Will eating late at night make you fat?

TL;DR: No.

Despite awkwardness caused by his recent rants on the topics of modern cooking techniques and more recently fandom, Alton Brown is still one of my favorite sources of culinary quotes. One of which, related to nutrition, is

There are no "bad foods", only bad food habits.

In keeping with my recent posts' unintentional gastronomic theme [1,2], I am going to dedicate this post to debunking a myth related to the above quote. I claim that there is no bad time to eat, only bad eating habits. Specifically, I'd like to dispell the common misconception that the time of one's meal alone can affect weight gain.

I recall hearing a story on NPR a year or two ago about a study which specifically tried to test the claim that eating late at night is unhealthy. The study concluded that there was absolutely no correlation between the proximity of mealtime to sleep and weight gain, other than the fact that people tend to choose to eat more unhealthy foods late at night. I can’t seem to find a reference to that study, however, so I am going to have to do some research myself. Right. Lets get stuck in.

The majority of our food is actually digested during sleep, so the common argument that “eating late at night is bad because our metabolism [slows or shuts down] during sleep” is incorrect. With that said, there is a correlation between night eating, low self-esteem, reduced daytime hunger, and weight loss among people who are already obese or prone to obesity, however, this correlation does not necessarily imply causation (i.e., the act of eating a late meal does not necessarily provoke these conditions). It may simply be the case that the types of foods that people prefer to eat late at night are less healthy. There is still much debate on the subject, however, many scientists agree that meal frequency, as opposed to time, is one of the best predictors for weight gain. For example, the time between meals is highly correlated to one’s waist size. This makes some intuitive sense, since eating more, smaller meals will help regulate insulin levels, and spikes in insulin levels (which can be caused by large meals and/or large gaps in time between meals) have been linked to weight gain.

A newer study followed the eating and sleeping patterns of 52 subjects over one week. They found a correlation between “late sleepers” (i.e., people who go to sleep late and wake up late) and high body mass index, and that eating after 8pm was associated with higher body mass index. A relatively recent New York Times article summarizing the results of the study makes the further claim that eating late at night leads to weight gain, however, I disagree with that claim on the grounds that correlation does not imply causation. In fact, the original study noted:

Late sleepers consumed more calories at dinner and after 8:00 PM, had higher fast food, full-calorie soda and lower fruit and vegetable consumption.

Therefore, I think the results of the study can be interpreted to mean that there is a correlation between eating/sleeping late and a poor diet.

Furthermore back in 2006, the same research team conducted a study on monkeys in which they were fed a diet similar to the average (i.e., high-fat) diet common in the USA. The only variable was the time of day that the monkeys were fed. With all else remaining constant, the researchers found no correlation between weight gain and time of feeding.

It was really interesting to see that the monkeys who ate most of their food at night were no more likely to gain weight than monkeys who rarely ate at night. This suggests that calories cause weight gain no matter when you eat them.

While I’m on a roll here, let me quickly dispel yet another myth. I know many people that adhere to the strict “calorie-in/calorie-out” nutritional theory. This seems particularly popular among mathy/engineering types. The idea is that if your body burns fewer calories than what it takes in through food then it will gain weight. This theory, in general, is fallacious. The body doesn’t necessarily consume all of the calories of the food we stick down our throats. The time between meals will, in effect, affect the way one’s body “decides” to digest the food. Furthermore, as this study and others like it suggest, not all types of calorie sources are equal when it comes to how the body processes them!

In conclusion, if you’re overweight, it’s not necessarily legitimate to blame your late-night eating schedule. You can’t even blame your high calorie diet (only the types of calories you choose to eat).

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