Vitamin D deficiency is becoming an increasingly important cause of poor bone health. The key points:
It is generally agreed that most western diets without fortification don't supply adequate amounts of vitamin D in those who have insufficient sun exposure. Oily fish is the best source with small amounts in eggs and meat. Many countries have vitamin D fortification of food but the actual practice differs widely from jurisdiction to jurisdiction. The most commonly fortified foods are milk, margarine and cereal products (Nowson 02). Even with Vitamin D fortification, many at risk people in western countries will not receive adequate amounts of vitamin D, particularly those who get little or no sun exposure or who consume little of the fortified foods. Even in a very sunny country as Australia, marginal deficiency of Vitamin D (defined as levels between 25 to 50 nmol/L) is common with reported rates between 23 and 43% (Nowson 02). Clearly the message to avoid sun exposure has got through but the negative effect of low vitamin D levels is now increasingly being recognized as a major problem. Bone disease associated with low levels of vitamin D. Vitamin D levels under 10nmol/L (severe deficiency) will lead to rickets in babies and osteomalacia in adults. Inadequate vitamin D levels above 10nmol/L are associated with osteoporosis. The exact cut off for prevention of osteoporosis is uncertain but in older people safe vitamin D levels probably should be above 100nmol/L. (See below) Blood levels of vitamin D are positively related to bone mineral density. The results of the National Health and Nutrition Examination Study III (NHANES III) suggest that bone mineral density would be improved if a higher target level of D were aimed for. For whites in the 20 to 49 year age group, continuing improvement in bone mineral density is seen at blood levels of vitamin D well above those commonly seen in many westerners. The clear implication is that pushing vitamin D levels beyond accepted lower normal limit of 50nmol/L will benefit many people.
Our closest relatives in the animal kingdom, the old world primates have much higher levels of vitamin D. Physiologically these animals are very similar to humans. The level of vitamin D in them has been measured at between 110 to 150 nmol/L, much higher than the levels commonly seen in many westerners. This suggests that we have evolved to have these sorts of concentrations of vitamin D in our blood and that the adoption of our current largely indoor life style with diets poor in vitamin D has left us with levels far below our evolutionary optimum. (Nutrition Action 06)
How much vitamin D is required to reduce the risk of hip and other non-vertebral fractures? A meta analysis of five randomized controlled trials for hip fracture prevention involving 9294 participants and seven randomized trails for non-vertebral fractures with 9820 participants was reported recently. A 26% reduction of hip fractures and 23% reduction of non-vertebral fractures was seen with oral supplementation of 18-20 mg (700-800IU) of D3 per day. No benefit was seen with a supplement of 10 mg (400IU) per day. Optimal prevention of fractures occurred with vitamin D blood level of around 100nmol/L. To achieve this level requires around 30 micrograms (1200 IU) per day of D3 if the pretreatment level was between 44 and 77nmol/L. For those whose starting levels were below 44nmol/L, even more D3 is required. Calcium supplementation gave no added benefit (Bischoff-Ferrari 06). This meta analysis evidence relating to the effectiveness of vitamin D supplementation has been further updated in 2009. Data from 20 randomized double-blinded trials was now included.
The top graph (A) shows that the relative risk of non-vertebral bone fracture drops progressively from around 300IU/day of vitamin D to about 0.6 relative risk (RR) at 750IU/day of vitamin D. In the lower graph (B), this is reflected in the actual concentration of vitamin D in the blood with the relative risk of fracture dropping progressively from 1.0 RR at 58nmol/L to about 0.6 RR at 112nmol/L. (Bischoff-Ferrari 09) There is strong evidence that higher levels of vitamin D are protective against a range of disorders which include poor muscle coordination in the elderly, colon cancer and periodontal disease.
What intake of vitamin D is required to increase levels to desirable 90 -100nmol/L? This is not been worked out exactly and depends to some extent on the background intake/production of vitamin D. Some indication can be gained from a variety of dosage studies as outlined in the following table.
Current US recommendations are for
8mg/day
(200IU) for young adults, 16mg/day(400IU)
for those aged 51-70 and 24 mg/day
(600IU) for the over 70.
References: (Bischoff-Ferrari 06) Heike A Bischoff-Ferrari, Edward Giovannucci, Walter C Willett, Thomas Dietrich, and Bess Dawson-Hughes. Estimation of optimal serum concentrations of 25-hydroxyvitamin D for multiple health outcomes. Am J Clin Nutr 2006;84:18–28. (Bischoff-Ferrari 09)Heike A. Bischoff-Ferrari, Walter C. Willett, John B. Wong, Andreas E. Stuck, Hannes B. Staehelin, E. John Orav, Anna Thoma, Douglas P. Kiel, Jana Henschkowski. Prevention of Nonvertebral Fractures With Oral Vitamin D and Dose Dependency. Arch Intern Med. 2009;169(6):551-561 (Nutrition Action 06) Centre for Science in the Public Interest. Are you D deficient? Nutrition Action Health Letter 2006;33:3-7 (Nowson 02) Caryl A Nowson, Claire Margerison. Vitamin D intake and vitamin D status of Australians. Medical Journal of Australia. 2002; 177:149-152 |
