Vitamin D in a healthy body
Most of us make vitamin D3 from cholesterol in our sun-exposed skin. We also get D3 from fish oils, as well as vitamin D2 from mushrooms and other plants. Both D2 and D3 travel in the blood and are activated by two chemical reactions: the first in the liver and the second in the kidney. The reviewers stated that D3 is three times as efficiently activated as D2.
Active vitamin D raises blood calcium levels by increasing dietary calcium absorption and decreasing urinary calcium loss. When adequate calcium levels are maintained, it is properly deposited in the bone. Vitamin D has a similar effect on phosphorus, another bone mineral.
In addition to regulating blood levels of calcium and phosphorus, the reviewers noted that vitamin D plays a role in controlling cell division and preventing cancer, strengthening the immune response while preventing chronic inflammation, stimulating insulin production, and maintaining muscle function and tone.
Vitamin D deficiency
Here are some risk factors for vitamin D deficiency:
- Advanced age. Our ability to make vitamin D decreases with age as our stores of the cholesterol-derived vitamin D precursor in the skin dwindle.
- Dark skin or sunscreen use. Skin pigment and sunscreens absorb the ultraviolet sunlight, making it less available for vitamin D production.
- Insufficient sun exposure. The low winter sun angle and intensity leave those living at higher latitudes, such as the northern US and Canada, unable to make vitamin D for four to six months each year. Even when the sun's intensity is sufficient, many people limit their sun exposure by staying indoors or covered up.
- Low dietary intake. People who don't take advantage of fortified foods (mainly milk) or take fish oil on a regular basis have poor dietary intake.
- Obesity. When the body has more fat stores, a greater percentage of vitamin D is stored in it, making it less available.
"The consequences of vitamin D deficiency are the painful bone disease osteomalacia, worsening of proximal muscle strength and postural sway, osteopenia, osteoporosis, and increased fracture risk," according to researchers in Nutrition and Clinical Practice, who pointed out that some research showing that nonspecific musculoskeletal pain can be a symptom of vitamin D deficiency and is sometimes misdiagnosed as fibromyalgia syndrome or myositis.
Improving your D status
"Vitamin D deficiency can be prevented through sensible sun exposure, adequate dietary and supplemental intake, or pharmacologic vitamin D treatment," they said. "Children and adults can expose arms and legs to sunlight 2 to 3 times a week for about 5 to 10 minutes, depending on time of the day, season, and latitude, before applying a sunscreen."
In addition, they recommend supplements for people with risk factors for deficiency, suggesting that 1,000 IU of D3 per day may be more effective at preventing deficiency than the current RDA of 600 IU per day for adults under 70 and 800 IU per day in those over 70. Finally, they recommend checking vitamin D status to be sure that lifestyle, diet, and supplements are effectively maintaining healthy vitamin D levels in the body.
(Nutr Clin Pract 2007;22:297-304)