Over 200 women took part in the three year study, published in the American Journal of Clinical Nutrition, which adds to an ever-growing body of science suggesting an urgent need to review current daily intakes of the vitamin. Vitamin D is produced in the skin on exposure to UVB radiation and can also be consumed in small amounts from the diet. However, increased skin pigmentation reduces the effect of UVB radiation meaning darker skinned people are more at risk of vitamin D deficiency. Vitamin D deficiency can lead to a range of health problems, including rickets, poor tooth formation, convulsions, general ill health, and stunted growth. It has also been linked to an increased risk of certain cancers, cardiovascular disease, diabetes, and osteoporosis. Researchers from Winthrop University Hospital, Mineola, New York performed a dose-response experiment with 208 healthy African-American postmenopausal women. Half the women were assigned to the vitamin D intervention arm of the trial and received daily supplements of 800 IU D3 (20 micrograms) for two years, and 2,000 IU (50 micrograms) for the final year. Generally, blood levels of 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, of 50 nanomoles per litre (nmol/L) are suggested as the lower limit of the normal range. Studies have reported however that levels of parathyroid hormone (PTH), a hormone that regulates calcium balance, and calcium absorption are not optimised below serum 25(OH)D levels of 80 nanomoles per litre. The authors, led by Sonia Talwar, report that the lower dose raised 25(OH)D levels from a baseline average of 47 nmol/L to 71.4 nmol/L after three months. After three months at the higher dose (2,000 IU), the average serum concentration of 25(OH)D was 87 nmol/L. Moreover, 95 per cent of the participants achieved a serum 25(OH)D concentration greater than 50 nmol/L, while levels greater than 75 nmol/L were achieved by only 60 per cent. "Supplementation with 50 micrograms per day (2000 IU/d) oral vitamin D3 is sufficient to raise serum 25-hydroxyvitamin D concentrations to greater than 50 nmol/L in almost all postmenopausal African American women," wrote Talwar. "However, higher doses were needed to achieve concentrations greater than 75 nmol/L in many women in this population." Taking their results one step further, they formulated an algorithm in order to allow for the prescription of vitamin D needed to achieve optimal serum concentrations. Talwar and co-workers report that a daily dose of 2800 IU is needed if the individual has a starting 25(OH)D level of at least 45 nmol/L, while a daily dose of 4000 IU is needed for individuals with 25(OH)D levels less than 45 nmol/L. Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. The former, produced in the skin on exposure to UVB radiation (290 to 320 nm), is said to be more bioactive. The latter is derived from plants and only enters the body via the diet. Both D3 and D2 precursors are hydroxylated in the liver and kidneys to form 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D), the biologically active form that is tightly controlled by the body. A recent review of the science reported that the tolerable upper intake level for oral vitamin D3 should be increased five-fold, from the current tolerable upper intake level (UL) in Europe and the US of 2000 International Units (IU), equivalent to 50 micrograms per day, to 10,000 IU (250 micrograms per day) (American Journal of Clinical Nutrition, March 2007, Vol. 85, pp 649-650). Source: American Journal of Clinical Nutrition December 2007, Volume 86, Number 6, Pages 1657-1662 "Dose response to vitamin D supplementation among postmenopausal African American women" Authors: S.A. Talwar, J.F. Aloia, S. Pollack and J.K. Yeh
Doses of vitamin D3 of 2,000 International Units (IU) - the current tolerable upper intake level (UL) in Europe and the US - are needed to ensure blood levels of the vitamin amongst post-menopausal African-American women, says a new study.