Vitamin D
Vitamin D is a fat-soluble nutrient. It is one of the 24 micronutrients critical for human survival. The sun is the major natural source of the nutrient, but vitamin D is also found naturally in fish and eggs. It is also added to dairy products.
Supplemental vitamin D is associated with a wide range of benefits, including increased cognition, immune health, bone health and well-being. Supplementation can also reduce the risks of cancer, heart disease, diabetes and multiple sclerosis. People deficient in vitamin D may also experience increased testosterone levels after supplementation.
The body produces vitamin D from cholesterol, provided there is an adequate amount of UV light from sun exposure. There is only a sufficient amount of UV light coming from the sun when the UV index is 3 or higher, which only occurs year-round near the equator, between the 37th parallels.
Most people are not deficient in vitamin D, but they do not have an optimal level of vitamin D either. Due to the many health benefits of vitamin D, supplementation is encouraged if optimal levels are not present in the body.
Some studies suggest that vitamin D may protect against colon cancer and perhaps even cancers of the prostate and breast. But higher levels of vitamin D in the blood have also been linked to higher rates of pancreatic cancer.
In humans, the primary function of vitamin D is to maintain normal levels of serum calcium and phosphorus concentrations by enhancing small intestine dietary absorption efficiency of these minerals. 25-hydroxyvitamin D [25(OH)D] enhances the efficiency of calcium and phosphorus absorption along the entire small intestine. When dietary calcium intake is insufficient, 25(OH)D and parathyroid hormone (PTH) mobilize monocytic stem cells in the bone marrow to become mature osteoclasts. These osteoclasts mobilize calcium from the bones, thereby maintaining blood calcium levels. Vitamin D is thought to have physiological effects in other parts of the body as vitamin D receptors are also found in the cells of other organs, like the intestine, kidney, stomach, brain, prostate, breast, and white blood cells.
The anticancer effect of vitamin D is thought to be due to induction of cell differentiation and antiproliferation. In lymphoma cells, interventional 25(OH)D3 to normalize levels (>30 ng/mL) resulted in significantly stronger antibody-dependent cell-mediated cytotoxicity, suggesting benefit in D-deficient individuals receiving chemotherapies such as rituximab that utilize this mechanism. In xenograft models of breast cancer, dietary D3 elevated circulating D levels and increased CYP27B1 expression in both tumor and intestines, suggesting it stimulates local calcitriol synthesis in the tumor microenvironment and promotes ensuing paracrine/autocrine actions that contribute to its anticancer activity. The upregulation of CYP27B1 expression by tumors was unique to D3 versus calcitriol in the same tissue. In other animal models, a positive feedback signaling loop between the serine-protein kinase ATM (ataxia telangiectasia mutated) and the vitamin D receptors was identified as critical for cancer chemoprevention by vitamin D.
Dosage
The recommended daily allowance for Vitamin D is currently set at 400-800IU/day, but more current research suggests this may be too low for adults. The safe upper limit is 10,000IU/day. For moderate supplementation, a 1,000-2,000IU dose of vitamin D3 is sufficient to meet the needs of most of the population.
Vitamin D3 supplementation (cholecalciferol) is recommended over D2 supplementation (ergocalciferol), since D3 is used more effectively in the body.
Vitamin D should be taken daily, with meals or a source of fat, like fish oil.
Safety
Toxicity may occur with doses greater than 10,000IU/day. Individuals with kidney stones, kidney disease, high blood calcium levels, gastrointestinal disease, heart disease, liver disease or other diseases associated with disorders of calcium metabolism should seek medical advice before taking supplemental vitamin D.