Your body keeps track of time in ways you never really notice. Beyond sleep and wake cycles, it responds to seasonal cues. New research suggests dietary fat plays a role in that timing, acting as a biological signal that helps the brain interpret the season. Certain processed fats appear to interfere with that process.
The study, led by researchers at the University of California, San Francisco, and published in Science, examined how mice adjust to seasonal changes in light. When lighting shifted to mimic winter, some mice adapted quickly. Others adjusted more slowly, maintaining higher body temperatures and delayed daily rhythms, patterns associated with summer physiology.
The difference wasn’t calorie intake. It was fat composition.
In natural environments, food sources change with the seasons. During colder months, plants and animals contain higher levels of polyunsaturated fats. Those fats help tissues function at lower temperatures and also serve as a signal. Higher levels correspond with winter conditions. Lower levels align with summer, a period associated with energy storage.
When researchers fed mice diets with identical calorie counts but different fat ratios, the outcomes diverged. Mice consuming fewer polyunsaturated fats took roughly 40 percent longer to adapt to winter lighting schedules. Their internal clocks lagged. Under summer lighting, the same mice adjusted more quickly, consistent with a physiology expecting abundance.
The researchers traced this response to a molecular switch in the hypothalamus, a brain region involved in metabolism and circadian timing. That switch responds to nutrient signals and influences how cells process fats and regulate body temperature. Diets low in polyunsaturated fats altered the switch’s activity and changed the expression of hundreds of genes tied to fat signaling.
To confirm the mechanism, the team studied genetically modified mice in which the switch couldn’t activate. Those animals adjusted to seasonal lighting at the same rate regardless of diet. In unmodified mice, adjustment speed varied based on fat type.
Food processing amplified the effect. When researchers compared natural corn oil with partially hydrogenated corn oil, the processed version eliminated the seasonal signal. Hydrogenation alters fat structure for shelf stability. In doing so, it removed the chemical cues associated with winter fats.
Humans share the same biological pathway. People with rare mutations affecting it develop sleep timing disorders, indicating that the circuitry exists in humans. Whether dietary fat influences human seasonal rhythms in the same way remains untested.
The authors emphasize that this research doesn’t translate directly into dietary advice. It does, however, raise a narrower point. Modern diets deliver altered fat profiles year-round, regardless of season. That constant signal may affect how internal clocks interpret time. For now, the finding adds another variable to how bodies respond to food in environments where seasons no longer shape what ends up on the plate.
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