Vitamin D is a fat-soluble vitamin that is naturally present in very few foods, added to others, and available as a dietary supplement. It is also produced endogenously when ultraviolet rays from sunlight strike the skin and trigger vitamin D synthesis. Vitamin D obtained from sun exposure, food, and supplements is biologically inert and must undergo two hydroxylations in the body for activation. The first occurs in the liver and converts vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. The second occurs primarily in the kidney and forms the physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol.
Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and to prevent hypocalcemic tetany. It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts . Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis.
Vitamin D has other roles in the body, including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation . Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vitamin D. Many cells have vitamin D receptors, and some convert 25(OH)D to 1,25(OH)2D.
Serum concentration of 25(OH)D is the best indicator of vitamin D status. It reflects vitamin D produced cutaneously and that obtained from food and supplements and has a fairly long circulating half-life of 15 days. 25(OH)D functions as a biomarker of exposure, but it is not clear to what extent 25(OH)D levels also serve as a biomarker of effect (i.e., relating to health status or outcomes) . Serum 25(OH)D levels do not indicate the amount of vitamin D stored in body tissues.
In contrast to 25(OH)D, circulating 1,25(OH)2D is generally not a good indicator of vitamin D status because it has a short half-life of 15 hours and serum concentrations are closely regulated by parathyroid hormone, calcium, and phosphate. Levels of 1,25(OH)2D do not typically decrease until vitamin D deficiency is severe.
There is considerable discussion of the serum concentrations of 25(OH)D associated with deficiency (e.g., rickets), adequacy for bone health, and optimal overall health, and cut points have not been developed by a scientific consensus process. Based on its review of data of vitamin D needs, a committee of the Institute of Medicine concluded that persons are at risk of vitamin D deficiency at serum 25(OH)D concentrations <30 nmol/L (<12 ng/mL). Some are potentially at risk for inadequacy at levels ranging from 30–50 nmol/L (12–20 ng/mL). Practically all people are sufficient at levels ≥50 nmol/L (≥20 ng/mL); the committee stated that 50 nmol/L is the serum 25(OH)D level that covers the needs of 97.5% of the population. Serum concentrations >125 nmol/L (>50 ng/mL) are associated with potential adverse effects (Table 1).
|<30||<12||Associated with vitamin D deficiency, leading to rickets
in infants and children and osteomalacia in adults
|30 to <50||12 to <20||Generally considered inadequate for bone and overall health
in healthy individuals
|≥50||≥20||Generally considered adequate for bone and overall health
in healthy individuals
|>125||>50||Emerging evidence links potential adverse effects to such
high levels, particularly >150 nmol/L (>60 ng/mL)
* Serum concentrations of 25(OH)D are reported in both nanomoles
per liter (nmol/L) and nanograms per milliliter (ng/mL).
** 1 nmol/L = 0.4 ng/mL
An additional complication in assessing vitamin D status is in the actual measurement of serum 25(OH)D concentrations. Considerable variability exists among the various assays available (the two most common methods being antibody based and liquid chromatography based) and among laboratories that conduct the analyses. This means that compared with the actual concentration of 25(OH)D in a sample of blood serum, a falsely low or falsely high value may be obtained depending on the assay or laboratory used. A standard reference material for 25(OH)D became available in July 2009 that permits standardization of values across laboratories and may improve method-related variability.
There are two major forms of vitamin D. Ergocalciferol and cholecalciferol are known as vitamins D2 and D3, respectively. Vitamin D2 is often man-made and used to fortify foods. Your body manufactures Vitamin D3 with the help of ultraviolet B (UVB) radiation. Just 15-30 minutes of sun exposure is enough to produce ample amounts of vitamin D. But how does that process work?
UVB rays from the sun react with a preform of vitamin D called 7-dehydrocholesterol that exists in your skin’s epidermis. When these rays hit 7-dehydrocholesterol, it changes the molecular structure to become a pre-activated form of vitamin D called 25-hydroxyvitamin D. Once the conversion is complete, 25-hydroxyvitamin D enters the blood stream. The liver and kidneys then get involved to ensure the circulating vitamin D is further activated into its fully usable form.
One of vitamin D’s main uses in your body is bone growth and bone-health maintenance. That’s based on vitamin D’s ability to help regulate the amount of calcium in your blood serum. This delicate calcium balance is controlled by the parathyroid glands. When low calcium serum levels are detected, the glands secrete a hormone that increases the amount of active vitamin D in the bloodstream.
The increase of vitamin D leads to changes that normalize the serum levels of calcium. Here’s how it happens:
- Increases the absorption of dietary calcium by the intestines.
- Promotes resorption of calcium filtered by the kidneys.
- Recruits calcium from the bone when dietary calcium levels are insufficient.
Vitamin D also has been shown to support healthy immune function, mood, and the maintenance of healthy blood pressure already in the normal range.
It’s true your body can make vitamin D with the sun’s help. But a lack of vitamin D—and all out deficiency—are common, especially the further you live from the equator. This has a lot to do with the angle of the sun—the lower the sun is in the sky, the more UVB the atmosphere filters out. That’s why vitamin D supplementation is an effective, and safe way to get the vitamin D you need.