Of all the “rules” floating around in the weight-loss advice world, one that frequently comes up is not to eat after 7 pm. Is there something magic about this time of day, or eating in the evening, in general? Does snacking before bedtime cause us to gain weight? Could there be any truth to this?
To be honest, I have no idea where this advice came from or who was the first to put forth this diet rule. The first time I remember hearing it was from Oprah, who after one of the times she lost a great deal of weight was dishing out weight loss advice to her audience. I’m sure she didn’t come up with this rule but in my head, I give her credit with at least making it popular.
My first reaction to this rule is, ‘no, of course eating after 7 pm or any other arbitrary point in the evening won’t make you gain weight’ but I wanted to be sure and see what that science says. So, what does the science say? Does eating in the evening, after dinner and before bed, cause us to gain weight or have a higher body mass index (BMI)?
If we go by large, epidemiological-type studies, the answer is no, eating late at night is not associated with weight gain or a higher BMI. In a study that included data from 7,147 adults, it was found that weight was not associated with evening food intake (food after 5 pm) and did not predict weight gain over a 10 year follow-up period. Another study which analyzed intake of 1802 adult women yet again did not find a relationship between intake of food after 5 pm and weight. Although these studies were quite large, they both looked at food intake after 5 pm which, in my opinion, isn’t super specific to those who eat late at night or before bedtime (say, after they have already eaten dinner). By including all meals after 5 pm, you are lumping in all people who eat dinner after they get home from work (perhaps these people have a 6 pm dinner) and who may or may not snack after eating dinner. In more recent research involving the eating habits of 768 children and 852 adolescents once again found no relation between time in which evening meals are eaten and weight. In this study, unlike the two mentioned previously, evening meals were classified as those after 8 pm.
Another study looking into eating behaviours and weight of 978 women found that eating before bedtime was not associated with having a higher BMI. Interestingly, the only behaviour they were able to isolate with carrying a higher likelihood of being overweight or obese was eating beyond satiation (i.e. continuing to eat after feeling full). In another study analyzing eating behaviours of 4249 adults, late evening meals were not associated with overweight status (BMI greater than 25). In this study, eating quickly was the only behaviour that was associated with a BMI greater than 25. Other research involving 499 participants followed for one year, backs up these findings as it was unable to find an association between eating before bed and risk of obesity.
In general, on a population level, it appears that eating in the evening doesn't influence weight gain or body weight. However, these epidemiological studies are not able to prove or disprove a cause and effect relationship and we are really in need of some good quality randomized controlled trials on this subject. I do think it's fair to say that on an individual level that there may be people whose weight is influenced by when they eat, or more accurately what they are eating at certain times of the day...but more on that a bit later.
In my research for this post, I found some interesting tidbits that made me question whether we could say definitively that eating in the night doesn’t cause weight gain or that there isn’t a relationship between evening snacking and weight. The next part of this post talks about a couple of things I discovered.
METABOLISM CHANGES AND TIME OF DAY
This I found pretty interesting. There are some studies that indicate that our body’s metabolism acts differently in the day than in the night (or more accurately, between your active and passive periods of the day).
It has been known for a while that humans and many other organisms have a circadian rhythm, or a daily 24-hour cycle of biological processes. In simple terms, our body has an internal clock in which many biological functions follow. There is also research that shows that in addition to a central clock, that there are “peripheral clocks” in various organs, including the pancreas and liver (these two organs are involved in metabolic and digestive processes, among other things). These peripheral clocks are interconnected and, apparently, are influenced by a number of external factors or “synchronizers”. Food has been said to be one of these external synchronizers. In laboratory studies involving mice and rats, digestive processes such as gastric emptying, intestinal motility, glucose absorption and enzymatic activity have been shown to be influenced (or synchronized) by feeding at different times. In one study in mice, it was found that changing feeding cycles from night to day (as mice are nocturnal) resulted in more weight gain than mice fed during the night (their natural feeding time), despite no differences in activity level or caloric intake.
While most studies seem to involve animals, there is a small human study that looked at metabolic changes when circadian rhythms were interrupted. When subjects were awake and eating in the middle of the night, researchers found that both glucose and insulin were significantly increased after meals, compared to when they would eat in the day. In 3 out of 8 (or 38%) participants, post-meal glucose levels mimicked those seen in pre-diabetes. This means that despite having higher insulin levels (the hormone which helps glucose move out of the blood and into the cells), participants had higher than normal blood glucose levels after meals. This could imply that some degree of insulin resistance was happening, a condition which in the long term is associated with weight gain.
