New research suggests a surprising factor in human longevity. A look at 200 years of birth records found that high solar activity in a person's birth year was correlated with a five-year reduction in lifespan.

Image credit: Sun Spot by Marsel Minga/flickr/CC By 2.0

Researchers studied more than 9,062 Norwegian births from 1676 to 1878, and published their results in "Solar activity at birth predicted infant survival and women's fertility in historical Norway," published in Proceedings of the Royal Society B. By using sunspots as a barometer for solar activity — and therefore higher levels of ultraviolet radiation in that year, the researchers were able to correlate lifespan to higher levels of sun activity at birth. They also controlled for class, ecology, age at first reproduction, and year of birthAnd with thoseconfounding factors controlled for, there was still a shorter average lifespan of 5.2 years for those born during years of high solar activity. By sex, it broke down to an average of 5.1 years shorter for females and 5.3 years shorter for males.

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The researchers theorize that higher levels of ultraviolet radiation degrades folate (Vitamin B), and folate is essential in fetal development. Therefore, women pregnant during years of high solar activity would be exposed to more radiation. And could have seen their folate levels drop, leading to less healthy babies.

They also looked at the effect high solar activity had on fertility. And, in that case, the results were very class stratified:

Our results revealed that low-status but not high-status mothers born in a solar maximum period had reduced fertility whereas high-status mothers did not, and that all mothers born in a solar maximum period had reduced [lifetime reproductive success], although the latter did not reach statistical significance. Our findings suggest that maternal exposure to solar activity during gestation can affect the fitness of female children. The effect of socioeconomic status on the relationship between solar activity and fertility suggests that high-status pregnant women were better able to avoid the adverse effects of high solar activity. One possible explanation for this is that poor pregnant women were exposed to higher doses of [ultraviolet radiation] than rich women because low-status women were outdoor workers, whereas pregnant high-status women could spend more time indoors.

Another possibility for the difference was that the healthier diets among higher-status women would protect them better from the effects, whereas lower-status women's less robust diets would leave them more susceptible to the effects of ultraviolet radiation.

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Not that this means that pregnant women these days should hide from the sun. Vitamin B supplements may mitigate the folate deficiency. And, says study author Frode Fossøy, an evolutionary biologist at the Norwegian University of Science and Technology, we need the sun to synthesize Vitamin D:

A lot of the media now has been that if you're pregnant or planning to become pregnant, then you should avoid sun, or not go south for the winter to get a lot of sun, especially if you're very pale. We also need sun, to get vitamin D—so it's a delicate balance.

[Solar activity at birth predicted infant survival and women's fertility in historical Norway via Scientific American]