In American culture, the idea that night time eating is somehow inherently bad is popular. Most of us have seen infomercials discussing the problems with eating past 7 pm, 9 pm, or some arbitrary time. In this series, I will discuss how night time eating affects metabolism and weight balance.
So, what are we really concerned about with night eating?
I would argue most people are simply concerned about weight gain or gaining inches around their waists. Although the average person doesn’t think about weight gain in such a way, greater weight gain and waist size go hand-in-hand with metabolic dysfunction, such as developing insulin resistance, high blood glucose, cholesterol and triglycerides . This cardiometabolic dysfunction leads to full on cardiometabolic diseases, such as diabetes and high blood pressure. Obviously, no one wants to ruin their metabolic health; weight gain just happens to be a less abstract way of thinking about things. So, with that said there are some studies demonstrating physiological effects that may make us want to avoid night time eating. We’ll discuss those first, and then move on to studies looking at night time eating and weight.
Physiological reasons to avoid eating at night
Glucose and triglycerides are primary sources of energy for our cells and insulin is analogous to the key that unlocks the cells to allow glucose and triglycerides in. As noted above, weight gain and obesity play a role in insulin resistance and the development of type II diabetes [2, 3]. When our cells do not respond to insulin, glucose and triglyceride levels in the blood can become dangerously high. In a classic study done by the prominent University of Chicago professor Eve Van Cauter and colleagues , subjects had glucose infused into their veins over 30 hours. It was found that glucose levels were highest in the night, specifically during their sleep, but there was not a clear trend for insulin. The timing of starting the infusions varied to ensure that measurements were not just dependent on the time elapsed since the beginning of the infusion. These findings suggest that glucose tolerance (the ability of cells to bring in glucose) is lower at night. In another study of 12 men, metabolic rate decreased by as much as 35% during sleep, with the decrease being less pronounced during REM sleep, when brain activity is higher .
Taken together, these studies suggest metabolic changes may occur at night that would make us want to avoid night time eating, but these studies could also just be tricking us in to making what is called an association fallacy, or guilt by association. In other words, they show physiological evidence that night time eating can lead to metabolic changes that may be associated with diabetes and other cardiometabolic disease, but that doesn’t mean that night time eating is necessarily linked to diabetes and other cardiometabolic disease. In order to determine this researchers would have to follow night eaters and non night eaters over time. That's where epidemiological studies come in handy (what is epidemiology?).
Epidemiological studies suggest that as the day progresses we tend to eat larger meals, and eat more frequently [6, 7]. Studies in free-living, healthy adults have shown that meal satiety (how full you feel) varies with the time of day. No surprise here, meals consumed during the night are less satiating on a per Calorie basis, and they lead to greater subsequent daily caloric intake compared to if the food was consumed in the morning hours . In a study of about 850 people, the proportion of Calories consumed in the morning was negatively associated with total caloric intake and the opposite was true for night time eating . In other words, eating more Calories at breakfast was associated with consuming fewer Calories overall, and eating more Calories at night was associated with consuming more total Calories.
Large meals in populations that consume a majority of their daily food intake during the night can be associated with weight gaining or holding on to excess weight
An important consideration here is will power reserve. As the day goes on, you deplete your will power to make good decisions. You sit in traffic, deal with a co-worker or co-workers that irk you, or if you work in certain fields, you deal with rude customers, students, etc. You may have to worry about transporting your kids, running errands, and so on. So, by the time you get home for the evening your will power to make good decisions is running low. I think simply knowing this could be empowering for many people. If you acknowledge that this could be a reason for excess night time eating, you can plan accordingly by preparing meals in advance, sticking to specific healthy nighttime snacks (e.g. an apple and protein shake), or planning a post dinner walk to destress; instead of watching Netflix and eating 2 cups of ice cream, handfuls of cookies, or whatever your go-to snack is. The evidence available suggests most nighttime eaters succumb to cravings for energy dense foods that lead to overeating.
This idea is supported in a study of over 20,000 Swedes. Night eaters were described as eating over 25% of Calories after dinner, or awakening at least once per week to eat . The prevalence of night time eating was 4.6% in men and 3.4% in women. Among obese men and women, the prevalence was much higher, 8.4% in men and 7.5% in women. Men and women with night time eating also had 3.4 and 3.6 times higher risk of binge eating compared to individuals who did not classify as night eaters, indicating these folks probably were not night eating carrot sticks and hummus at night. In another controlled study, participants ate from a computer-operated vending machine, and night eaters consumed roughly 15% (690 kcal) of their daily energy during nighttime eating episodes. Interestingly, they gained more weight over time than non-night time eaters and the additional ~700 Calories consumed at night accounted for more than all of the extra total Calories they ate compared to non-night time eaters (night time eaters on average ate an extra 500 total Calories per day) .
In a study that investigated the effects of night time eating habits on weight gain over several years, obese women who ate at night gained an average of about 11 lbs over a follow up period of six years, whereas obese women who did not eat at night gained only about two lbs over the same amount of time . No statistically significant associations were found for women, in general, when the researcher pooled normal weight and obese women. Night eating was not associated with weight change among men. So, this study indicates only certain sub-populations are susceptible to weight gain from nighttime eating. More on this later.
Adults aren’t the only ones susceptible to late night eating. Kids also succumb to night time cravings. In a study investigating the time of food intake as a potential risk factor for childhood obesity, data from over 11,000 2-18 year olds was extracted via nutrition surveys collected for the NHANES program . This particular study looked at food consumed in 2-hour time blocks starting at 4-6 pm up to 10-11:59 pm. Not surprisingly there was a lot of variance based on sex, ethnic group, and age. In regard to the researchers’ primary question, overweight 6-11 year olds consumed a significantly higher proportion of their Calories as the night went on. In contrast, overweight adolescents consumed significantly lower proportions of their Calories with each advancing two-hour time block. Personally, I was surprised to see no significant differences were found between the proportion of energy consumed between 8 pm and midnight and body weight status. I speculate that the lack of differences reported in this particular study could be to the myriad of other factors that may supersede the importance of night eating in regard to childhood obesity. Numerous factors contribute to this problem including composition of the child’s overall diet quality, total caloric intake, exercise patterns, and sedentary time [12, 13, 14].
So thus far we have reviewed studies suggesting that night time can be detrimental, but the data seems kind of “messy”. We’ve seen that night time eating appears to be associated with overweight and obesity in some sub-populations, but not all. We’ve also seen that night eating is associated with binge eating bouts. While the studies reviewed thus have focused on when Calories were consumed, they didn’t focus on the type of food being consumed. It’s pretty well accepted now that energy balance is the most import factor in determining a person’s weight (making sure Calories in match Calories out at a healthy body weight), so the type of foods being eaten and the context of night time eating should be the most important factors in determining if its detrimental or not. Interestingly that is exactly what more recent studies are starting to show. Stay tuned for part II...
*My initial reading on this topic and a lot of the papers I reviewed were cited in the following review paper, The Health Impact of Nighttime Eating: Old and New Perspectives .