How did we arrive at a worldwide epidemic of vitamin D deficiency?
Our levels of 25-hydroxyvitamin D are distressingly low, putting pregnant women and newborns, among others, at risk
Which women are at high risk of vitamin D deficiency?
Women whose skin is darkly pigmented are at high risk of vitamin D deficiency; they require approximately three times the amount of exposure to sunlight as women with lightly pigmented skin to generate the same amount of vitamin D.10 In one study of pregnant adolescent African-American women, 46% had a 25OH vitamin D level <20 ng/mL.11
Women who wear concealing clothes, such as a burka, are also at increased risk of vitamin D deficiency.12,13
Women living in poverty may have dietary and lifestyle patterns that limit vitamin D intake and exposure to sun. In a study conducted in Camden, New Jersey, total vitamin D intake was reported to be low in African-American and Hispanic women.14
Musculoskeletal health in women. Many young women are deficient in vitamin D. In a recent study of 16- to 22-year-old women living in sun-drenched California, 59% of subjects had, surprisingly, a 25OH vitamin D level <30 ng/mL; 41% had a level <20 ng/mL.15
Of interest, women in this study who had a low vitamin D level tended to have increased fat infiltration in muscle at the mid-thigh (detected by computed tomographic scanning). Based on other studies, it is now thought that fat infiltration reduces muscle strength and undermines physical performance, including athletic performance. In a study of young adolescents, a positive relationship was detected between the vitamin D level and enhanced muscle function, including muscle power, velocity, and jump height.16
Osteoporosis. Many postmenopausal women are vitamin D deficient. A low level of vitamin D is associated with decreased intestinal calcium absorption, a negative calcium balance, and a rise in the parathyroid hormone level, which accelerates bone resorption.
A total calcium intake of approximately 1,500 mg/d in postmenopausal women is associated with positive calcium balance. A serum 25OH vitamin D level of about 20 to 40 ng/mL maximally suppresses PTH secretion.
A low 25OH vitamin D level is associated with an increased risk of hip fracture17; adequate calcium and vitamin D supplementation reduces the risk of osteoporotic fractures in the elderly. The authors of a meta-analysis of seven randomized trials reported that the risk of fracture was reduced about 35% when women were given vitamin D supplementation at 700 to 800 IU/d—but that risk was not reduced at a dosage of 400 IU/d.18 Similar findings have been reported in other meta-analyses.19
A note of caution: In one randomized trial, supplementation with vitamin D and calcium was associated with a 17% increase in the risk of kidney stones.20
Colon cancer. In prospective observational studies, a strong inverse relationship has been observed between levels of 25OH vitamin D and the risk of colon cancer.
For example, in the European Prospective Investigation into Cancer and Nutrition (EPIC) study, the vitamin D level was measured in health study participants, and analysis of the relationship between this level and new, incident cases of colon cancer revealed that 25OH vitamin D levels >30 ng/mL were associated with a 12% decrease in the risk of colon cancer, compared to subjects with levels of 20 to 30 ng/mL.21 For subjects who had a 25OH vitamin D level >40 ng/mL, the risk of colon cancer was reduced by 23%.
A prospective randomized trial would be required, however, to prove that vitamin D has a protective effect on the risk of colon cancer.
A taste one doesn’t soon forget—forgotten
Throughout the 1950s, I remember the mandatory weekly dose of natural cod liver oil, a rich source of vitamin D. Somehow, with a movement away from that weekly regimen, and miscalculation of what constitutes optimal vitamin D supplementation, we’ve entered a period of worldwide vitamin D deficiency.
It is clear that for most women, vitamin D supplementation at 400 IU/d is inadequate to prevent deficiency. Most women should consider a vitamin D dosage of 800 to 1,000 IU/d. Measuring the 25OH vitamin D level, with the aim of providing supplemental vitamin D to achieve a value >30 ng/mL, will help end the epidemic.22