Another finding in this study was that levels of leptin (the hormone that signals to your body that you are full and don’t need more food) were lower when subjects were eating in the middle of the night. This could mean that our body has a harder time giving us the “I’m full” signal at night, causing us to overeat. The lower levels of leptin at night is an interesting finding as it supports other research that shows that the satiating ability of food (i.e. a food’s ability to satisfy our hunger) drops throughout the day and is lowest at night. A 2016 review of both animal and human studies backs up these findings showing that limiting food to the active part of the day leads to less metabolic disturbances.
While none of the above studies can without a doubt prove that eating in the evening or anytime before bed causes weight gain, I think they provide some evidence that eating outside of a natural feeding time can impact us physiologically. It should be noted, however, that the subjects in many of these studies were put into very unnatural situations, changing sleep and eating to a completely different time of day (or night) and the results may not be completely transferable to those eating a snack before bed (they may be more relevant to those working shift work or those frequently travelling across time zones). That being said, I do think it is plausible to say, in some people, that eating late at night could have negative metabolic effects or impact weight but more quality research is needed in this area to be certain.
COULD IT BE JUST GENETICS?
What if the tendency to eat at night was in part due to genetics? And what if those who have these genetics tend to be overweight?
According to some research, there are specific genes that are associated with being either a morning person or an evening person. People with the “evening genotype” tend to stay up late in the night and sleep in late the morning. They often tend to skip breakfast, or if breakfast is eaten they have a lower percentage of their calories at breakfast time. Interestingly, compared to those with a “morning genotype”, the evening people tended to be heavier and during a 20-week dietary trial, had a harder time losing weight than early eaters. In a related study, it was also found that those with this evening genotype were more resistant to weight loss and had higher circulating levels of the hormone ghrelin (a hormone which gives your body the “hungry” signal).
What does this mean? Well, genetics could play a role in determining a person’s tendency to eat in the late evening. These same genetics could be related to having a larger body, and it may not necessarily be the time in which people are eating that causes weight gain or a higher BMI. Now, I did not study genetics (I’m a dietitian!) so I don’t pretend to fully understand the complexity of this topic. I do, however, think it is interesting to think that our genes could play a role in our preference of when we eat. I also think it’s important to consider the possibility that our weight is the result of a number of modifiable and non-modifiable factors, such as genetics.
What does this all mean? Does eating before we go to bed cause us to gain weight?
Eating after 7 pm (or whatever arbitrary evening time you pick) won’t necessarily lead to weight gain. I could not find one large study that showed an association between evening meals or snacks and higher body weights. That being said, I think the answer can also be yes. In some people what they eat, after dinner and before bed, can contribute to weight gain. It is sometimes a simple equation of your evening snack exceeding your daily caloric requirements. Doing this too often can very well contribute to unwanted weight gain.
The other thing to consider is the food itself being eaten in the evenings. Which foods are most likely to be eaten at night? Potato chips or cut up veggies? Ice cream or a handful of nuts? Hot buttery popcorn or a bowl of blueberries? We know the answer, don’t we? (hint: it’s usually not the healthier option). From my personal observations, the foods consumed after dinner tend to be higher in empty calories. So it might not necessarily be a question of when but what people are eating in the evening time and whether these foods cause one to exceed daily caloric needs.
The evening is also a period in which a lot of mindless eating takes place. Ever find yourself turning on the TV and automatically grabbing a snack to munch on, even when you’re not hungry? This is all too common. Many of us have developed a habit of having a nighttime snack, regardless of our hunger levels. As we learned earlier, eating beyond the point of fullness has been associated with a higher BMI, and this is much easier to do when you aren't eating mindfully. Learning to recognize and honour our hunger and fullness is an important step when we start to examine our eating behaviours and its relationship to our health.
To summarize, eating after 7 pm isn't going to cause an automatic weight gain. However, our food choices in the evening could very well be causing us to gain weight. While there is some evidence that disrupting our circadian rhythm by eating at unnatural times of the day can cause some metabolic disturbances, I'm not convinced that eating at 7 pm (or in the early evening) will have the same metabolic effects as being awake and eating in the middle of the night (as was the situation in many of the studies I reviewed).
So, as the research stands, there is no need to feel any guilt if you are eating a portion of your calories after 7 pm, no matter your body size. When we are evaluating our health we have many factors to consider, including our eating habits, activity level, genetics (and a bunch of other factors, too). Weight is not a simple factor of when we eat. If you are concerned about your weight or your eating, have a chat with a Registered Dietitian who can assess your eating habits and provide you with individualized guidance.