Vitamin D refers to a group of lipid-soluble nutrients responsible for enhancing intestinal absorption of calcium, iron, magnesium, phosphate and zinc.
In humans, the most important compounds in this group are vitamin D3 and vitamin D2. They can be ingested from the diet and from supplements. Few foods contain vitamin D; synthesis of vitamin D (specifically cholecalciferol) in the skin is the major natural source of the vitamin. Dermal synthesis of vitamin D from cholesterol is dependent on sun exposure.
Dietary Reference Intake for vitamin D assumes no synthesis occurs and all of a person's vitamin D is from food intake, although that will rarely occur in practice. But the DRI is so low because it was developed to prevent rickets, a disease we don’t see too often in modern, developed economies, not to optimize health.
Vitamin D has a significant role in calcium homeostasis and metabolism. There is growing evidence that vitamin D may reduce the risk of a variety of health concerns.
Vitamin D is very important for bone health. It helps build stronger bones, partly by increasing the absorption of calcium. Vitamin D also improves the function of muscles, which in turn improves balance and decreases the likelihood of falling.
Healthy adults between 19-50 years old, including pregnant or breast-feeding women, require a minimum of 400 IU daily. And it used to be thought that taking more than that would make the body retain too much calcium, a condition called hypercalcemia. According to the Vitamin D Council, “Although most people take vitamin D supplements without any problems, it’s possible to take too much. This is called vitamin D toxicity. Vitamin D toxicity, where vitamin D can be harmful, usually happens if you take 40,000 IU per day for a couple of months or longer, or take a very large one-time dose.”
In the last several years, there has been a flood of health news about studies linking relatively low vitamin D with neoplasms, cardiovascular diseases, diabetes, metabolic disorders, depression, infectious diseases, autoimmune diseases, mortality and even autism.
A search on PubMed for vitamin D finds many hundreds of academic papers.
John J. Cannell, MD, Founder and Executive Director of the Vitamin D Council, says, “Many people are turned off by these claims and say it’s impossible that one thing is involved in so many different disease processes, but they are unaware of the mechanism of vitamin D. It is actually a steroid hormone that turns genes on and off. There are at least a thousand genes that are directly regulated by vitamin D.”
Studies for Vitamin D
Cholesterol
Vitamin D may improve cholesterol numbers. A study analyzed data from 576 postmenopausal women who were part of the National Institute of Health’s Women’s Health Initiative trial. Women who took 400 IU of vitamin D plus 1,000 mg of calcium daily showed a significantly higher blood level of vitamin D after two years, compared to the control group who took a placebo. Interestingly, those who had higher blood levels of vitamin D also had better lipid profiles, showing increased high-density (“good”) cholesterol), decreased low-density (“bad”) cholesterol and lower triglycerides. The researchers acknowledge that their findings are not conclusive about how vitamin D affects cardiovascular health over the long term. It is clear that longer studies are needed. However, given that these results were from blood work for women followed for several years, there is a relationship here that merits further research. The study was published in the March 2014 issue of Menopause.
A meta-analysis combined data from more than 4,500 breast cancer patients from 5 observational studies to see if higher vitamin D levels at the time of breast cancer diagnosis were associated with longer patient survival times. Over a 9-year period, patients in the group with the highest blood level of vitamin D had about half the fatality rate compared to those in the group with the lowest level. In the paper, researchers reported that other lab studies have shown that vitamin D has anticancer effects, arresting tumor growth in 3 critical phases of development. While these results are encouraging, the researchers caution that a causal conclusion is not possible and that a randomized controlled trial is needed to shed further light on the findings.
Risk of Mortality
Low vitamin D levels carry a greater risk of death. In a large systematic review and meta-analysis published in the British Medical Journal, researchers looked at the link between vitamin D and chronic diseases to assess mortality risk. They combined data from several large databases of studies, including Medline, Embase and the Cochrane Library. Low blood levels of vitamin D were associated with a greater risk of mortality from cardiovascular disease, cancer and other causes. Calculations showed that each decline of vitamin D of the kind measure in blood tests was associated with a 16% greater risk of mortality and that supplementation with vitamin D3 reduced mortality risk by 11%.
Vitamin D may help lower blood pressure, decrease the risk of diabetes, and lower the chances of heart attacks, rheumatoid arthritis, or multiple sclerosis.