Your season of conception could shape your body’s calorie-burning ability

By Vijay Kumar MalesuReviewed by Susha Cheriyedath, M.Sc.Apr 8 2025

Could a winter conception shape your metabolism for life? A Japanese study reveals that exposure to cold before conception may ‘pre-program’ your body to burn more calories decades later.

Study: Pre-fertilization-origin preservation of brown fat-mediated energy expenditure in humans. Image Credit: Victor Josan / Shutterstock

In a recent study published in the journal Nature Metabolism, researchers in Japan explored whether exposure to colder temperatures before conception boosts brown fat activity and lifelong energy expenditure in humans.

Background

Why do some people seem to stay slim despite eating more? A surprising factor might be the weather before they were even conceived. Brown adipose tissue (BAT), or brown fat, helps our bodies burn calories by generating heat, especially in cold conditions. It is more active in babies and declines with age, but not for everyone. In mice, cold environments before pregnancy can "pre-program" fat-burning traits in offspring. Could the same be true for humans? In a world facing rising obesity and climate change, understanding how early environmental cues shape metabolism could offer new tools for prevention. More research is needed to explore this link.

About the study

The present study analyzed 748 healthy adults (from an initial pool of over 900 screened) across five Japanese cohorts. Researchers estimated participants' fertilization seasons based on birth dates and standard gestational length, categorizing them as "cold" (October 17–April 15) or "warm" (April 16–October 16) based on Japan’s seasonal temperature patterns. They used fluorodeoxyglucose-positron emission tomography/computed tomography (FDG-PET/CT) to visualize brown fat activity, near-infrared time-resolved spectroscopy (NIR-TRS) to assess brown fat density, and doubly labeled water (DLW) to track total daily energy expenditure (TEE) under real-life conditions.

Participants also underwent cold-induced thermogenesis (CIT) and diet-induced thermogenesis (DIT) assessments using indirect calorimetry. These tests showed how much extra energy they burned when exposed to mild cold or after eating. Energy data were adjusted for fat-free mass and physical activity levels. Meteorological data, including outdoor temperatures and day-night fluctuations, were matched with each participant’s birth region and conception timeline. A structural equation model was used to understand how fertilization season, BAT activity, and body mass index (BMI) were linked. Researchers controlled for age, sex, body size, and other lifestyle factors. All participants were medication-free and had no known metabolic disorders.

Study results

People conceived in colder months consistently had more active brown fat in adulthood. Using FDG-PET/CT in Cohort 1, researchers found 78% of individuals from the cold-fertilization group showed detectable brown fat activity, versus only 66% from the warm group. This had nothing to do with birth season; only the time of conception mattered.

Cohort 2 confirmed the trend using NIR-TRS. Participants conceived in cold months had significantly higher brown fat density, especially in the neck and shoulder area, where brown fat is most common. This pattern was seen in both men and women in Japan, strengthening its generalizability.

In Cohort 3, researchers tested how well participants burned calories in response to cold air (CIT). Those conceived during the cold season burned significantly more energy after mild cold exposure—1.5 times more than those from the warm season, with the strongest effects observed in winter. At room temperature, energy use was the same across both groups, showing that the effect is tied to cold-induced thermogenesis, not baseline metabolism.

Cohort 4 explored energy use after eating (DIT). Again, those from the cold-fertilization group burned more calories post-meal. In Cohort 5, the DLW method showed these individuals had higher TEE in daily life, even after adjusting for physical activity and body composition.

But what does this mean for long-term health? Cohort 2, which included adults of all ages, showed that cold-conceived individuals had lower body mass index, less visceral fat, and smaller waistlines. These benefits were linked to increased brown fat activity, as confirmed by structural equation modeling. Interestingly, in younger participants (Cohort 1: males aged 18–25), BMI differences were minimal, likely because they had not yet experienced age-related fat gain.

What caused this effect? A deep dive into weather data found that lower outdoor temperatures and wider day-night temperature swings during the months before conception were the strongest predictors of adult brown fat activity. These patterns were not seen during pregnancy, suggesting that the critical window for this programming happens even before fertilization, potentially through hypothesized epigenetic changes in sperm driven by paternal cold exposure, a mechanism observed in mice but not yet confirmed in humans.

In other words, if your parents conceived you during a cold snap, your body might be better at burning calories and resisting weight gain. This finding may have wide-reaching implications for understanding population-level trends in obesity and energy metabolism, but requires validation in diverse global populations.

Conclusions

This study suggests that being conceived during colder months boosts brown fat activity and long-term energy expenditure, lowering the risk of obesity in adulthood. By identifying the preconception period as the key window, it highlights a novel concept: Pre-fertilization Origins of Health and Disease (PfOHaD). These insights deepen our understanding of how climate and environment shape health across generations. As global temperatures rise and obesity rates climb, this research prompts urgent questions about how early environmental exposures may influence our metabolic destiny and underscores the need for studies beyond Japan. Understanding this link could pave the way for new preventive strategies against metabolic diseases.