So where did this "no carbs at night" thing come from?
In order to properly assess this fitness "fact," we need to understand why limiting carbs at night is recommended in the first place. Most "experts" who recommend limiting carbs at night do so because their assertion is that since you will be going to sleep soon, your metabolism will slow down and those carbohydrates will have a greater chance at being stored as fat compared to if they were consumed earlier in the day, where they would have a greater probability of being burned. This seems reasonable, but broscience always does. They also often assert that insulin sensitivity is reduced at night, shifting your carb-storing directionality towards fat and away from muscle.
Let's tackle the issue of metabolic rate slowing down at night time first. The logic behind this theory seems reasonable enough: You lie down in a bed and don't really move, just sleep, so obviously you are burning fewer calories than if you are awake doing stuff. Even if you are just sitting in a chair or couch resting, you have to burn more calories than just sleeping right?
"The Idea that you should avoid carbs at night because your metabolism slows down and you won't 'Burn Them Off' definitely doesn't pass the litmus test."
At first glance this seems to jive with work from Katoyose et al. which showed that energy expenditure decreased during the first half of sleep approximately 35 percent.1 However, these researchers did show that during the latter half of sleep energy expenditure significantly increased associated with REM sleep. So, there are rises and falls in sleeping metabolic rate (SMR), but what is the overall effect? Interestingly, at the very least it does not appear that the average overall energy expenditure during sleep is any different than resting metabolic rate (RMR) during the day.2,3 Additionally, it appears that exercise increases sleeping metabolic rate significantly, leading to greater fat oxidation during sleep.4 This seems to be in line with data from Zhang et al. which demonstrated that obese individuals had sleeping metabolic rates lower than their resting metabolic rates, whereas lean individuals had sleeping metabolic rates significantly greater than their resting metabolic rate.3 So unless you are obese, not only does your metabolism not slow down during sleep, it actually increases!
The idea that you should avoid carbs at night because your metabolism slows down and you won't "burn them off" definitely doesn't pass the litmus test.
So the whole "don't eat carbs at night" thing is definitely broscience, right?
So far, the fear of carbs at night certainly smells like broscience, but before we render a verdict, let's examine things further. There is also the issue of insulin sensitivity and glucose tolerance to address. This is where things get interesting. Compared to morning meals, levels of blood glucose and blood insulin definitely remain elevated longer with evening meals.5,6 Aha! There it is, proof, that you shouldn't consume carbs at night right? Not so fast. Though insulin sensitivity and glucose tolerance appear to be worse at night compared to a morning meal, it is important to keep in mind that a morning meal is after an overnight fast and the fast may improve insulin sensitivity. Perhaps a more fair comparison is a mid-day meal vs. a nighttime meal. In this case there is actually no difference in insulin sensitivity or glucose tolerance.5
Therefore, it appears that insulin sensitivity and glucose tolerance are not necessarily impaired at night, but rather are merely enhanced by an overnight fast.
Does any of this science mumbo jumbo actually make a difference?
While it is great to talk about mechanisms and nitpick every intricate detail about metabolism, at the end of the day, we have to examine whether or not any of this stuff makes any difference. Fortunately for us, a recent study published in the Journal of Obesity examined this very question.7 Researchers from Israel put people on a calorically restricted diet for six months and split them into two groups, a control group and an experimental group. Each group consumed the same amount of calories, protein, carbohydrates, and fat but they distributed their carbohydrate intake very differently. One group (control) ate carbs throughout the day, whereas the experimental group consumed the majority of their carbohydrate intake (approximately 80 percent of the total) at the night. What they found after six months may shock you.
Not only did the experimental group consuming the majority of their carbs at night lose significantly more weight and bodyfat than the control group, they also experienced better satiety and less hunger!
Whoa, hold up. Less hunger? I don't buy it
You heard me right, they were less hungry. Now I'm sure all of you that have been following typical fitness protocols where you eat six times per day and have most of your carbs earlier in the day are thinking, "Man, if I went more than 2-3 hours without carbs, I'd be starving!" Well, my friends, you are buying into a vicious cycle. Let me explain: When you eat small amounts of carbs frequently, you are basically titrating in glucose to your system.
To dispose of this glucose, your body releases insulin to drive blood glucose into cells. Over-secretion of insulin, however, may cause hunger to rise (typically about 2-3 hours post meal, the approximate time course of an insulin response), but—no problem—you are eating every 2-3 hours anyway, right? Just titrate in some more glucose. Unfortunately this makes you crave and consume glucose like clockwork. It tricks many people into thinking that they need carbs every 2-3 hours or they would be hungry when, in fact, the opposite is true.
If you ate carbs less frequently with more time between carb dosings, you would be less hungry because your own body would ramp up systems that deal with endogenous glucose production, and keep your blood glucose steady. When you consume carbs every 2-3 hours however this system of glucose production (gluconeogenesis) becomes chronically down regulated and you must rely on exogenous carb intake to maintain your blood glucose levels. Now if you transition from eating carbs every 2-3 hours to further apart for the first few days you may be hungry until your body has adjusted to using gluconeogenesis to maintain blood glucose rather than just eating carbs every 2-3 hours, but once you do adjust, you will find that you are far less hungry.
Bringing things full circle, this is exactly what the researchers found! These subjects were hungrier in the first week of the diet compared to 90 and 180 days into the diet where they were much more satiated.