The modern conundrum surrounding when and how much to eat at each meal has belabored the phrase: “breakfast like a king, lunch like a prince, dinner like a pauper.”
I have no idea whence this adage emerged, but its proponents suggest that the ideal eating pattern involves a large breakfast (the most important meal of the day!), a moderate lunch, and a meager dinner.
Likely not rooted in scientific reason (how long has it been since paupers were relevant?), this age-old recommendation may actually make sense — from a biological point of view.
A large swath of recent evidence suggests that our body’s metabolism is tailored to consume most of our calories (i.e. energy) earlier in the day as compared to “backloading” calories later on. Interestingly, and perhaps unfortunately, modern society has gotten this backwards — we often consume our largest meal of the day at dinner. To be quite frank, all of our meals are large — we eat breakfast, lunch, and dinner like kings; perhaps better.
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The concept of time restricted feeding (TRF) — where one consumes his or her calories within a “restricted” daily eating window — has gained popularity among biohackers and anyone aspiring to lose their “dad (or mom) bod.” A form of intermittent fasting, TRF is espoused as an efficient, effective, and non-laborious way to reduce one’s daily caloric intake (if desired) while boosting metabolism of fat through a consistent daily fasting period.
An iteration of TRF, known as early time-restricted feeding (eTRF) has also gained notoriety due to the idea that one’s calories should be eaten earlier vs. later in the day.
eTRF has been shown in several recent studies (which I’ve covered in this blog) to be superior to later TRF in terms of improving insulin sensitivity and cardiovascular risk factors like blood pressure.
The mechanism behind eTRF’s potency for improving metabolic health centers around our biological clocks — aka our circadian (daily) rhythms. We have circadian rhythms in nearly every cell in our body. Metabolism is no different — how much and what fuels we use for energy can fluctuate widely throughout a 24-hour day.
Insulin sensitivity, appetite hormones, and gene expression for enzymes that control the breakdown of fats, carbohydrates, and proteins also exhibit circadian rhythms. For this reason, WHEN we eat might be just as important as WHAT we eat.
In regards to circadian rhythms, eTRF is claimed to be more beneficial for a few reasons. One is that insulin sensitivity is higher in the morning vs. the evening, in general. Thus, we might be better at “handling” carbohydrate intake at this time; more effective at moderating our blood glucose levels in response to a meal.
Another reason involves fat oxidation. Eating earlier in the day, in theory, allows for a longer fasting window throughout the late-afternoon and evening. Ending your dinner early and fasting until breakfast (or lunch) the next day provides an ample opportunity for your body to “run low” on glucose and glycogen (stored glucose) and begin to metabolize a larger percent of stored body fat.
And, as a new study suggests, late-night eating might actually inhibit our ability to metabolize more fat while we sleep, preventing this natural “metabolic switch” from occurring.
Published in PLOS Biology, this study examined the metabolic effects of breakfast eating vs. skipping using “an experimental approach to test the metabolic consequences of a straightforward exchange of equivalent nutritional intake between early morning (8am) and bedtime (10pm).”
Put another way: in the context of equal calorie consumption and meal frequency, what are the metabolic effects of eating breakfast and skipping a late-night snack compared to skipping breakfast and eating a late-night snack?
Using a randomized, cross-over design (this means that the order of the subject’s visits was determined by chance, but everyone eventually was exposed to both experimental interventions), participant’s completed two 56-hour stays inside a metabolic chamber separated by a short “washout period.”
Throughout the 56-hour stint, each individual’s total energy expenditure, respiratory exchange ratio (RER; the amount of Carbon Dioxide produced/Oxygen consumed), carbohydrate oxidation (CO) and lipid oxidation (LO) were calculated by using measures of carbon dioxide output and oxygen consumption — what is known as indirect calorimetry.
While not getting into details, indirect calorimetry can assess how much total energy someone is using, and where that energy is coming from (i.e. what fuels are being broken down).
For reference, a low RER (around 0.7) indicates a predominance of fat metabolism, while an RER around 1.0 indicates that carbohydrates are the main fuel source being used. Most humans hover somewhere around 0.85 — utilizing a mix of fuel sources at any given time, with fluctuations throughout the day (remember those rhythms!)
Let’s take a look at the eating schedule, the main focus of this study.
During each intervention, participants were given 3 daily meals. However, WHEN these meals were served depended on the specific condition.
The “breakfast group” received a ~700 calorie breakfast (8am), ~800-calorie lunch (12:30), and ~1000 calorie dinner (5:45).
In the alternate intervention (the “late-night snack group”), they were given the same three meals but with one variation; the 700-calorie breakfast was substituted for a 700-calorie late-night snack (10pm) with a similar nutritional composition. Instead, plain tea or coffee was given for “breakfast” during the 3-day stay in the chamber.
