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Vitamin D deficiency is becoming an increasingly important cause of poor bone health.

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The key points:

  • Optimum vitamin D level is probably around 100nmol/L with most people in western society being below this.
  • It is very difficult to calculate from dietary intake and sun-exposure what your blood level will be.
  • Most people would benefit from an additional 25 mg (1000 IU) of vitamin D3 a day. Gaining this from dietary sources is difficult and sun exposure is not recommended. It is probably easiest to take this as a supplement. (Vitamin D2 is 25% less potent than D3).
  • If in doubt, have your vitamin D level measured. (Medical opinion should be sought in those who have kidney disease, renal stones or parathyroid disease.)
  • Vitamin D is a factor in prevention of other disorders which include poor muscle tone and coordination in the elderly, periodontal gum disease and colon cancer.
Vitamin D deficiency. Probably the biggest public health problem associated with bone health is vitamin D deficiency related the combination of low dietary  intake and reduced natural production because of limited sunlight exposure. In people who spend significant periods of time outdoors, the  major source of vitamin D is the natural conversion of the cholesterol metabolite, 7-dihydrocholesterol, to cholecalciferol (vitamin D3). This is then converted in the liver to 25-hydoxy cholecalciferol, the substance that is commonly assayed for in measuring vitamin D levels. A further step then occurs in the kidneys with the conversion of 25hydoxy cholecalciferol to 1,25-dihydroxy cholecalciferol (calcitriol) which is the main active form of the vitamin. An excellent discussion of vitamin D can be found in Wikipedia (http://en.wikipedia.org/wiki/Vitamin_D)

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.

Graph showing the increase in bone mineral density with increasing levels of Vitamin D in adults 20-49. Circles are whites, squares are Mexican Americans and triangles are African Americans in the NHANES III study. (Bischoff-Ferrari 06)

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. 

Composite graph for various disorders in older people with progressive increase in vitamin D levels:
Top green line shows improvement in bone mineral density.
Reduction in relative risk of fracture (red line) and colon cancer (blue line) (The smaller the value of this measurement, the less the chance of developing that disorder.)
Improvement in muscle tone/coordination by reduction in time to walk 8ft line (yellow line).
Improvement of periodontal disease by reduction in attachment loss of gum to teeth (orange line). (Bischoff-Ferrari 06)
 

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.

Increase in vitamin D level Average level achieved Intake of vitamin D
Around 10-40nmol/L (older people) Around 60nmol/L 10mg (400IU)
31nmol/L 79nmol/L 15mg (600IU)
50-65nmol/L Around 100nmol/L 20mg (800IU)
56nmol/L (Younger people)Around 100nmol/L 100mg (4000IU)

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.
Calculations done on the NHANES III data suggest that a daily oral dose of 50
mg/day (2000IU) may shift the distribution of vitamin D levels seen in the population so that only 10-15% would have levels below 75nmol/L. This dosage would increase the level of vitamin D in already replete people to levels between 110 to 175 nmol/L. As comparison, average levels of vitamin D in farmers was 135nmol/L and lifeguards,163nmol/L. Toxicity is seen in people with levels of 220nmol/L which usually required dosages of 200mg or more of vitamin D (Bischoff-Ferrari 06).

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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