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  • Writer's pictureRyan Allen

Timing meals for metabolism, body composition, circadian rhythm, and more

Much of the talk in the world of nutrition focuses on what to eat, what not to eat, and a little bit on how much you should eat. Less often is the question posed of when to eat. What time of day should one eat? When should one eat in relation to exercise, sleep, and other activities? Should meals be spread out across the day or consumed all at once? This turns out to be a fairly complicated subject with far from perfect data, although there are plenty of actionable insights from the scientific literature.

Most importantly, we should emphasize that your meal-timing strategy will depend on what you are optimizing for. Similarly to the ever-present debates over the best diet, there does not appear to be a universal eating schedule that will work for everyone. Rather, it is about finding what works best for you, and your goals at a given time. Personally, my eating schedule fluctuates quite a bit depending on my objective (gaining muscle, losing fat, overall metabolic health and nutrient cycling, sleep and circadian rhythm definition, etc.). Lastly, if you’re finding that your eating habits do not align with those touted in this post for your respective goal, do not worry. These suggestions are just those strategies that seem optimal based on the data we have available; they are by no means exclusive methods to achieve their corresponding goals.

Let’s start off with what most people are curious about, which is healthy weight loss. Meal timing does not appear to have a major impact on this. In general, our framework for healthy weight loss follows the principle of calorie balance: you must be expending more energy than you are consuming. There are three ways to attain this. The first, caloric restriction, means tracking your total calories in and out, making sure you are in a deficit (as you might guess, accuracy can be challenging to attain here). Next, dietary restriction involves limiting the kinds of foods you eat as a means of restricting caloric intake (for example, if someone eats fewer calories on a low-carbohydrate diet, or a vegan diet, or a Mediterranean diet, etc.). Time restriction, or time-restricted eating (TRE), involves eating within a certain range of time during the day. In recent years, studies have shown that TRE does not offer additional weight loss or metabolic benefits above caloric restriction itself. In other words, eating one large meal a day does not appear to facilitate weight loss or measurable improvements in health any more than eating the same amount of calories spread out across the day. The key takeaway here is that it is the calories that matter, not the time at which you eat them. That said, I have personally utilized TRE frequently as an effective tool to achieve caloric restriction, because I can only eat so many calories within the small time window that I allow myself. One final thing to add on this so-called “intermittent fasting” is that time restriction could still be metabolically relevant when taken to greater extremes. Likely only in periods of prolonged fasting for at least a few days, interesting switches to our metabolism take place, including ketogenesis and upregulated autophagy, which have a number of proposed benefits for disease prevention, optimizing metabolic function, and longevity. For this reason, I have introduced 3-day fasts to my regimen, but with no serious frequency; my loose goal is to do one about every other month.

When it comes to building muscle, the long-standing dogma has been that, among the most important things nutritionally, one wants to consume protein in close proximity to exercise. Some argue in favor of eating protein before a workout, so as to prevent catabolism (muscle breakdown) from taking place at all. Dr. Don Layman, Professor of Food Science and Human Nutrition at the University of Illinois and expert on dietary protein, does not believe this to be the case. While he acknowledges ongoing debate in the literature on this topic, he states that his research has shown exercise to be clearly catabolic no matter what, if someone is fed or fasted. Thus, the time to build is after activity, not before. This goes with the old adage of promptly getting a post-workout protein shake, so as not to miss your “anabolic window,” or period of time in which your muscle is rebuilding following the stimulus of the workout. For that reason, I will usually follow my workouts up with protein consumption if it’s not too disruptive for me. Layman’s work has demonstrated a roughly two-hour post-workout window where one sees the biggest anabolic (building up) effect from eating protein for untrained individuals. In regularly training individuals, though, getting protein immediately in this window does not seem to produce much of a difference in muscle mass or strength, provided sufficient protein intake throughout the day. As we have previously mentioned, this optimal protein intake for muscle growth seems to involve spreading out protein doses of roughly 20-60 g each across the day, for a total of about 1 g per pound of body weight.

Finishing on the timing relationship between eating and exercise, you might recall the impact of low-intensity cardiovascular exercise on blood sugar control. Low-to-moderate intensity cardiovascular exercise is an excellent stimulator of something called insulin-independent glucose uptake. This is when, as the name suggests, sugar from the bloodstream gets taken into cells without the use of the hormone insulin, which performs the same action. When we eat, those nutrients get absorbed into our bloodstream from our gut, and we get a postprandial (after-meal) spike in blood sugar. Low-intensity exercise after the meal can therefore help blunt this peak of blood sugar, thereby lessening vascular damage that can be caused by frequent bouts of high blood sugar. So, in a way, the old adage of taking a walk after you eat to stay healthy seems quite valid.

Finally, when we eat can have a major impact on our body’s internal clock, or circadian rhythm. This cycle we keep ourselves on does a lot more than just maintain states of sleep and wake, but regulates hormone release, alters levels of energy and arousal, and plays critical roles in preserving homeostasis. As we have touched on before, it is ideal for sleep purposes to avoid eating in close proximity to bedtime, largely due to the thermogenesis (heat production) of food, when we want to lower our core body temperature to fall asleep. Dr. Satchin Panda, Professor at the Salk Institute in San Diego and expert in circadian biology, relates that this same concept may apply more generally to circadian rhythm, that it may be optimal for circadian regulation to shift one’s calories earlier in the day. Thinking solely in terms of temperature, this more or less makes sense, as we generally want to warm up at the start of the day to promote wakefulness and arousal, and cool down as we approach bedtime to drift into sleep. It seems intuitive that you would then want your thermogenesis from food as early in the day as possible. Fascinatingly, there are also observations that shifting caloric intake earlier in the day promotes better blood glucose control as well. There are many hypotheses as to why this may be, or why we may be more insulin-sensitive earlier in the day. One such theory is actually that melatonin, our sleep-promoting hormone, actually interferes with appropriate insulin secretion in response to an increase in blood sugar. This is one reason why late-night eating can be particularly harmful in producing massive, poorly-regulated spikes in blood sugar. Much of this circadian data requires further study, although I must say circadian biology is one of the fields of preventive health that intrigues me most these days, as I think it is a powerful force to harness for our lifestyle optimization.


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