate, protein and fat) and used or stored as needed.

I've found no evidence that the way we digest food or store it varies based on the time of day or how close feeding time is to bedtime.

Considerable research has evaluated the timing of meals. Most of this research has focused on the effects of skipping breakfast, not eating at night. The results suggest that your mother was right (at least about this): Skipping breakfast is not good. Mental functioning (as measured by tests of memory and grades) seems better among school-age children who eat breakfast; little research on adults exists.

Research also has demonstrated that skipping breakfast is associated with seemingly unrelated behaviors that may have a negative impact on health, including smoking, lack of exercise and snacking on foods with poor nutrient value.

For example, a 2003 study from the University of Massachusetts found that people who skipped breakfast regularly had a higher incidence of obesity than those who ate breakfast regularly. But the type of breakfast may also matter: A separate study noted higher body weight among adults who skipped breakfast or ate meat or eggs compared with those who ate cereal or breads for breakfast.

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Mealtimes and Weight

Researchers have analyzed the effects of "nibbling" (frequent small meals) versus "gorging" (fewer, large meals), a regular versus irregular meal pattern, and skipping versus eating breakfast among people who are lean, obese or neither. The results have been mixed.

For example, the University of Massachusetts study that linked obesity with skipping breakfast also found that people who ate frequent small meals were less likely to be obese. Yet other research suggests frequent snacking does contribute to obesity — clearly, it matters how much and what you eat — but the timing may be less important.

Other recent research suggests that irregular intake of meals may be associated with insulin resistance (a feature of type 2 diabetes) and higher cholesterol. And irregular meal intake may reduce the "thermic effect" of food — that's the number of calories burned by the act of eating, processing, digesting and storing nutrients. It seems that by eating at irregular intervals you may burn fewer calories than if your intake is more regular.

But where does that leave "night eating"? I have been unable to find any study that specifically asked and answered this question: When total calories are kept constant, does eating at night (whether just before bed or in the middle of the night) lead to weight gain? But here's what I did find:

    • A 1997 review article summarizing the results of past research concluded that meal frequency — whether during the day or at night — had no clear effect on body weight. That is, people who chose to eat frequently, regardless of the time of day, had no higher rate of obesity than those who ate fewer large meals.
    • In a study of more than 2,000 middle-aged men and women, 9% of women and 7.4% of men described "getting up at night to eat." Overall, there was no difference in weight gain between those who did and those who did not.
    • A study in 2004 of more than 800 men and women noted that the more people ate in the morning, the less they tended to eat during the course of a day, while people who ate more at night tended to eat more overall.
    • A trial from January 2005 found that eating late at night was not associated with weight gain even among people with "night eating syndrome" (see below).

Most of these studies relied on diet diaries or recall and might not be accurate reflections of actual eating habits. And while they are interesting, none of these specifically addresses the question about whether it's a good or bad idea to eat at night. A 2009 study in mice found that the time of day did matter: Mice who ate during the day (rather than their usual nighttime dining) gained more weight. However, results in mice do not always predict results in humans.

In fact, I could find no compelling evidence that eating late at night or just before bed matters one way or the other. It is likely that total intake over a 24-hour period balanced against calories burned through one's daily activities matters much more than what time a snack or meal is consumed.

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Other Health Effects

There are reasons other than concerns about weight to be careful when you eat. For people prone to heartburn (also called gastroesophageal reflux disease, or GERD), eating just before bed may cause trouble. When you lie down to sleep, the connection between the esophagus and stomach becomes horizontal instead of vertical. People with GERD often have more symptoms if they lie down after eating, whether it's bedtime or not. That's why people with GERD are routinely advised to raise the head of the bed (to maintain some element of gravity to keep acid in the stomach where it belongs) and not to eat shortly before going to bed.

I also have read reports of poor-quality sleep and nightmares as a consequence of eating before bed, but these are not well supported by evidence either. Perhaps the best advice in this regard is to rely on your experience. If eating at night seems to make it harder to sleep or has been associated with bad dreams, it's probably a good idea to avoid that late night snack. And if spicy or fatty foods seem particularly troublesome, avoid those.

Caffeine and alcohol deserve special mention. People who are sensitive to the stimulant effects of caffeine should avoid intake one to three hours before bed (or even longer if experiencing insomnia). While alcohol is a sedative that may promote sleep, alcohol-induced sleep commonly leads to awakening just a few hours later and difficulty getting back to sleep.

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Eating at Night: A Real Disease?

There is one type of nighttime eating that some experts consider an actual disease, night eating syndrome (NES). Its definition is still evolving, but it generally describes significant food intake that interrupts sleep.

One definition requires that a significant portion of the daily calorie intake occur after dinner, and that sleep is disrupted. An inability to return to sleep without eating is another feature of this condition.

There is no clear consensus regarding how common it is, its cause and how (or if) it should be treated. Preliminary studies suggest that antidepressants in the category of fluoxetine (Prozac) may help, but rigorous scientific trials have not yet been published.

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The Bottom Line

It is possible that, for some people, eating at night is associated with weight gain. Perhaps they find it easier to be careful about portion size and food choices during the day but simply "lose it" at day's end. For some, the structure of three meals a day may make it easier to avoid excessive calorie intake.

But it's probably a myth that eating before bed has a unique ability to promote weight gain compared with eating at other times of the day. Although scientific studies someday may prove that calories ingested before bed are handled differently than calories ingested at other times, evidence for this commonly held belief is lacking. For now, it's safe to assume that one's weight reflects the balance between calories burned and calories consumed over time, regardless of when you choose to eat.

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Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

Kathy McManus, M.S., R.D., L.D.N. is the director of the department of nutrition at Brigham and Women's Hospital and director of nutrition and behavioral modification program for the Program for Weight Management at Brigham and Women's Hospital.

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