Circadian rhythms are physical, behavioral, and mental changes that follow a 24-hour cycle.
They oversee the daily rise and fall of hormone levels, body temperature, metabolism, and much more.
Your circadian rhythm helps your body predict what will happen next and prepare for it.
These cycles are controlled by “clocks,” which scientists have spotted in the cells of nearly all your organs and tissues. And these clocks are governed by a “master clock” in your brain.
But what keeps the master clock in check? The main signal that “trains” the central clock is light.
Dedicated receptors in our eyes send information to your central clock to let it know when it’s light — it does this even when your eyes are closed.
Where does nutrition come into it?
Chrononutrition is the science of how food, metabolism, meal timing, and your body clock interact.
Your circadian rhythm influences many aspects of your physiology, including digestion, hunger, and metabolism.
But the relationship between food and your body clock is a two-way street:
Your circadian rhythms influence how your body handles food at different times of the day, but eating also helps train your body clock (although to a lesser extent than light).
Among other questions, chrononutrition researchers are trying to understand the impact of time-restricted eating, fasting, how regularly you eat, what time you eat, and how your body responds to food at different times of the day.
In particular, they are interested in whether these factors influence metabolic health and the risk of developing obesity and type 2 diabetes.
Here, we’ll dip our toes into some of the ongoing research.
Looking at rodents
Much of the research into chrononutrition involves animals. It’s difficult to apply the findings to humans, but they do offer some clues about how food and circadian rhythms might be linked.
For instance, when scientists provide animals access to food during a short window of time each day, their body clock adjusts.
After around 1 week on the same schedule, researchers can measure changes in the animals before feeding time.
Firstly, they become more physically active 2–3 hours ahead of their meal time. Scientists call this food anticipatory activity (FAA).
Alongside the increased activity, the animals’ body clock prepares it for the coming meal — their body temperature rises, hormone levels change, and levels of digestive enzymes increase.
If the scientists then change the time that they feed the animals, FAA will still continue at the same time every day until their body clocks eventually adjust to the new schedule.
When do you take in energy?
In many Western countries, we tend to eat our largest meal toward the end of the day. So, chrononutrition researchers are trying to understand whether it matters what time you consume most of your energy.
One group of scientists followed 420 people while they embarked on a 20-week weight-loss plan. The researchers split the groups into those who ate their main meal earlier and those who ate it later.
They found that those who ate their main meal earlier in the day lost more weight than the late eaters, even though both groups consumed similar amounts of energy.
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At ZOE, we know that calorie counting isn’t the best way to manage your weight. Your whole diet is what counts. Focusing on good quality fresh foods is more important than adding up calories.
Other factors beyond diet are also important, such as exercise and sleep.
What makes this study particularly interesting is that both groups ate similar types of food, had similar levels of activity, and slept for a similar amount of time.
Although other studies have reached similar conclusions, the authors call for more research. Still, this is an intriguing finding.
There are significant challenges in studying chrononutrition in humans at the population level.
People lead different lives and have individual lifestyles. Our biology varies, too (more on that in the next section).
As an example, the authors of a review on the timing of food intake and obesity explored global eating patterns.
They found that in Eastern Europe, people consume roughly equal amounts of calories for breakfast, lunch, and dinner.
In North America and Northern Europe, people eat significantly more calories at dinner than either breakfast or lunch. And in Southern Europe, lunch tends to be the most energy-dense meal of the day.
To add to the complexity, there’s variability within these regions, too.
Overall, in agreement with the weight-loss study above, the authors of the review concluded that eating a larger meal later in the day appears to be associated with an increased risk of obesity.
However, they also explain that scientists need to carry out more research to confirm these findings. Chrononutrition is still a young science.
When your body breaks down a meal, sugars enter your blood as glucose. Your body responds to this rise in blood sugar by releasing insulin.
Insulin lowers blood sugar levels by helping glucose enter cells where it’s used as energy.
Scientists have shown that our body’s ability to control blood sugar varies across the day.
On average, blood glucose responses appear to be better in the morning than in the afternoon.
In fact, as part of ZOE’s PREDICT1 study, which involved more than 1,000 participants, we investigated this question.
The researchers gave participants an identical meal at either breakfast or lunch. They found that, on average, when a participant ate the meal for lunch, their glucose response was two times higher than when they ate the meal for breakfast.
However, the authors note that there was “wide individual variation.”
What about fat?
Similar to blood glucose levels, after eating, levels of fat in the blood rise. However, they rise and fall much slower than blood glucose, taking hours to peak and return to normal.
Evidence suggests that our blood fat responses also change across the day.
For instance, a study on healthy men found that their blood fat response was more pronounced after lunch than after breakfast — even when the two meals were identical.
Other research has shown that blood fat responses are poorer when males eat a meal at night than during the day. However, this study didn’t see the same effect in females.
ZOE runs the largest ongoing nutrition science study. As part of this, we’ve shown that everyone’s response to food differs — even identical twins can respond differently to the same foods.
Chrononutrition adds an extra level of complexity: It’s not just what you eat, it’s when you eat it and how your unique biology responds. That’s why we believe that personalized nutrition is the future.
What should you do?
We’ve only managed to scratch the surface of the wildly varied and complex world of chrononutrition. However, it seems pretty clear that there’s a lot going on.
Researchers are working hard to tie loose ends together, but there’s a great deal of work to be done.
When you eat your food might influence how your body handles it. And how your body changes across the day might affect what and when you eat.
Experts believe that as our understanding of chrononutrition develops, we can use this knowledge to help reduce the risk of metabolic disease. But, for now, we have to wait for the data to roll in.
At ZOE, we know that every body is different. There’s no one-size-fits-all approach to nutrition. We can help you understand how to eat for your body. Chrononutrition is just another piece of the already complex puzzle.
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