Vitamin D has long been linked to bone health; when researchers discovered that rickets was caused by a deficiency of the vitamin, public health experts urged the fortification of this staple with vitamin D, which made milk, already naturally rich in calcium, virtually synonymous with bone health. More recently, however, researchers are finding more evidence that vitamin D is a critical nutrient; it is also essential for muscle, nerve and immune system function and may play roles in cancer, dementia and depression. Vitamin D is also produced by skin cells, in the presence of sunlight, though our ability to manufacture the vitamin declines with age. Different forms of vitamin D can be measured in the blood (including active versions and inactive precursors); testing is likely to increase as research reveals the importance of the vitamin to overall health.