Researchers at UCSF in San Francisco say the types of fat on your plate can act like a seasonal calendar for the brain, and heavily processed fats may keep the body stuck in a summertime state. In lab mice, diets rich in hydrogenated or saturated processed fats slowed the animals' transition to winter-like nights and increased nighttime body temperature. The team reports that the work adds a dietary twist to how light and food interact to shape sleep and metabolism, although it remains unclear whether people respond in the same way.
The team, led by Ying-Hui Fu and Louis J. Ptáček, published the findings in a paper in Science. The authors trace the effect to phosphorylation of the clock protein PERIOD2 (PER2) in the hypothalamus, a molecular switch that helps set both daily and seasonal rhythms. Their experiments tracked running-wheel activity and core body temperature while mice were exposed to light cycles that mimicked long summer days and short winter days.
What the mice showed
When researchers swapped regular corn oil for partially hydrogenated corn oil, the mice behaved as if the nights were still short. They started their nightly running hours later and kept warmer body temperatures, signs that the animals were effectively stuck in a summer state. As described by UCSF News, the processed oil wiped out the winter signal in the brain and increased PER2 phosphorylation in the hypothalamus.
Genetic tests backed up the causal link. Mice engineered so that PER2 could not be phosphorylated at serine 662 did not change their seasonal adjustment when fed different fats, showing that this chemical switch is required for the diet effect. The team also found that calorie restriction and brief fasting shifted the switch toward a winter-like timing and moved daily activity schedules earlier. Media summaries note that mice on lower-PUFA diets adjusted to winter about 40% more slowly than those on higher-PUFA diets. For more details, see the University of Copenhagen and a lay summary at StudyFinds.
Processed fats and your pantry
Partial hydrogenation, the industrial process that produces trans fats, is the kind of change the lab used to mimic processed foods, and U.S. regulators have been trying to push those oils off the menu for years. The FDA declared partially hydrogenated oils no longer GRAS in 2015 and set compliance deadlines for industry reformulation. At the same time, many packaged foods tweak natural fat ratios for shelf life and texture, a reality that science writers say could let modern, year-round diets send scrambled seasonal signals to the brain. See the FDA's Federal Register notice and coverage in BBC Science Focus.
Limits and takeaways
The authors stress that this was a mouse study using male animals from a single genetic strain, and that laboratory lighting and diets differ from everyday human life, so drawing direct conclusions for people is premature. The paper and related press materials note that the same molecular pathway exists in humans, but there are still no direct tests showing that dietary fat composition changes seasonal timing in people, so more human research is needed.
For now, the local message from the lab is fairly modest: more whole foods and fewer industrially processed fats may help keep internal timing in sync. As postdoctoral scholar Dan Levine told UCSF News, "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 team says more work is needed to find out whether the mouse results map onto human sleep, metabolism, and weight patterns.
