Vitamin D Summary
Vitamin D can be synthesized in the skin through a process that is dependent on sun exposure to ultraviolet (UV) radiation. Vitamin D is converted by the body to 25-hydroxyvitamin D, which is measured in blood tests to determine vitamin D status, and then to 1,25-dihydroxyvitamin D, which is the biologically active form. Vitamin D circulates as a hormone in the blood, and has a major role in regulating calcium, magnesium, and phosphate, which have important impacts on bone. In addition, vitamin D has several other effects, including on cell growth, proliferation, and differentiation, calcium signaling, brain development, neurotrophic and neuroprotective actions, neurotransmission, synaptic plasticity, neuromuscular and immune functions, reduction of inflammation, and regulation of gene expression. Deficiency of vitamin D is caused by inadequate sun exposure and/or inadequate dietary intake. Deficiency is common even among individuals with light skin tones with the modern custom of extensive skin coverage by clothes and sunscreens, and increasingly sedentary indoor lifestyles. Vitamin D is present in appreciable quantities in only a few foods; in the American diet many of the highest sources, such as milk, are often not ingested in appreciable amounts. While sun exposure is an excellent way to boast vitamin D levels, practical concerns in many disease populations, and fear of cancer, have led many to recommend supplementation and the monitoring of this therapy by blood levels. Vitamin D deficiency typically results in impaired bone mineralization, which leads to bone fractures and bone-softening diseases including osteomalacia in adults, and rickets in children. Thus, vitamin D is often used to treat bone-related conditions, although it is often recommended for many other conditions beyond the skeleton. There is good evidence of an association of vitamin D deficiency with a variety of brain disorders, including ADD/ADHD and autism. Appropriate blood 25-hydroxyvitamin D levels are controversial, with some studies recommending levels over 40 ng/ml. Vitamin D toxicity is very rare, even in studies using extremely high doses by injection, although levels of 25-hydroxyvitamin D above an unclear threshold may be too high.