How the Season You Were Conceived May Shape Metabolism and Brown Adipose Tissue

The time of year in which you were conceived may shape how your metabolism functions in adulthood. In the largest research effort of its kind, scientists report that people conceived during colder periods appear to store fat differently from those conceived in warmer periods.

Season of conception and brown adipose tissue (BAT)

Working with 356 healthy young male volunteers, a research team in Japan observed that participants conceived in a cold season tended to show comparatively higher brown adipose tissue activity. Brown adipose tissue is an energy-burning fat that helps keep the body warm and supports blood sugar regulation.

Alongside this elevated brown fat activity, the same group also demonstrated higher energy expenditure, a lower body mass index (BMI), and reduced fat build-up around internal organs - a pattern consistent with better overall metabolic health.

By contrast, in people classed as overweight or obese, brown fat activity is frequently reduced.

Extending the findings beyond one group

To see whether the pattern might hold more broadly, the researchers also analysed a second cohort comprising 286 adults (men and women) spanning a range of ages. In this wider sample, they ultimately reported "modest yet significant associations" between being conceived in cold seasons and brown fat activity, together with lower BMI, a smaller visceral fat area, and a reduced waist circumference.

From their modelling, the team believes increased brown adipose tissue (BAT) activity is likely the factor underpinning the other metabolic differences. BMI alone, for example, did not show a direct link to the season of conception.

These results are correlational rather than proof of cause and effect. Even so, they reinforce and expand on other observational work suggesting that the time of year someone is born could influence health outcomes later in life.

Notably, this study did not identify a meaningful association between an individual’s birthday and their metabolic health. Instead, the relationship appeared tied to the season in which conception occurred - 266 days prior to birth.

What the Japan study measured and how

In the Japanese study, the male participants were exposed to a cool environment of 19 °C (66 °F) for 2 hours. The researchers then evaluated both brown fat activity and metabolic activity after the cold exposure, and also after the participants had been seated at room temperature.

Brown fat activity was clearly higher among those conceived in the Northern Hemisphere during the cold windows of January 1 to April 15 and October 17 to December 31, compared with those conceived during the warmer interval from April 16 to October 16.

The team also looked at real-world weather conditions around the time of each birth, and found a comparable association centred on the conception period when examining daytime temperatures.

"A major strength of our study is the thorough assessment of BAT using the gold-standard method and a large sample size of healthy participants," the authors write.

"Our well-designed approach and the largest sample size in this field allow us to certify the intergenerational influence of cold stress on BAT activity in humans."

How cold exposure before conception might influence offspring

The findings are consistent with the idea that cold conditions could alter gene expression in male sperm or female eggs, with those alterations potentially being transmitted to children when the two germ cells meet and fertilisation occurs.

According to the authors - led by biomedical scientist Takeshi Yoneshiro of the University of Tokyo - this could represent a "sophisticated predictive cold adaptation" that is passed between generations and may help offspring cope better in colder climates.

Further studies will be required to test and develop this idea, which the researchers describe as "Pre-fertilization Origins of Health and Disease".

Support for the broader concept comes from earlier mouse work indicating that exposure to specific weather conditions before conception can boost offspring metabolism, potentially through epigenetic switches in the father’s sperm.

When the authors of that earlier study translated their results to humans, they reported that people conceived during cold months were 3.2 percent more likely to have active brown fat tissue, while those conceived during warmer months were more likely to lack active brown fat.

The new work from Japan builds on and extends those earlier results.

Yoneshiro and colleagues also argue that truly explaining these patterns will require a stronger grasp of how cells retain and pass on “memories” across generations - and how environmental influences such as cold exposure, exercise, or diet might affect sperm, eggs, and ultimately the next generation.

"Indeed," writes Helmholtz Munich epigeneticist Raffaele Teperino in an independent review of the research, "parental health and exposure to environmental challenges at conception, and maternal health and exposures during gestation and breastfeeding, are emerging as key determinants of offspring health and prenatal risks of complex, noncommunicable diseases."

The study was published in Nature Metabolism.