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A few takeaways:
Vitamin D deficiency was initially linked to the childhood disease rickets, which causes the softening and weakening of bones, often leading to fractures. (In adults, rickets is called osteomalacia.)
But vitamin D researchers have discovered other important functions as they began questioning the inverse relationship between sunlight exposure and increased incidence of many chronic diseases, such as cancer, autoimmune diseases, diabetes, and cardiovascular disease.
As the serum 25(OH)D concentration rose above 80 nmol/L, there was a 50% reduction in the colorectal cancer risk.
The risk of prostate cancer is reduced by 50% when serum vitamin D levels are greater than 50 nmol/L.
Vitamin D is integral to the normal function of the immune response.
Also, while not listed in this article, D deficiency has also been linked to depression (which, even in mild forms, can cause fatigue).