[♪ INTRO]
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]