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We’re all pretty familiar  with cold and flu season.  But a few decades ago, public health experts  began to notice that some non-infectious diseases, like breast cancer, followed  a seasonal pattern too.  Which might seem strange, because we  don’t talk about a “cancer season”   the way we talk about flu season. But these annual spikes can teach us a lot about  how our bodies interact with the environment.

Scientists explain the seasonality  of infectious diseases like the flu   by looking at a bunch of different factors. Things like how the host’s  behavior spread the disease,   or how the biology of the virus  changes over the course of the year. The influenza virus is transmitted  most effectively in cool, dry weather.

So even though you can get the flu whenever,  fall and winter are its favorite time of year.  But diseases that aren’t caused by  germs can see regular seasonal spikes   for totally different reasons...  like the influence of the sun. We’ve known for a long time that  excessive exposure to sunlight   increases someone’s chances  of developing skin cancer.  That’s because ultraviolet  radiation from the sun damages DNA   and interferes with its ability to repair itself, a fairly routine process that DNA does constantly. And more people are diagnosed with skin cancer  in the summer months than in the winter.  In the long term, we also see more skin  cancer diagnoses along with the solar cycle,   an eleven year fluctuation of solar activity.

Simply enough, when the sun emits  more UV, we see more skin cancer. The solar cycle doesn’t affect your individual  risk as much as, say, wearing sunscreen. But there’s enough of an impact that we  can see it in the overall population.

While skin cancer has a pretty  well-understood mechanism, the spike in coronary artery disease that doctors  see every winter is not as straightforward.  Our circulatory systems respond to drops in  temperature by constricting our blood vessels.  When that happens, the heart has to  pump blood against more resistance,   which increases blood pressure, and high blood pressure is an important  risk factor in cardiovascular disease. And studies in rodents suggest  that reduced temperature also   impairs the body’s ability to make nitric oxide, a chemical that expands blood  vessels and reduces blood pressure. Certain hormones also fluctuate with temperature.

For instance, thyroid hormone  helps regulate how forcefully   our hearts contract and expands blood vessels. And exposure to cold conditions  decreases thyroid hormone levels. Cardiovascular disease is caused by the  interaction of so many different factors, that it’s hard to point to just one  thing that makes the biggest difference.  But these factors might give  insight to a larger trend.    Breast cancer is another disease that sees a   seasonal spike in diagnoses  in the spring and fall, and a patient’s chances of survival are usually   better if it’s diagnosed in  the summer than in the winter.

And that puzzles scientists because  breast cancer can be developing for   years before it’s detectable, so it  shouldn’t matter when you find it. Breast cancer can grow quickly, though,   and something has to explain those  clear seasonal peaks and valleys. So in the last few years, researchers have   been looking for anything that  might cause faster growth rates, and push more breast cancers to the point where  they're more easily detectable on a mammogram.

We know that some types of breast cancers can grow   faster thanks to estrogen  receptors on their cells. Those cells tend to grow faster  in the presence of estrogen.  Since estrogen sees annual peaks and valleys,   the seasonality of the hormone might’ve  explained the seasonality of the disease.  But the same seasonal spikes in breast cancer  diagnoses are seen in people with ovaries,   which make estrogen, both  pre- and post-menopause.  Since the body makes much less  estrogen during menopause,   we'd expect to see some kind of difference  if it could be explained by that hormone.  So scientists have looked into other  substances that fluctuate throughout the year,   including vitamin D and melatonin. And here’s where it gets a bit complicated.

Our bodies create more vitamin D during  the summer thanks to more sun exposure, and mammals secrete melatonin at different  times of the day in response to darkness.  So some researchers think that the rise in  vitamin D protects us during the summer,   while melatonin protects us during the winter, thus explaining the spring and  fall peaks in breast cancer.  And in the last few decades,   different studies have shown that treating  breast cancer cells with a Vitamin D derivative might prevent them from growing  and encourage them to die outright. Breast tissue has an enzyme that converts   a vitamin D precursor molecule into  that supposedly beneficial derivative. And those cells can also have the  ability to pick up vitamin D as well.

A 2019 meta-analysis of seventy  studies showed that low vitamin  . D levels in the blood were associated  with increased risk of breast cancer.  And it’s the same story with melatonin. Early studies showed that melatonin slowed  the growth of breast cancer cells in the lab, and studies since then have  shown that it can modify estrogen   receptors on certain types of breast cancers.

But that’s not the end of the story.   There’s some support for both of these  mechanisms, but overall the evidence is mixed. A meta-analysis from 2017  found no association between   urinary melatonin levels and breast cancer risk.  But previous meta-analyses found that less  melatonin did increase breast cancer risk. Researchers have also conducted  randomized controlled trials,   the gold standard for investigating  a causal relationship, and found vitamin D supplementation had no  effect on the incidence of breast cancer.  That means we’re not really sure yet why breast  cancer has that weird double seasonal peak.

It’ll take a lot more research to know for sure. In general, it’s a combination of factors  that predispose us to certain diseases,   so teasing out causation from  correlation can be tricky.  Regardless, there are a lot more patterns,   and a lot more reasons for patterns,  in illness than we might have realized. Thanks for watching this episode of SciShow, which  was brought to you with the help of our patrons.

Patrons help us make fascinating,  free videos for everybody. And they get access to cool perks  too, like monthly blooper reels. If you’d like to join in,  check out patreon.com/SciShow. [♪ OUTRO].