How did these individuals become vitamin D deficient in the first place? According to Bredesen, it’s a combination of lifestyle habits and nutritional choices: “They’re living indoors, they’re not getting out enough, they’re not taking vitamin D, or they’re not absorbing the vitamin D they are taking.” 

While getting outdoors and spending some time in the sun—safely, of course—is always recommended, the sun isn’t a historically reliable source of vitamin D. (And for the record, diet isn’t either!)

So, we’re left with supplementation. But as Bredesen stated, some people are taking vitamin D supplements and seeing no improvement to their D levels—or their cognitive function.