- What is the function of vitamin D?
Maintaining proper calcium levels in the blood
Although typically categorized as a fat-soluble vitamin, vitamin D actually functions more like a hormone. Calcitriol, the most metabolically active form of vitamin D, works with parathyroid hormone (PTH) to maintain proper levels of calcium in the blood. Low levels of calcium in the blood stimulate the secretion of PTH from the parathyroid gland. PTH then stimulates the conversion of inactive forms of vitamin D to calcitriol. Calcitriol acts to increase the intestinal absorption of calcium, increase the resorption of calcium by the kidneys, and stimulate the release of calcium from the bone, thereby increasing blood calcium levels. Alternatively, when blood levels of calcium are too high, calcitriol decreases the intestinal absorption of calcium and stimulates the bones to take up calcium, thereby decreasing blood calcium levels.
Maintaining proper phosphorus levels in the blood
When vitamin D levels remain problematically low, the parathyroid gland becomes overactive, a condition known as hyperparathyroidism. PTH levels rise, and blood levels of phosphorus drop. Without adequate phosphorus, bone cannot be properly mineralized, which contributes to the defects seen in osteomalacia. In addition, the new bone cells being laid down by the osteoblasts (the cells that create new bone) absorb more water and swell, causing the bone pain associated with osteomalacia.
Maintaining normal cellular growth and function
Scientists have recently discovered that our bodies convert vitamin D3 into calcitriol not only in the kidneys, but in other tissues as well, including the lymph glands and skin. Because calcitriol is so closely involved with the lifecycles of our cells, this discovery has led scientists to believe that vitamin D may be very important in the prevention and treatment of certain cancers.
Vitamin D may also play a role in regulating cellular growth and function in our brain cells. In mice studies, vitamin D has been found to have a significant effect on brain cell (neuronal) growth and division. The brains of vitamin D deficient rats look different. If vitamin D is not present during gestation, long lasting defects occur. Future research may well determine that development of the human brain is also closely tied to our vitamin D status.
Maintaining healthy immune function and preventing excessive inflammation
Vitamin D also helps regulate immune system activity, preventing an excessive or prolonged inflammatory response. Our immune cells, specifically our active T-cells, have receptors for vitamin D. This is important because autoimmune diseases-including multiple sclerosis, diabetes, rheumatoid arthritis, irritable bowel diseases (such as Crohn's and ulcerative colitis-all have a T-cell component of inflammation.
- What are deficiency symptoms for vitamin D?
Vitamin D deficiency results in decreased absorption of calcium and phosphorus. As a result, prolonged vitamin D deficiency has a negative impact on bone mineralization. In infants and children, such a deficiency manifests itself as rickets, a condition characterized by bone deformities and growth retardation. Adults with vitamin D deficiency may experience bone pain and/or osteomalacia (soft bone).
- What are toxicity symptoms for vitamin D?
Excessive dietary intake of vitamin D can be toxic. Toxicity of vitamin D can come from either its plant-based (D2) or animal-based (D3) form. Symptoms of toxicity include loss of appetite, nausea, vomiting, high blood pressure, kidney malfunction, and failure to thrive.
- What factors might contribute to a deficiency of vitamin D?
Insufficient sun exposure
It is especially important for individuals with limited sun exposure to include good sources of vitamin D in their diets. Homebound individuals, people living in northern latitudes, individuals who wear clothing that completely covers the body, and individuals who always use sunscreen or work in occupations that prevent exposure to sunlight are at significant risk for vitamin D deficiency.
Breast feeding, if the mother is vitamin D deficient
If a mother is deficient in vitamin D and her infant is exclusively breast-fed, that infant may require a vitamin D supplement in order to avoid vitamin D deficiency.
Obesity is also associated with vitamin D deficiency. Since vitamin D is fat-soluble, it can be taken into fat cells and stored, thus making it potentially less available in our body's metabolism. Production of vitamin D may also be reduced in obese individuals. For example, in one study, when normal and obese subjects were exposed to the same amount of radiation from natural sunlight, obese subjects only produced 55% the amount of vitamin D as normal weight subjects.
Insufficient dietary fat or inability to absorb dietary fat
Since vitamin D is a fat-soluble vitamin, a diet that is extremely low in fat, and/or the presence of certain medical conditions that cause a reduction in the ability to absorb dietary fat, may cause vitamin D deficiency. These medical conditions include pancreatic enzyme deficiency, Crohn's disease, celiac sprue, cystic fibrosis, surgical removal of part or all of the stomach, gall bladder disease, and liver disease. Symptoms of fat malabsorption include diarrhea and greasy stools.
Health conditions that involve the parathyroid gland or kidney
Under certain circumstances, the conversion of inactive forms of vitamin D to calcitriol is impaired. For example, diseases that affect the parathyroid gland, liver and/or kidney impair the synthesis of the active form of vitamin D.
The production of vitamin D precursors in the skin decreases with age, and the kidney is less able to convert vitamin D to its active hormone form.
Some individual's genetic inheritance includes genetic polymorphisms that result in the production of vitamin D receptors (VDR) that don't work very well. To help compensate for such VDR defects, these individuals need more vitamin D than would normally be necessary.