When you were conceived could impact your weight

Get the Popular Science daily newsletter💡

Global rates of metabolic disorders have been rising for decades. High blood pressure, high cholesterol, obesity, and diabetes have all become much more common since the mid-20th century. These metabolic disorders  now impact more than one in three Americans, in what’s widely described as an epidemic. In turn, the risk of deadly heart disease, stroke and certain cancers rises. 

While societal, genetic, and lifestyle factors all play a major role in this collectively rising risk, another surprising variable may also influence our metabolic fate. The time of year and weather leading up to an individual’s conception shifts how their body uses energy and ultimately potentially their lifelong health, per a study published April 7 in the journal Nature Metabolism.

“The season in which a person was conceived has a lasting impact on their metabolism,” lead study author Takeshi Yoneshiro, a molecular physiologist and metabolism researcher at the Tohoku University School of Medicine in Japan, tells Popular Science. “Our study suggests that the environment experienced by parents even before pregnancy can shape the child’s metabolic health later in life”

What is brown fat?

More specifically, Yoneshiro and his co-researchers found that those conceived during colder months have more brown fat, a type of tissue with well-established health benefits. 

Brown fat’s primary biological purpose is thought to be regulating body temperature. Compared with regular fat, which is mostly passive energy storage, brown fat is far more active– burning through calories to create heat. “Calories-in, calories-out” is an oversimplification, but it’s true that excess weight and visceral fat– the particularly harmful fat that collects in the abdomen around organs– are largely determined by the amount of energy we consume and the amount we burn. The idea of dieting is to minimize calories in (though often this backfires). But brown fat helps on the other side of the equation by using up more of the energy we ingest. It also may have more direct metabolic benefits by burning up the harmful byproducts of other cellular processes.   

For those conceived during colder months, an increase in brown fat also appears to correlate with higher daily energy use, lower body mass index (BMI), and less fat accumulation around internal organs, compared with their summer or spring counterparts. 

[ Related: There are two types of fat cells. Here’s how to get more of the good stuff. ]

Conception season vs. birth season

In the new study, the researchers assessed four separate cohorts, totalling more than 500 people. They used the birthdate of each participant to retroactively calculate the approximate time of their conception.That conception estimate was paired with weather data, measurements of metabolic activity, energy expenditure, BMI, abdominal fat, and brown fat levels, to see if there were any trends. 

While birth season had no direct significant effect on a subject’s metabolic health or brown fat level, the timing of their conception did, according to the results. It might seem like a contradiction, but can be explained by the fact that human gestation is 9-months long. Someone can be conceived in a cold month, but born at a much warmer time and vice versa. 

Additionally, the weather and season during in utero development showed no significant influence. This implies that the preconception period is the most critical time for determining future brown fat activity levels in offspring. Higher temperature variability (estimated as the difference between the recorded cold and high temperature for a given day) also showed a positive pre-conception impact on brown fat. 

Depending on the cohort assessed, the difference in brown fat between the warm and cold season groups was between 11.9 and 14.6 percent. Daily energy expenditure was about 5.8 percent higher in the cold conception group. 

The epigenetic factor

It can sound hard to believe–almost akin to the pseudoscience of horoscopes–that seasonal trends before you’re born could have any bearing on your health. However, thanks to decades of research, scientists now know that how your genes are expressed can be altered by environmental cues, in a process known as epigenetic modification. 

Through epigenetics, the actual DNA sequence goes unchanged, but certain genes may be deactivated or intensified as molecules differentially bond and react to particular stretches of DNA. A 2018 study in mice found that epigenetic changes in sperm cells, in response to temperature, can influence brown fat levels in offspring. 

[ Related: Scientists can now tell if you had a ‘vanishing’ twin in the womb. ]

That same 2018 study, performed by an unaffiliated group of scientists, similarly found a small but significant correlation between conception month and BMI in a single human cohort. This new research builds on those findings, by conducting additional tests on more people, assessing four independent groups, and collecting data on brown fat prevalence, energy expenditure, and visceral fat measurements to connect the dots between BMI and birth season. 

“We did a little bit of human work, but not nearly to this extent,” says Christian Wolfrum, a co-author of the 2018 paper and a molecular biologist at ETH Zurich, who was uninvolved in the new research. With the new “impressive” study, “we can now make a strong statement about humans,” Wolfrum adds. 

‘We generally look for bad things’

However,  this new study is still largely observational. Yoneshiro and his colleagues didn’t prove cause and effect, nor dig into the underlying mechanisms by which parental temperature exposure and brown fat are linked. But the authors suggest an epigenetic path, similar to what was found in the 2018 research in mice, is the most likely explanation in humans. In other words: paternal cold exposure leading up to conception is a key factor in dictating a child’s amount of brown fat activity, and ultimately metabolic health through life. 

“When we look at parental programming, we generally look for bad things,” says Raffaele Teperino, a developmental physiologist and epigeneticist at the Helmholtz Munich, a biomedical research center in Germany. Often, he says, epigenetics researchers are identifying changes that lead to disease and reduced quality of life. Teperino was not involved in the new research, but wrote an accompanying commentary on it, publishing alongside the study in Nature Metabolism. One of the most interesting things about the study in his view, is that “the authors report a preconceptual challenge which is actually favorable for the next generation.” It’s the rare instance where an epigenetic modification might offer a health benefit.

Eyes on paternal health 

The findings, now repeated across two studies, have big implications for would-be parents and for our understanding of human health. For one, the link between temperature and brown fat means climate change could further challenge our ability to stay healthy across generations, says Teperino. The study offers a potential explanation for differences in metabolic disease rates across longitudinal and climate gradients. Finally, it highlights that paternal health can shape a child’s future in unexpected ways. “Paternal health has been largely overlooked…but science is showing that it’s relevant,” he explains. “We need to rebalance the responsibility for reproduction and offspring health, so it’s not everything on the moms.”

[ Related: Unhealthy sperm can play a role in lost pregnancies. ]

But it’s too early to know exactly if or how those hoping to conceive should integrate these findings  into their lives, says Teperino. “I’m not sure we have enough solid evidence to give instructions or recommendations yet.” 

We do know that sperm production cycles take weeks, not hours, so a single cold shower won’t do much to ensure the health of future offspring. But beyond that, more work, in more diverse populations is needed to understand if or how we could consciously harness the epigenetic benefits of cold, say both Teperino and Wolfrum. Perhaps cutting back on sauna time ahead of conceiving is a good idea, Teperino suggests, but don’t necessarily reach for the ice bucket yet.

 

More deals, reviews, and buying guides

The PopSci team has tested hundreds of products and spent thousands of hours trying to find the best gear and gadgets you can buy.