Photo/Illutration Participants in the Shibare Festival in Rikubetsu, Hokkaido, spend time on Feb. 1 in the event’s main attraction, where they are challenged to endure a frigid day in a row of ice domes. (Chieko Hara)

Children conceived during the colder months of the year are less likely to become overweight than their siblings who are conceived in warmer weather, according to researchers.

Their findings indicate that higher temperatures stemming from climate change may, conversely, contribute to an increase in the number of individuals vulnerable to obesity.

Environmental stimuli to which parents are exposed at low temperatures appear to be transmitted to their offspring through reproductive cells, enabling their children to develop a constitution that more easily burns fat off as heat.

The findings of a team of scientists from Tohoku University and other institutes were published in the specialized journal Nature Metabolism at (https://www.nature.com/articles/s42255-025-01249-2).

The team focused its research on brown fat, because the substance is thought to prevent becoming overweight by consuming fatty acids and other chemicals to generate heat.

With the consent of 356 adult men living in Japan's northernmost main island of Hokkaido, the team examined the function of brown fat via diagnostic imaging.

The scientists simultaneously calculated the dates of fertilization of sperms and eggs based on information on the subjects’ birth delivery times.

That way, the research team determined whether the birth and fertilization timing of individuals with active brown fat function fell within the cold period from mid-October to mid-April or during the remaining warmer span in the year.

The results revealed that the participants whose fertilization was estimated to have occurred in a colder climate had active brown fat in 78.2 percent of cases, compared with 66.0 percent for those conceived at higher temperatures. The difference between the two groups was statistically significant.

In the meantime, no correlation was detected between the subjects’ birth dates and brown fat activity.

The research team similarly tested another method other than diagnostic imaging on 286 adult men and women in and around Tokyo for brown fat analysis. The subjects conceived during the cold span turned out to have a higher density of brown fat, too.

Additionally, the overall energy consumption for people conceived in the shivering period proved to be higher, after statistical adjustments were made to rule out the effects of their physique and physical activity.

Takeshi Yoneshiro, an associate professor of medicine at Tohoku University, noted that the degree of obesity and the amount of visceral fat were lower among individuals fertilized during the colder months of the year. 

All these results were mostly attributed to not only low readings prior to the day of their insemination but also large daily temperature fluctuations around that time.

Parents' exposure to frigid climate and temperature fluctuations is believed to stimulate reproductive cells, so that their offspring can “withstand the cold.” This biological message is likely transmitted through reproductive cells to the fertilized eggs and subsequently to their newborns.

As a result, the newly born kids are more unlikely to develop obesity due to their enhanced ability to burn fat and thereby raise their body temperature, in the face of freezing atmospheric conditions.

As climate change is expected to reduce the number of cold days and mitigate daily temperature fluctuations, the team stated that it may be increasingly difficult at some point for babies to benefit from this important message that helps prevent obesity.

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