What is Chrononutrition?

November 29, 2022

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Hands holding alarm clock with fork and knife. Chrononutrition is a relatively new specialty in the fields of nutrition and biology that tries to understand how the timing of food ingestion affects health. The central idea here is that metabolic health, cardiovascular health, and body composition come down not just to what and how much we eat but also when we eat.

You are, of course, aware that our bodies operate on a roughly 24-hour rhythm governed by circadian clocks. The sleep-wake cycle is the most obvious example. Many other aspects of human biology are also governed by 24-hour clocks operating both in the central nervous system and the peripheral organs and tissues. Chrononutrition seeks to answer two broad and related questions:

How do the body’s natural clocks affect food choices and metabolism?
How does food timing affect circadian rhythmicity and, consequently, various health markers?

The latter is particularly relevant to people who, probably like you, strive to make food, movement, and lifestyle decisions to maximize their health and longevity. Although the topic of chrononutrition has only gained traction within the past decade, evidence increasingly suggests that we may be able to manipulate food timing to improve well-being.

Today, I’ll briefly review the underlying premise of chrononutrition and return to a question that has come up many times in our community: Should I be eating or skipping breakfast if my goal is optimal health now and for decades to come?

Chrononutrition 101

Here’s what you need to know to understand chrononutrition:

First, many biological functions are guided by central and peripheral clocks. I already mentioned sleep-wake. Body temperature is another example. Body temperature peaks in the afternoon and decreases overnight, hitting its nadir in the early morning hours. More to the point of this post, many aspects of metabolism also operate on a circadian rhythm. These include¹

Saliva production
Gastric emptying and gut motility (the movement of food through the digestive tract)
The release of digestive enzymes
Nutrient absorption
Beta cell function (insulin release from the pancreas)
Glucose tolerance
Hunger

Second, that elusive and enigmatic target we call “health” depends on proper circadian rhythm alignment—everything happening when it should. Research shows, for example, that circadian misalignment, as happens with shift work and eating at the wrong times, leads to impaired immune function.²

Third, we stay “on time” thanks in part to behaviors that tell the body’s clocks what time it is. These behaviors, like sleeping at night and getting early morning sun exposure, are called zeitgebers. Eating at the proper times is another zeitgeber that keeps our circadian rhythms aligned, contributing to physiologic homeostasis. Conversely, eating (or sleeping or getting light exposure) at the wrong times causes misalignment and dysfunction.

The implication, then, is that we can use what we know about the body’s natural rhythms to figure out the best and worst times to eat, and the consequences of getting it wrong. That’s chrononutrition.

So What ARE the Right and Wrong Times to Eat?

There are few things that scientists agree on, but I bet you’d be hard-pressed to find a scientist who thinks that eating at night is healthy, or even health-neutral. All the evidence from shift workers, mice, and human research subjects says eat during the day, don’t eat at night (actually, the reverse for mice since they’re nocturnal, but the point still stands).

That’s a pretty broad statement, though. We’d like to know more specifically, is it better to eat more of our calories in the morning, mid-day, or evening? Should we be loading carbs (or protein or fat) into our first meal of the day or closer to bedtime? These are exactly the types of questions chrononutrition researchers are investigating.

Observational data from epidemiological and prospective studies suggest that eating earlier in the day (i.e., eating breakfast) is associated with better glycemic control and less type 2 diabetes,³ better cardiovascular health,⁴ and less adiposity (lower body fat).⁵ Now, I know a lot of you practice time-restricted eating and frequently skip breakfast. Before you get too concerned, let me qualify this statement with some big caveats.

First, let’s remember that observational studies can’t establish causality. These findings tell us nothing about whether eating or skipping breakfast leads to better or worse health outcomes, just that they may be correlated. Only randomized controlled trials can point to causation, and that’s where these observations start to break down. RCTs looking at weight loss and cardiometabolic risk, for example, have yielded conflicting results. And two recent meta-analyses of RCTs found no consistent relationship between eating versus skipping breakfast and body composition.⁶ ⁷

Also, the participants in these observational studies represent cross-sections of the population. By and large, they do not reflect the average health-conscious Primal individual who is fat-adapted and practicing intermittent fasting for the benefits. Quite the opposite. Take a new analysis of the large NHANES database that linked skipping breakfast with greater risk for cardiovascular disease-related mortality.⁸ In this sample, people who skipped meals were also more likely to smoke, drink excessively, have poorer diet quality overall, and face food insecurity—all of which are independently associated with cardiovascular disease. The authors even state that “skipping meals, in particular skipping breakfast, might also be a behavioral marker for unhealthy dietary and lifestyle habits.”

In other words, breakfast skippers—meaning people who simply don’t eat breakfast, not people who intentionally practice time-restricted eating—have more risk factors overall compared to their breakfast-eating counterparts. How much, then, can we say that skipping breakfast is to blame for their poor health outcomes?

What Does This Mean for Skipping Breakfast?

Should you or shouldn’t you skip breakfast? At this point, it’s hard to say for sure. It’s still the early days of chrononutrition, much too soon to crown breakfast the most important meal of the day.

That said, the evidence is already pretty solid that humans are more insulin sensitive in the morning. Folks with insulin resistance and type 2 diabetes might therefore have an easier time controlling blood sugar if they load more of their carbs, and perhaps more of their total calories, earlier in the day. Alternatively, if you’re going to ingest more carbs in the afternoon, try to time them around exercise to take advantage of insulin-independent glucose uptake.

For everyone else, I’d say continue to do what feels right to you, but be open to experimenting. It doesn’t hurt to try switching up your eating window if you’re currently skipping breakfast and still dealing with high fasting blood sugar, poor energy during the day, or other stubborn health issues.

I’m open to the possibility that as more human studies roll in, we may find that there are some advantages to an earlier eating window for just about everyone. Or we may find that it doesn’t really matter whether you eat breakfast as long as you aren’t eating too late. If skipping breakfast means your eating window gets pushed back, so you’re eating large meals close to bedtime, that may be the bigger problem.

Ultimately, the answer probably won’t be simple. The best and worst times for any given individual to eat are almost certainly a function of genetic predisposition, lifestyle factors (what is most feasible and least stressful), personal preference, and extant health. And, I expect, meal and macronutrient timing will always be farther down the list of things to worry about than what we eat and how much.

Self-experimentation is Still the Best Answer

If the epidemiological data have you feeling a little unsure about your breakfast skipping ways, by all means, go ahead and see what happens if you start eating breakfast. Maybe you’ll notice a big difference. Or you won’t, and you can go back to skipping breakfast if you so desire.

The one caveat here is that research also suggests that consistent meal times are important for circadian rhythm health. I wouldn’t recommend skipping breakfast one day, skipping dinner the next day, and then eating from 8 a.m. to 8 p.m. on day three. Pick a schedule and stick with it for, say, a month (a length of time I picked somewhat arbitrarily). Then try the other eating window for the same amount of time, and compare.

See if you notice any differences and how you feel, look, or perform in your workouts. Which one is easier for you given your work and family obligations? Importantly, is your sleep quality improved on one versus the other? Maybe you’ll even want to check blood markers and see how lipids or insulin (HbA1c) are affected.

If you feel and perform best skipping or eat breakfast, that’s your answer.

What’s your n=1 data? Have any readers had good results from going back to eating breakfast after a period of skipping? How about the opposite?