Your conception time could shape your body's calorie-burning ability

Your conception time could shape your body's calorie-burning ability

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

In a study recently published in the journalNatural metabolismResearchers in Japan examined whether exposure to colder temperatures before conception boosts brown fat activity and lifetime energy expenditure in humans.

background

Why do some people seem to stay slim despite eating more? One surprising factor could 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 decreases with age, but not for everyone. In mice, cold pre-pregnancy environments can alter fat-burning characteristics in pre-program offspring. Could the same be true for humans? In a world facing rising obesity and climate change, understanding how early environmental cues could shape metabolism provide new tools for prevention. Further research is needed to investigate this link.

About the study

The diurnal temperature fluctuations (day-night differences) before conception mattered more than average temperatures—a gap of 10 °C doubled brown fat activity in the offspring.

The present study analyzed 748 healthy adults (from an initial pool of over 900 screened) in five Japanese cohorts. Researchers estimated participants' fertilization season using birth dates and standard gestational length and categorized them as "cold" (October 17 to April 15) or "warm" (April 16 to 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, time-resolved spectroscopy (NIR-TRs) to assess brown fat density, and double-labeled water (DLW) under daily energy (TEE) conditions.

Participants also underwent cold-induced thermogenesis (CIT) and diet-induced thermogenesis (DIT) 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-evening variations, were aligned 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 related. Researchers controlled for age, gender, height and other lifestyle factors. All participants were medication-free and had no known metabolic disorders.

Study results

Cohort 2 included adults aged 20 to 78 years, showing that the protective effect of brown fat against obesity becomes stronger with age and is highest in middle adulthood.

People in colder months consistently had more active brown fat in adulthood. Using FDG-PET/CT in Cohort 1, researchers found that 78% of people from the cold fat group had detectable brown fat activity, versus just 66% from the warm group. This had nothing to do with birth season; Only the time of conception was important.

Cohort 2 confirmed the trend with NIR-TRs. Participants conceived in cold months had significantly higher brown fat density, particularly in the neck and shoulder area, where brown fat is most common. This pattern was observed 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 designed 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 expenditure was the same in both groups, showing that the effect is tied to cold-induced thermogenesis, not baseline metabolism.

Cohort 4 explored post-meal energy expenditure (DIT). Here, too, those in the cold fattening group burned more calories after mowing. In Cohort 5, the DLW method showed that these individuals had higher T-shirt in daily life, even after adjusting for physical activity and body composition.

Cohort 5 followed toddlers aged 3 to 6 years and showed higher brown fatty acid activity in cold-controlled children even before puberty.

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

What caused this effect? A deep dive into weather data found that lower outdoor temperatures and broader day-night temperature swings in the months before conception were the strongest predictors of adult brown fat activity. These patterns were not observed during pregnancy, suggesting that the critical window for this programming occurs before fertilization, possibly through hypothetical sperm epigenetic changes 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 may be better at burning calories and resisting weight gain. This finding may have broad implications for improving population-level understanding of obesity and energy metabolism, but requires validation in diverse global populations.

Conclusions

This study suggests that conceived time in colder months increases brown fat activity and long-term energy expenditure and reduces the risk of obesity in adulthood. Identifying preconception as a key window highlights a novel concept: prerefinement of the origins of health and disease (PFOHAD). These findings deepen our understanding of how climate and environment shape health across generations. As global temperatures rise and obesity rates rise, this research raises urgent questions about how early environmental exposures may influence our metabolic fate and underscores the need for studies beyond Japan. Understanding this connection could pave the way for new prevention strategies against metabolic diseases.

Sources: