The sun is a major producer of ultraviolet radiation.
The study, published in the Proceedings of the Royal Society B, shows that increased levels of ultraviolet radiation (UVR) may have a marked effect on human fertility across generations and mortality.
"Several long-term studies on a wide variety of species, including humans, have revealed that the environment an organism is exposed to early in life may influence adult life-history traits, such as survival, fertility and lifetime reproductive success," write the authors.
Church records from 1750-1900 taken from two different areas of Norway and involving 9,062 people were assessed by the researchers. They examined several life history variables such as the age at which women had their children, how many children survived after birth and how many of them went on to marry and have children themselves.
Gine Roll Skjærvø and her colleagues then compared this information with environmental data for the time periods in question. They discovered that children born in years with high levels of solar activity had a greater risk of mortality compared with children born in years with less solar activity.
Shorter lifespans, reduced fertility
Solar activity is measured by the number of sunspots observed on the surface of the sun. These vary in accordance with an 11-year cycle - 8 years of low activity followed by 3 years of high activity.
Skjærvø calculated the amount of UVR for any given year based on this information. The more solar activity that occurs, the more UVR is present on Earth.
The researchers found that the lifespan of children born during years with high solar activity was an average of 5.2 years shorter than other children. Differences in mortality were greater during the first 2 years of life.
"One proximate explanation for the relationship between solar activity and infant mortality may be an effect of folate degradation (vitamin B) caused by UVR," suggest the authors. Folate, or folic acid, is very important for pregnant women as it helps to prevent birth defects such as spina bifida.
The authors also found that children born in years with high solar activity tended to have fewer children, who in turn would reproduce less themselves. "In addition, fertility and lifetime reproductive success were reduced among low-status women born in years with high solar activity," they write.
Study author warns against sunbathing while pregnant
Families with a low socioeconomic status were most affected by UVR. During the period studied by Skjærvø and the team, class distinction was clear and low-status women got more sun exposure from working in fields.
The authors acknowledge that diet quality may also have been an influential factor as high-quality food could be more likely to prevent DNA damage to fetuses. They state that more research is required to clearly establish the mechanism at work here, however.
UVR exposure before and after birth cannot be distinguished by the authors using the available data. As the gestation period is the most vulnerable developmental stage for children, however, the authors believe that their findings are due to the effects of UVR during pregnancy as opposed to after.
"There are probably many factors that come into play, but we have measured a long-term effect over generations," says Skjærvø. "The conclusion of our study is that you should not sunbathe if you are pregnant and want to have a lot of grandchildren."
Recently, Medical News Today published an article detailing the health benefits of folic acid, including recommended intake levels, potential health risks and a list of foods containing high levels of the vitamin.