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Circadian rhythm

Is it really true that evening snacks are more fattening?

Next to the intestines, the liver is the first organ to meet all the nutrients we absorb from food. To function optimally, the liver must have proper working hours – otherwise the risk of becoming overweight and developing diabetes increases.

By Birgitte Svennevig, , 11/17/2021

Is it true that circadian rhythm affects how my liver works?

Your liver has a circadian rhythm, which, among other things, is controlled by a hormone called cortisol. Cortisol is also known as a stress hormone, and the adrenal glands secrete large amounts of it when the body experiences prolonged stress. But it is also a hormone that is found in the body under normal circumstances, and one of its tasks is to tell the liver whether it is time to rest or work.

The production of cortisol peaks early in the morning, just before we wake up, and when we have fasted all night. It prepares the liver to respond to this fasting condition.

This is seen not only in humans, but also other mammals such as mice. Interestingly, our new research in mice shows that cortisol also prepares the liver for the coming meal, which makes the liver ready to absorb and store glucose more efficiently.

When we have eaten our last meal of the day at dinner, the cortisol also bids us goodnight, and the liver prepares itself for fasting again – after all, you are in for a good night’s sleep. If you do choose to eat after dinner or because you wake up and get bored at night, the liver is not geared for handling more nutrients, and this is unhealthy in the long run.

Did you know your liver is not the only organ that has a circadian rhythm? Every single cell in your body has a molecular biological circadian rhythm, and together they ensure that you, as an organism, function according to the 24-hour circadian rhythm.

How do you know that eating at night is unhealthy?

Experiments with mice have shown that the liver becomes ill when the intake of high-calorie food (rich in fat and sugar) is out of sync with the normal circadian rhythm. For mice, which are nocturnal, this means they primarily consume food at night.

For example, an American study divided mice into two groups. Both groups received the same high amount of calories, but one group was allowed to eat whenever it wanted, while the other group was only fed during its active hours (i.e. at night).

The mice with free access to the high-calorie food around the clock became overweight, diabetic and had a diseased fatty liver. In contrast, the mice that ate only at night were healthy. Since the two groups received the same number of calories, this study is indicative of the importance of when we eat. Spreading one’s calorie intake around the clock is unhealthy.

What does this mean for me and my evening snacks?

The livers of mice and humans are so similar to each other that these animal studies translate well to humans. We also know from studies of people who work nightshifts that the risk of, for example, diabetes and overweight increases. These individuals eat at times when, according to their circadian rhythm, they ought to sleep.

Overall, this means that you need to ensure you adhere to your liver’s rest and working hours by following your circadian rhythm.

To elaborate, this means that you must fast during your liver’s rest period when it is not geared for receiving nutrients from food, but on the contrary is geared for fasting until morning. That means zero calories after dinner at 6-7 o’clock until the next morning. And the notion about not being able to sleep because you feel hungry, and then it’s better to get up and snack a little? It’s all in your head. Your body doesn’t need more food, so it’s just a bad habit you need to quit.

Would you like to know more?

The work of understanding the connection between circadian rhythm and weight loss is a huge international field of research.

If you are interested in seeking more scientific input, try using the search terms ‘time restricted eating’ and ‘chrononutrition’.

For example, check out this article in the journal Nutrition &Diabetes.

Or this podcast interview.

Meet the reseacher

Lars Grøntved is an Associate Professor and Head of Research at the Department of Biochemistry and Molecular Biology and ATLAS – Center for Functional Genomics and Tissue Plasticity. His research is supported by, among others, Danish Diabetes Academy, the Novo Nordisk Foundation, the Danish National Research Foundation and the Independent Research Fund Denmark.

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Editing was completed: 17.11.2021