In the first few weeks of spring, a 50-degree-Fahrenheit (10-degree-Celsius) day might call for a light jacket or no jacket—or even short sleeves, depending on the person. But in the fall, the same weather might have you reaching for a parka.
It’s not just in your head. The relative warmth of spring is physiological as well as psychological; after a long, biting winter, your body has changed in ways that can make 50 degrees F seem downright balmy.
“I fully experience this on a regular basis with my work,” says Cara Ocobock, an anthropologist at the University of Notre Dame, who studies how the human body adapts to cold. Her work often takes her to Finland, where she studies populations of reindeer herders who spend lots of time in extreme cold.
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“The human body is very good at acclimatizing to different environmental situations that are not permanent—and the changes that your body experiences during this time also aren’t permanent,” she says. Some of these changes involve a heat-generating organ that was only recently discovered in adults.
Scientific American spoke with Ocobock to learn more about the changes our bodies undergo during winter—including to that strange, newfound organ—and how these changes affect us when the winter chill finally gives way to the warmth of spring.
Have you personally experienced this “50 degrees feels warm” phenomenon?
Yes, I have a story from my last trip to Finland. I was 300 kilometers [185 miles] north of the Arctic Circle during what should have been the coldest time of the year. There were maybe four or five days where it didn’t get above –20 degrees Fahrenheit [–29 degrees Celsius]. But then five days later, it was in the positive 40s Fahrenheit [or five to 10 degrees C], which should not exist that far north that time of year. After those days of extreme cold, I started sweating [when it went] above freezing. I wouldn’t even wear a coat. My body just kind of reversed course—like, “We need to cool you down; this is not what we have been used to.”
How quickly do these physiological changes happen when someone is exposed to more extreme temperatures?
There’s always going to be individual and populational variation, but we see the changes start happening pretty quickly. It can start within 24 hours, but they don’t fully set in for about seven to 10 days. You will maintain those changes until you go and switch environments again, and then you’ll lose your acclimatization. This can be to heat, cold, humidity, dryness or high altitude as well. For example, when I [returned to sea level from] field work in the Rocky Mountains, I was able to do two full lengths of an Olympic swimming pool without breathing. Within two weeks, that was gone.
So how do our bodies change when we are exposed to cold weather?
There’s a constant balancing of several different systems going on here. One of the quick changes is an increase in your resting metabolic rate—the baseline number of calories your body burns in order to survive. Your body is kind of increasing its own thermostat to produce more heat because you are losing more heat to the environment.
We also see changes in the way your blood vessels [tighten or expand] to respond to the cold. In the cold, [vessels constrict to] reduce how much blood is flowing through and the heat that can potentially be lost to the environment. And when you’re cold, blood will be shunted more to the deep blood vessels that are further away from the surface, whereas in a hot climate, the opposite happens.
We also see and increase in brown adipose tissue activity—this is an active area of research. “Brown fat,” as we call it colloquially, is a type of fat that burns only to keep you warm during acute cold exposure. In adult humans, it’s located [just above your clavicles], as well as along your major deep blood vessels. This organ, and we do consider it kind of its own organ, uses energy to produce heat—not energy to [activate your muscles] to go run a mile or anything like that. We used to think that human adults never have brown fat. We knew that babies have it [for the first few months of life], but we thought that once they burned through it, that was it. But we are now seeing brown adipose tissue everywhere we look in adult human populations.
How is brown fat different from regular fat?
Brown adipose tissue is very, very rich in mitochondria. Instead of being the powerhouse of the cell, those mitochondria are the furnace. It basically short-circuits the typical process so that this tissue produces heat rather than energy.
In adults, to date, we have seen brown fat in populations in Russia and Finland—cold climates, which makes sense. We’ve seen it in Albany, N.Y.—temperate climate but cold winters. And we’ve also seen it in Samoa—a tropical island climate. So we’re beginning to think that brown adipose tissue might be a very deeply ancient tissue and that it could have been around in our evolutionary history for a very long time.
How does brown fat activity change during cold seasons?
One study on seasonal changes in brown adipose tissue [was] conducted by my former graduate student, Alexandra Niclou. She looked at seasonal variation in a brown adipose tissue among folks in Albany. She found that people were able to maintain higher body temperatures from brown fat in the winter but at a reduced caloric cost. And so it seemed the brown fat actually got more efficient the more it was being used to maintain body temperature in the winter. So there does seem to be a physiological difference in how brown fat is responding between the seasons. I’m going back to Finland this spring [to measure this further] among reindeer herders and indoor workers.
Given all of those factors, what do you think is happening to our bodies on that first “warm” spring day?
In the winter, you’re going to have an increase in resting metabolism. You might see an increase in your brown adipose tissue activity in order to keep you warm. Then all of a sudden it’s 50 degrees Fahrenheit outside, but your resting metabolic rate is still going to be higher, [and your brown fat might be more active], which means your body is producing more heat than it typically would have been. That’s probably why you feel like it’s way warmer out and start sweating. That acclimatization process is going to take a week or more to get you used to this new, warmer temperature setting.
There’s also a developmental aspect of this—where you grew up likely has a massive, massive impact on how your body responds to different extremes and changes in seasonal temperatures. I’m a college professor [in Indiana], and walking around campus this time of year, you can tell the kids from the East Coast and the Midwest versus those from the South and the West Coast [by who is wearing] short T-shirts and sandals when it’s, like, 50 degrees and [who is] still in puff jackets. It always cracks me up. And we might actually see happening with brown adipose tissue as well—that the more you are exposed to cold during critical developmental periods as a child, the more active and responsive your brown adipose tissue may be as an adult.
Do these seasonal changes still impact you if you spend most of the winter indoors?
They are definitely still impacting you. It might not be as much, obviously, and this is part of what we’re doing with our work in Finland with reindeer herders, who spend more time outside in the extreme cold, and indoor office workers in the same region. But because you still go outside, you still experience acute cold, [even if it’s not] for hours and hours on end.
Why is it important to understand how our bodies acclimatize to extreme temperatures?
Understanding how bodies rapidly respond [to changes in temperature] is going to be even more important in the face of climate change, when we have highly and dramatically variable environments —where you get ice storms in Texas, for example. [Helping people acclimatize via what we know about] biology, behavior and technology is going to be critical, I think, because no matter what, our bodies are going to be physiologically limited in coping with both extreme cold and extreme heat. Our bodies are not limitless, so we have [to adjust our] behavior and turn to technology to make up for what our bodies can’t do.