People born in the fall and winter are more likely to have allergies than people born in the spring and summer. While that might be news to you, the scientific community has known it for years. What hadn't been explored before was how this happened. That's what a group of scientists in the U.S. and Europe set out to discover.
Many of us have a unique physical identifier — perhaps a noticeable birth mark. We also have certain "markers" on our DNA that can determine things like our weight and height, our mood swings and even our risk for some conditions, like schizophrenia.
The researchers found out that those markers — called epigenetic marks — can help determine whether we're at risk for allergies. "Epigenetic marks are a way to regulate the expression of genes without altering the underlying DNA sequence," explains Dr. Gabrielle Lockett, lead author of the study, via email. (Gene expression is a process where information stored in our DNA is translated into instructions for making proteins or other molecules.)
Lockett, a postdoctoral research fellow in the faculty of medicine at England's University of Southampton, says that researchers have long known that your birth season, as well as certain environmental exposures, like smoking, famine or even your social environment, are associated with certain epigenetic marks and can alter gene expression. They just didn't know why.
"Epigenetic marks were a good candidate for the connecting mechanism [between birth season and allergies] because they can be altered by environmental exposures, can influence gene expression, and can last many years," she says.
Lockett and her team scanned DNA samples from 367 18-year-olds who were born on the Isle of Wight in England. They paid particular attention to certain markers called DNA methylation in the samples. DNA methylation is one type of epigenetic mark. Methyl groups help cells reproduce normally, and they can literally turn genes "on" or "off." When your methyl groups are depleted, "bad" genes, like those that cause cancer, are turned "on."
Researchers wanted to know whether certain markers could be linked to the time of year a person was born and whether those people suffered from allergies, like eczema and asthma. "We ... found that, yes, DNA methylation at certain places in the genome is consistently associated with season of birth," Lockett says. "We went on to discover that these birth seasonal epigenetic marks are associated with gene expression, and could potentially provide a link to allergic disease."
Lockett says her team found similar results when they duplicated the study on a group of 8-year-olds from Holland. But the markers weren't present when the team tested a group of newborns. Lockett says this suggests that epigenetic markers arise after birth, or maybe even as a result of one's environment. Her study was published in the European Journal of Allergy and Clinical Immunology.
The link between birth month and allergy risk applies to all kinds of allergies. "For decades, studies have shown that people born in autumn and winter are at increased risk of not only rhinitis (hay fever), but also food allergy, asthma and eczema," says Lockett. "One large Dutch study found that people born in autumn and winter were at increased risk of having immune responses to non-seasonal allergens (such as egg white and cow's milk) as well as to seasonal allergens (pollen, cat, dog, and house dust mite). This suggests that season of birth is associated with a person's risk of allergy in general: The effect is not restricted to seasonal allergens."
So why would your birth season affect your risk of allergies? Lockett lists some of the theories. "Lower sunlight exposure during autumn and winter might reduce vitamin D levels, which have a known connection to allergy risk. Seasonal fluctuations in the levels of certain allergens, such as pollen and house dust mites, or the age at which a baby gets their first rhinovirus [hay fever] infection (more common in winter) may also contribute to allergic disease risk. Also, the seasonal availability of different fruits and vegetables eaten by the pregnant mother could alter the nutrition that babies born in different seasons receive during important stages of early development."
But more research is needed before we can know for sure.