Your Body Fat May Be Shaped by The Month You Were Conceived

The season in which you were conceived could have a surprising impact on how your metabolism works today. The largest study of its kind has found that individuals conceived in colder months store fat differently from those conceived in warmer months.

Among 356 healthy young male volunteers, researchers in Japan found that those who were conceived in a cold season showed relatively higher brown adipose tissue activity. Brown adipose tissue is a type of fat that burns energy, keeps us warm, and helps regulate blood sugar.

Along with increased brown fat activity, participants also showed increased energy expenditure, a lower body mass index (BMI), and less fat accumulation around their organs, which indicates better metabolic health overall.

In those who are deemed overweight or obese, brown fat activity is often lacking.

To extend the results to a wider population, the team of researchers considered a second cohort of 286 male and female adults of various ages. Ultimately, they found "modest yet significant associations" between conception during cold seasons and brown fat activity, along with decreased BMI, visceral fat area, and waist circumference.

Based on their models, the team suspects that the increase in brown adipose tissue (BAT) activity is what is driving the other metabolic outcomes. BMI on its own, for instance, was not directly associated with the season of conception.

The findings are only correlational, but they support and extend other observational studies, which have found that the season in which someone is born may impact their health outcomes later in life.

The current study did not find a significant link between a person's birthday and their metabolic health, but it did find a link with their season of conception, 266 days before birth.

The findings suggest that cold weather may impact the genetic expression of male sperm or female eggs, and these changes may be passed along to offspring when the two germ cells meet and fertilize.

This could be a "sophisticated predictive cold adaptation" passed through the generations, which better enables offspring to survive in cold climates, according to the authors, led by biomedical scientist Takeshi Yoneshiro from the University of Tokyo.

More research is needed to explore that concept, which the authors call "Pre-fertilization Origins of Health and Disease".

But the findings are supported by past research on mice, which found that being exposed to certain weather conditions during preconception can enhance the metabolism of offspring, possibly because of epigenetic switches in their father's sperm.

When the authors of that same study applied their findings to humans, they found that those conceived during cold months were 3.2 percent more likely to possess active brown fat tissue, whereas those conceived in warmer months were more likely to lack active brown fat.

Researchers in Japan have now extended those results. In their study, they exposed male participants to a chilly 19 °C (66 °F) for 2 hours. Brown fat activity and metabolic activity were assessed after cold exposure and after sitting at room temperature.

Brown fat activity was notably higher in those who were conceived in the Northern Hemisphere between January 1 and April 15 and October 17 and December 31, as opposed to those conceived in a warm period between April 16 and October 16.

The research team also considered real-world weather before and after each birth, finding a similar association in the conception period with 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."

Yoneshiro and colleagues argue that we need a better understanding of how cellular memories are stored and inherited to truly understand how environmental factors, like cold weather, exercise, or nutrition, can impact sperm, eggs, and 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.