Over an 11-year follow-up period, doctors compared the levels of vitamin D circulating in the blood to the amount of time until a first defining medical event. Researchers found that the chances for any major disease event decreased significantly when vitamin D levels rose above 20 nanograms per milliliter of blood, which is the same as 50 nanomoles per liter of blood. Labs measure vitamin D both ways.
Doctors also said that vitamin D levels varied seasonally, with the highest levels in summer, the lowest in winter, and autumn generally higher than springtime.