Fatty foods may trick your body into thinking it’s a different season

Fat doesn’t just add flavor – it also sends powerful signals to your body. Scientists have found that when we eat foods high in saturated fat, our bodies react as if it’s summertime – the season of abundance – and start storing fat for the months ahead.

For animals, summer is the time to bulk up before food becomes scarce in winter. That rhythm once helped mammals survive. But in modern life, with endless light and food, that ancient system might be working against us.

Fatty foods and the body clock

Researchers at UC San Francisco recently discovered that saturated fat influences how a protein called PER2 behaves. This protein acts like a switchboard for fat metabolism and the body’s internal clock.

When saturated fat levels rise, PER2 encourages the body to store energy – a signal that would make sense in the wild when summer brings more food.

When unsaturated fat dominates, PER2 shifts gears and helps the body burn stored fat, mirroring the energy-scarce months of fall and winter.

As the study explains, plants also change their fat makeup through the seasons. They produce more saturated fats in warmer months and more unsaturated fats when temperatures drop.

Mammals eating those plants pick up on these natural cues, which then guide their own seasonal behavior.

Seasonal behavior and diet

“It makes a lot of sense that both nutrition and the length of the day would guide seasonal behavior,” said Dr. Louis Ptacek, professor of Neurology and a senior author of the study.

“If it’s fall and there are still plenty of nuts and berries to eat, the bear might as well keep eating rather than settle in for winter sleep, even while it senses that the days are getting shorter.”

The study was conducted in mice and supported in part by the National Institutes of Health.

While the findings are preliminary for humans, they offer new insights into how modern diets might be confusing the body’s natural rhythms – and possibly contributing to conditions like obesity and type 2 diabetes.

Tracking seasonal changes

Dr. Ptacek and co-senior author Dr. Ying-Hui Fu have spent years studying PER2. In their previous work, they demonstrated that this gene is important to the body’s 24-hour sleep-wake cycle.

Recently, the protein also been linked to the way the body processes fat, which prompted the researchers to investigate further the connection among light, diet, and metabolism.

In their recent research, the team simulated seasonal changes in mice by varying the duration of light each mouse was exposed to daily.

The researchers simulated 12 hours of light to represent fall and spring, 20 hours of light for summer, and 20 hours of darkness for winter.

Fatty foods disrupt seasonal rhythms

Mice fed on moderate-fat foods easily adjusted to these changes. When nights became longer, they grew nocturnal and exercised on their wheels, true to their natural tendencies.

But those on high-fat diets struggled. They delayed activity, unable to sense the changing length of nights.

When the researchers compared different fat sources, the results grew clearer. Mice fed hydrogenated fats, the kind found in many processed snacks, were slow to respond to winter’s extended darkness.

“These types of fats seem to prevent mice from being able to sense the early nights of winter,” said postdoctoral scholar Dr. Dan Levine. “It begs the question of whether the same thing is happening for people snacking on processed food.”

The modern mismatch

Our ancestors’ eating habits were guided by daylight, temperature, and food availability. Today, artificial lighting and endless grocery store shelves erase those natural cues. We can eat and stay active long past sunset – a lifestyle that our bodies never evolved for.

Hydrogenated fats and constant exposure to light disrupt more than seasonal rhythms. They can throw off the same biological cycles that control sleep, appetite, and energy.

Over time, this imbalance may contribute to metabolic and mental health problems. “Eating a lot of food becomes maladaptive when there’s no escape from temptation,” said Dr. Levine.

Avoid fatty foods to restore balance

Researchers say resetting these rhythms – by limiting processed foods, getting enough natural light, and eating in sync with the day-night cycle – could help restore balance. Better alignment may improve energy, sleep, and overall health.

As Levine put it, even one indulgence can start the pattern: “That one holiday cookie could turn into two cookies the next day, because you’ve now tricked your circadian clock into thinking it’s summer.”

The full study was published in the journal Science Translational Medicine.

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