Feed on

In 2001, Oprah claimed that she doesn’t eat anything after 7:30pm because her fitness advisor said that you need at least two hours in between eating and sleeping in order to digest properly(1). As with everything Oprah says, this led to many Americans reiterating her claim that eating late at night is unhealthy.

The claim is based around the perception that the food eaten late at night takes longer to digest and the calories are less likely to be used as energy and more likely to be stored as fat. However, the scientific community claims that the body’s metabolism works just as well during sleep and that calories are still calories no matter what point in the day they are consumed. The research is convincing, however there are studies that show associations that point to both sides of the argument being correct.


There are many articles on the web from so-called “mythbusters” that disagree with Oprah and her followers based on the theory that ‘a calorie is a calorie’ and they point to several studies that disprove the correlation between weight gain and late night snacking(2) However, the studies that are referenced do not specifically test the relationship between weight gain and late night snacking and make it difficult to completely disregard the myth.

A study that is referenced frequently cited is the Gustaf study(3), published in 1996, that compares the meal patterns between obese and normal weight men. The study took Swedish men aged 20-60 years old, and consisted of 86 obese men and 61 normal weight men, all randomly selected. The study focused on the energy intake, eating pattern, and number of food intakes per day, and concluded that there was no ‘obese eating style’ because of the narrow differences in meals per day and eating patterns.

But what does this study say about late night snacking? The subject pool is fairly small, but the study concludes there is no significant difference in eating pattern from obese and normal weight men. So from the subjects they had, the obese men did not eat at night a significant amount more than the normal weight men. The study is not large or extensive enough to prove or disprove the myth, however it is still reasonable to conclude that obese men do not consume significantly more calories late at night, which contributes to the argument that late night snacking does affect health.

An argument commonly used by writers that say eating late at night is unhealthy is that the body does not expend less energy late at night. What happens in an experiment that tests how energy is expended when people are fed at different times in the day? An Italian study tests the effect of scheduling meal times on circadian pattern of energy expenditure, which is the daily cycle of how people consume their calories, by feeding small groups of people at different periods in the day(4). The study was able to find a significant relationship between all of the circadian patterns, which allowed them to determine that the meal times do not affect the pattern of energy expenditure, and even that this pattern might be biological in nature and irrelevant of the time of food consumption.

While this is significant evidence to disprove the myth, there are some issues with the study and its relation to late night snacking. The subject pools were broken into one meal at 10am, one meal at 6pm, three meals 10am, 2pm, 6pm, and fasting for 36 hours. None of these time periods are close to when the average person would be going to sleep, and it also only tests the subjects for three days. While the study supports the argument that different meal times do not affect the body’s pattern for metabolism, it does not give results on what happens if people are eating right before bed.

There are studies that do seem to show more evidently the lack of a relationship between late night eating and weight gain. A study from 2007 takes dietary data from 1792 people aged 20-59 and 893 people aged 60-80 and looking at the relationship between energy intake and BMI(5). There were associations between eating frequency and obesity, but no one eating occasion contributed more than any other to extra weight.

Another study done on female rhesus monkeys specifically targets the effects of nighttime caloric intake(6). Sixteen monkeys were fed high fat diets to increase calorie intake and gain weight, and there was no correlation between the percentage of calories consumed at night and body weight or weight gain.

Both of these studies have more convincing methods behind their conclusions, but both also have some issues. The BMI study has a huge subject pool and takes into account separate age groups, however the age groups have 40 and 20 year ranges, respectively. They also don’t use any control groups or mention any randomization. With such a broad unorganized group of people, it seems like it would be difficult to find a correlation between any two variables that aren’t already very obviously correlated. The monkey study has convincing data, however there are not many convincing studies done on the metabolisms of monkeys compared to humans. These two studies have important conclusions toward disproving the myth, but the processes should be questioned.

There are many sites that claim Oprah is correct, and that people should avoid eating late at night. Many of these sites, site personal experience and are mostly filled with irrelevant personal stories(7), however there are some studies that show associations between obesity and late night eating.

In a Swedish study done in 2002, a randomly selected pool of Swedish women were selected to fill out questionnaire about their meal patterns(8). The study found that caloric consumption was higher in obese women and that meals were consumed later in the day. There is no doubt that there is an association between eating late at night and obesity. However, the study only controlled for weight and not for the meal period, it is impossible to determine whether it is the amount of calories or the time at which they were consumed that has a greater effect.

Another study looks at the role of sleep timing in caloric intake and BMI(9). Again, it is difficult to say that caloric intake after 8pm is the cause of a high weight, but the study does say that after controlling for sleep timing and duration, caloric intake after 8pm does increase the chances of being obese. But it is difficult to use this study as strong evidence for a few reasons. The study only consists of 52 participants, and only states that caloric intake after 8pm may increase chances of obesity. It is also unknown whether there are biological factors from eating late at night that might affect sleep and therefore cause the body to process food differently.

Very few studies that support the myth can explain why it might be true, and each of the most relevant studies has flaws when it comes to proving a significant or insignificant relationship between the two. The studies that show a significant relationship do not control for other factors, and the results could be influenced by outside variables such as higher overall caloric intake and unhealthy sleeping habits. However the studies that show no relationship are too complicated to address solely nighttime caloric intake and seem to only prove certain aspects of the theory. The logic behind ‘a calorie is a calorie’ is sound, but the studies don’t exactly back up the theory.

In conclusion, the lifestyle of eating more meals, eating late at night, and sleeping poorly significantly affect weight, however it is not clear how much of an impact late night eating has on weight. In some studies, there is no relationship shown between BMI and meal time, however there are enough faults in the studies to cast doubts on the results. Based on the studies already done, the majority of the evidence does not support the myth. However, there needs to be more extensive studies to specifically test the impact of late night eating on weight.

1. http://www.oprah.com/spirit/How-Oprah-Does-It-All-Women-with-Drive

2. http://www.playnormous.com/blog/?p=503

3. http://www.ncbi.nlm.nih.gov/pubmed/8909929?dopt=Abstract

4. http://www.ncbi.nlm.nih.gov/pubmed/6802148?dopt=Abstract

5.  http://www.ncbi.nlm.nih.gov/pubmed/16953255?dopt=Abstract

6. http://www.ncbi.nlm.nih.gov/pubmed/16421340


8. http://www.ncbi.nlm.nih.gov/pubmed/12122550?dopt=Abstract

9. http://www.ncbi.nlm.nih.gov/pubmed/21527892


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