Our blood pressure rises in the morning, our brain releases sleep hormones before bedtime, and our body temperature drops during sleep. These are all regulated by our internal clock – the human circadian rhythm – which ensures that bodily functions are timed to meet bodily needs at the right time of day.
The internal clock uses daylight to synchronize and anticipate daily bodily routines and to adjust to the changing length of days across seasons.
Most organisms have a biological clock in their cells, synchronized by the brain. Its function is to regulate, coordinate and predict the regular variations in sleep, body temperature, food intake, behavior and activity in accordance with the circadian rhythm.
The expression of around 30% of all our genes are regulated by the biological clock. Disturbances to the circadian rhythm, for example due to shift work, predispose to a number of diseases, such as cardiovascular diseases, cancer, depression and diabetes.
All organisms living on the surface of the Earth are affected by varying day lengths depending on the seasons. It has long been believed that the length of daylight was the only factor that adjusts the rhythm of our biological clock to the seasons.
Source: Professor Thomas Mandrup-Poulsen
Now, new research from scientists at the University of California San Francisco and the University of Copenhagen shows that our diet may also play a crucial role in the biological clock’s ability to adapt to summer and winter.
In a study on mice, researchers examined how different types of fats affected their internal clock.
“It has long been believed that only changes in the length of daylight regulate the adaptation of our biological rhythms to the change of seasons,” says Professor Thomas Mandrup-Poulsen from the Department of Biomedical Sciences at the University of Copenhagen, who contributed to the study recently published in the journal Science.
“But in our study, we found that mice fed a ‘summer diet’ – high in calories or low in polyunsaturated fatty acids – adapted more quickly to a simulated ‘summer’ diurnal rhythm, where such diets are naturally prevalent, but more slowly to ‘winter’, when diets are typically low in energy and rich in polyunsaturated fats,” he adds.
For example, mice on the summer diet adjusted more quickly to waking up and being active at the appropriate time during the simulated summer.
It may be advantageous for the mouse with multiple signals helping the biological clock adapt to the seasons – for example when food is scarce.
“If a harsh winter is followed by an extremely dry spring with poor plant growth, the mouse will not encounter the usual increase in availability of foods with high energy density and saturated fat. The biological clock picks up on this signal, telling the mouse to wake earlier to spend more time searching for food to compensate for lack of food,” says Thomas Mandrup-Poulsen.
Humans also have an internal clock that functions similarly to that of mice. This raises the question of whether modern dietary patterns adversely affect our circadian rhythms.
Unlike mice, most people have access to the same types of food year-round, and we tend to eat energy- and fat-rich foods – as if it were summer all year round.
Fat is an essential nutrient for the body and a key source of energy. However, excessive fat intake increases the risk of cardiovascular diseases, among other conditions. Fats are categorized as saturated and unsaturated.
Saturated fat raises cholesterol levels and promotes atherosclerosis and blood clots, while unsaturated fat generally has the opposite effect. Saturated fat is found in dairy products and meat, while unsaturated fat is primarily found in fish and plants.
Unsaturated fat can be further divided into monounsaturated and polyunsaturated fatty acids. Monounsaturated fats are found in olive oil, nuts, avocados, seeds, and kernels. Polyunsaturated fats are found in fish, as well as in nuts, avocados, and plant oils.
Source: The Danish Heart Foundation
Researchers believe this could create a mismatch between the seasonal light signals our internal clock receives from the sun and the signals received from our diet.
“Our work shows that seasonal variation in the availability of polyunsaturated fatty acids relative to saturated fatty acids in the diet affects signals to the clock, including the phosphorylation of PER2. This could mean that consuming foods low in polyunsaturated fatty acids year-round could lock people into a persistent ‘summer circadian rhythm’,” says lead author Daniel Levine, postdoctoral fellow at the Department of Neurology, University of California San Francisco.
“During winter, a mismatch may occur between shorter light exposure and food intake that signals summer. This desynchronization of two key stimuli for the internal clock could be problematic,” adds Thomas Mandrup-Poulsen.
It’s well known that disruption of the biological clock – such as shift work – increases the risk of various lifestyle diseases, explains Thomas Mandrup-Poulsen. And the mismatch between dietary and light signals may lead to a kind of biological jetlag.
Biological jetlag could contribute to disrupted appetite regulation, potentially leading to obesity and subsequently metabolic, cardiovascular, and psychiatric disorders.
“If our body, due to our diet, thinks it’s summer when it’s actually winter, we may consume and store more energy – like if a squirrel could hoard food year-round. This biological jetlag could contribute to disrupted appetite regulation, potentially leading to obesity and subsequently metabolic, cardiovascular, and psychiatric disorders,” he says.
The next step is to investigate whether consuming more seasonal foods with energy density and fat composition determined by the season can improve our biological clock’s seasonal adaptation.
“Mice are forced to eat local and seasonal food, but humans can go to the supermarket year-round and buy energy-rich food from all over the world. What would happen if supermarkets only offered seasonal, locally grown foods – like the Mediterranean diet or the new Nordic cuisine? We’d likely be more in sync with the food we consume and the light we experience. And it would even be more environmentally sustainable,” says Thomas Mandrup-Poulsen.
Read the study ”Unsaturated fat alters clock phosphorylation to align rhythms to the season in mice”.
The researchers fed mice food with different energy densities or different ratios of saturated to unsaturated fatty acids. They found that mice fed an energy-dense diet with many calories or a diet low in polyunsaturated fatty acids adapted more quickly to the “summer” that the researchers had simulated with light, but more slowly to the “winter”.
Researchers discovered that changes in the mice’s biological clock adaptation to the “seasons” were due to alterations in the phosphorylation of a central clock component called PERIOD2 (or PER2) in the hypothalamus of the brain. Phosphorylation is a mechanism that switches various cellular processes on and off.
The altered phosphorylation of PER2 led to changes in the hypothalamus’s conversion of polyunsaturated fatty acids into a specific type of signaling molecules called oxylipins, which contributed to the biological clock’s seasonal adaptability.
The study was conducted as a collaboration between researchers at the University of Copenhagen and researchers at the University of California, San Francisco, who led the research effort.