In one study, researchers measured vitamin D levels in 40 men and found that those with 75 nanomoles of vitamin D per liter of blood (nmol/L) had much higher sperm motility compared to men with 25 nmol/L or less.
Motility is the ability of sperm to move properly toward the egg. As levels of vitamin D increased, more of the sperm were motile, and had normal structure, shape, and size.