Vitamin D Deficiencies Linked To Miscarriage And Reduced Fertility

One of the big mysteries of the Bronze Age is why lactose persistence and light skin color were so strongly selected for in Europe. Both traits reduce the risk of Vitamin D deficiency. But, why would Vitamin D deficiency have such a strong selective effect?

One reason is that Vitamin D deficiency is linked to miscarriage and to reduced fertility.

Women who had sufficient preconception vitamin D concentrations were 10 percent more likely to become pregnant and 15 percent more likely to have a live birth, compared to those with insufficient concentrations of the vitamin. Among women who became pregnant, each 10 nanogram per milliliter increase in preconception vitamin D was associated with a 12-percent lower risk of pregnancy loss.

A typical woman can have about eight kids plus a normal number of miscarriages in a lifetime. This would have been common for Bronze age farmers (herders spaced their children a bit further apart than farmers did, making each lost child even more devastating). Also Bronze Age women who didn't have enough Vitamin D probably had far less Vitamin D in their systems than modern women with Vitamin D deficiencies do.

When you are aiming for two or three children as most modern European families do, an extra couple of miscarriages will cause grief but won't impact your lifetime impact on the gene pool very much. But, if everyone needs to have eight children just to have two of them live to reach adulthood and have children of their own, extra miscarriages, together with increased difficulty getting pregnant, can dramatically reduce your reproductive fitness.

If miscarriage and reduced fertility were the primary problem with reduced Vitamin D, another secondary issue may have been immune system health which Vitamin D enhances, in a world with no vaccines or antibiotics to address infectious diseases, and where Vitamin C, another immune system enhancer, would have been almost entirely absent for much of the year in Northern Europe.

At any rate, this data point should be useful for quantifying the selective fitness impact of LP and light skin color genes in Northern Europe during the Bronze Age.