The content of the meals and total calories consumed were kept equal across all visits — so how MUCH someone ate wouldn’t confound the study measures. Basically, the only variable we are concerned with here is the timing of food intake.
A few important things to consider before delving into the relevant findings:
- The fasting period in the breakfast vs. late-night snack group was similar — about 14 hours in each (dinner to breakfast in one group and late-night snack to lunch in the other).
- During the study, there were no differences in participants’ physical activity levels, sleep quantity/quality, core body temperature, or circadian rhythms (assessed using melatonin and cortisol secretion).
For this reason, we can reliable assume that any changes are the cause of alterations in meal timing rather than some other extraneous variable!
The Daily Rhythm of Substrate Utilization
In the “breakfast” arm of the study, participants displayed a strong rhythmicity in their RER — it was LOW throughout the “lights off” sleeping period and HIGH during the active daytime period. In other words — fat metabolism was high at night while carbohydrate utilization dominated during the day.
These results indicate a strong oscillating rhythm in how our body uses different fuel sources throughout the day and night.
A clear “spike” in carbohydrate metabolism occurred after each meal in the breakfast group, a result that makes sense given the fact that carbohydrate availability would be high during this post-meal period.
But…these rhythms weren’t as robust in the late-night snack group — they had a lower amplitude in their RER rhythms.
And, while similar “spikes” in carbohydrate utilization occurred with each meal, there was a noticeable peak just after the evening snack!
The above differences in carbohydrate metabolism had implications for the total amount of fat that subjects burned throughout the study!
For example, in the breakfast group, RER fell just before sleep — they entered their “peak” fat oxidation period during the night. Metabolism “switched over.”
Consuming a late-night snack attenuated the reduction in RER just before bed, preventing the “metabolic switch” into fat metabolism during the night.
In fact, when comparing the two groups, the biggest difference in RER occurred during sleep — specifically between 10pm — 2am.
While RER was low (fat oxidation high) when participants skipped the late-night snack, it remained higher (fat oxidation lower) when a late-night snack was consumed — fat oxidation was inhibited by a late night meal!
It is interesting to note that throughout the entire 24-hour day, there were NO differences in carbohydrate utilization between participants when they consumed breakfast or a late-night snack.
The only overall differences across a 24-hour day were in lipid (aka fat) oxidation!
In fact, the breakfast group burned about 15g of fat MORE per day vs. the late-night snack group. This was only over a 3-day stay in the chamber! If you extrapolate these findings out over months, even years, this could add up to major metabolic benefits!
The authors conclude by saying that “the time of meal placement can cause variation in the amount of lipids oxidized regardless of the nutritional or caloric content of the meal.”
Burning the Fat at Both Ends
Why was fat oxidation enhanced in the breakfast group vs. the late-night snack group? At first it may seem odd — both groups had a similar daily fasting window (~14 hours) and both consumed identical diets.
This brings us back to circadian rhythms. What the authors in this study note is that “normally”, humans demonstrate a nightly drop in RER, indicating an increase in fat oxidation. Since it is “natural” to go throughout the night without food, our metabolism MUST switch over during the night to using our stored body fat to support its functions. This is why lipid oxidation dominates during sleep!
When you eat a late-night snack, you “replenish” some of your fuel sources right before bed. By maintaining a high carbohydrate availability when entering the sleep period, metabolism can be supported through oxidation of carbohydrates, at least for a while. This is why the study observed a stark difference in substrate metabolism between 10pm and 2am.
The interaction between diet and our internal circadian rhythms is best described by a quote from the paper:
“Timing of nutrient availability coupled to circadian control of metabolism drives a switch in substrate preferences such that a late-evening snack results in significantly lower lipid oxidation compared to a breakfast session [and no late night snack].” (Addition mine)
All of this isn’t to say you shouldn’t eat at night. For instance, having a low-carbohydrate late-night snack might alter these results, but that hasn’t been studied.
However, in light of the recent trend of time-restricted feeding, it sure does make sense to think about where you want to place your feeding window. Earlier, it seems, might be optimal in terms of when you want to take in most of your calories. That is…if your goal is optimizing fat metabolism as in this study.
Personally, I prefer to end dinner around 5-6pm, allowing enough time for the food to digest before I head to bed. I find that this improves my sleep quality and leaves me feeling better during my workouts the following morning.
But everyone’s preferences (and rhythms) are different. It will likely will take a bit of self-experimentation before you or I find the “perfect” meal timing schedule.
Not to sound metaphysical, but the more we learn about how our bodies function, it becomes quite clear that what is optimal is a synchronicity between our internal microcosm and the environmental macrocosm from which we evolved.
Kelly KP, Mcguinness OP, Buchowski M, et al. Eating breakfast and avoiding late-evening snacking sustains lipid oxidation. PLoS Biol. 2020;18(2):e3000622.