Fatty snacks in winter may trick the body into packing on the pounds

Next time you're tempted to raid the pantry for snacks loaded with saturated fat - especially in winter - you might want to consider that the result could be cravings for more high-calorie nibbles.

That's because, from the body's perspective, saturated fat - which includes the hydrogenated fats found in many snacks - signals the approach of summertime, when food is naturally plentiful and bulking up for the lean winter months is important.

Until recently, scientists thought that mammals get their cues on how much to eat only from the length of the day. Black bears, for example, gorge on berries and nuts during the lengthening days of spring and summer but fast during their long winter hibernation. But a new study from UC San Francisco shows that it may have as much or more to do with the balance of saturated and unsaturated fats in their diet.

The researchers discovered that saturated fat affects the behavior of a protein called PER2, which orchestrates fat metabolism and circadian rhythms. Depending on how much saturated fat is being consumed, this protein may either tell the body to burn fat or store it.

As summer approaches, plants begin producing more saturated fat. In mammals, the saturated fat signals the season of abundance and PER2 encourages the body to store energy that it will use in winter, when food is scarce.

As autumn approaches, plants produce more unsaturated fat, which helps them function better in chilly conditions. Eating more of this type of fat - instead of the saturated variety - signals that summer is coming to an end. As food supplies dwindle, PER2 prepares the body to tap into its fat stores.

"It makes a lot of sense that both nutrition and the length of the day would guide seasonal behavior," said Louis Ptacek, M.D., professor of Neurology and a senior author of the study, noting the example of hibernating bears. "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, which was done in mice, is the first to investigate the role of nutrition in how mammals adapt with the seasons. The findings appear Oct. 23 in Science and could inspire new approaches to treating obesity and type 2 diabetes. The research was supported in part by the National Institutes of Health (NIH).

Fat matters

Ptacek and co-senior author Ying-Hui Fu, Ph.D., have studied PER2 since their 2001 discovery that the gene and its protein control our 24-hour sleep-wake cycles. About a decade later, PER2 was found to play a role in fat metabolism as well, leading researchers to suspect that our biological clocks do more than keep time.

The team decided to investigate the combined impact of fat and light on mice. The researchers simulated the cycle of the seasons by exposing the mice to 12 hours of light and dark each day to approximate the fall and summer equinox; then, they shifted to 20 hours of light to simulate summer, and 20 hours of dark for winter.

Mice that ate a diet that was not too high in fat or calories adapted easily to the seasonal changes. As the nights got longer, they began to run on their exercise wheels as soon as it got dark, which is normal behavior for a nocturnal mouse.

In contrast, mice that ate a high-fat diet were unable to wake up and start running until a few hours after darkness set in.

Next, the team compared the effect of food rich in unsaturated fats, like those found in seeds and nuts, to food containing hydrogenated fat like that found in processed foods. The mice that ate more hydrogenated fat did not adjust well to the extended dark of winter and did not wake up and start running on their wheels until many hours after the start of night.

"These types of fats seem to prevent mice from being able to sense the early nights of winter," said postdoctoral scholar Dan Levine, PhD, who brought his interest in seasonal cycling to Ptacek's lab. "It begs the question of whether the same thing is happening for people snacking on processed food."

Electric light and constant calories

Hydrogenated fats aren't the only aspects of modern life that can disrupt seasonal rhythms. Electric lighting provides light all year long, and grocery stores make food abundant year-round.

Because we've evolved to eat a lot in the summer and store it to live on in the winter, "Eating a lot of food becomes maladaptive when there's no escape from temptation," Levine said.

Disruptions in biological rhythms have been linked to sleep disorders, obesity, diabetes, and mental health problems, and the researchers said the same is likely true for out-of-whack seasonal rhythms. Correcting these imbalances could improve sleep and energy levels, prevent chronic disease, and help shift workers and people with jet lag.

Levine advised resisting the urge to graze on fatty treats, especially in winter.

"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.