Winter Snacks May Trick Your Body Into Summer Mode

That handful of chips or stack of cookies you’re reaching for on a cold winter evening could be sending your body a confusing message: it’s time to bulk up for the lean months ahead. The only problem? Those lean months are already here.

A new study from UC San Francisco reveals that mammals, including mice and possibly humans, don’t just rely on daylight to tell what season it is. They’re also reading chemical signals from their diet, particularly the balance between saturated and unsaturated fats. And when modern processed foods pump us full of the wrong fats at the wrong time of year, our internal clocks get scrambled.

The research, published in Science, centers on a protein called PER2 that orchestrates both our daily sleep-wake cycles and how we metabolize fat. Scientists have known since 2001 that PER2 keeps our 24-hour rhythms ticking. What they’re learning now is that this same protein reads nutritional cues to figure out what season it is, and then tells the body whether to store energy or burn it.

Plants Telegraph the Seasons Through Fat

Here’s how it works in nature: As summer approaches, plants begin producing more saturated fat. Mammals eating those plants get a chemical signal that abundance is near, and PER2 flips a metabolic switch. Time to store calories for the hard times ahead. But when autumn rolls around, plants shift their chemistry, producing more unsaturated fats to help them withstand cold temperatures. Animals picking up on this change prepare to tap into their reserves.

Louis Ptacek, a professor of neurology at UCSF and senior author on the study, points to hibernating bears as a vivid example of this system at work. “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,” he said.

But what happens when the food supply never dwindles and the fat composition never changes? That’s the experiment modern humans are running on themselves every day.

The research team simulated seasonal changes in the lab by adjusting light exposure for mice: 12 hours of light and dark for the equinoxes, 20 hours of light for summer, and 20 hours of darkness for winter. Mice eating a normal diet adapted smoothly. As nights lengthened, they’d hop on their running wheels right at the start of darkness, exhibiting typical nocturnal behavior.

Hydrogenated Fats Jam the Signal

But mice fed high-fat diets, especially those rich in hydrogenated fats like the kind found in processed snacks, couldn’t adjust. When the long winter nights arrived, these mice stayed sluggish, unable to wake up and start their normal activities until hours after darkness fell. It was as if their bodies couldn’t sense the season at all.

The team dug deeper, comparing the effects of unsaturated fats from seeds and nuts against hydrogenated fats from processed foods. The results were stark. Mice eating more hydrogenated fat simply couldn’t adapt to extended winter darkness. Their wheels sat motionless for hours into the night.

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

Dan Levine, a postdoctoral scholar who led the study, suspects the answer is yes. The mechanism operates through a specific phosphorylation event on PER2, a chemical modification that acts like a molecular switch. When mice ate diets high in unsaturated fats, particularly polyunsaturated fatty acids, phosphorylation of PER2 decreased. This helped them advance their behavioral rhythms to match winter’s longer nights. But when they ate partially hydrogenated fats, phosphorylation increased, making it harder to shift into winter mode and easier to slide into summer patterns.

The implications stretch beyond mice in laboratory cages. Humans evolved in environments where food abundance and composition varied dramatically with the seasons. Our bodies expect to gorge in summer and draw down reserves in winter. But grocery stores and processed foods have essentially locked us into permanent summer, at least from our metabolism’s perspective.

“Eating a lot of food becomes maladaptive when there’s no escape from temptation.”

Add in electric lighting, which extends our perceived daylight hours year-round, and you have a recipe for what Levine calls “seasonal misalignment,” a chronic state where our internal clocks never sync with our actual environment. This misalignment has been linked to sleep disorders, obesity, diabetes, and mental health problems.

The finding could open new approaches to treating metabolic diseases. If researchers can figure out how to manipulate the PER2 phosphorylation switch, they might help reset circadian rhythms in shift workers, ease jet lag, or even develop interventions for obesity and type 2 diabetes.

For now, though, Levine offers simpler advice: resist the winter snack attack. “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,” he said. Your body might be preparing for a long, cold winter, but that bag of chips is telling it summer camp is just getting started.

Science: 10.1126/science.adp3065