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Vitamin D target level surrounded by uncertainty

Most Australians get more than 90% of their vitamin D through sun exposure of the skin. Tricia Wang

A new joint position statement on target levels for vitamin D, published in the Medical Journal of Australia (MJA) by the Australian and New Zealand Bone and Mineral Society and Osteoporosis Australia today, maintains the currently accepted vitamin D target level, despite recent calls for it to be raised. This is a hotly debated issue, with considerable support worldwide for target levels, at least in adults, to be raised to much higher levels than many researchers believe are required for good health.

The position statement is explicitly aimed at vitamin D levels during pregnancy and in infants, children and adolescents. Low vitamin D levels are common in Australia: 48% of pregnant women whose vitamin D status was tested had low levels, and the vitamin D levels of babies reflect those of their mother during pregnancy and at birth. Importantly, the position paper provides well-rounded advice about appropriate sun exposure as well as vitamin D supplementation for these populations.

Most Australians get more than 90% of their vitamin D through sun exposure of the skin. Once formed in the skin, vitamin D is taken up into the blood stream and then has to go through two biochemical reactions – firstly in the liver to form 25-hydroxyvitamin D (25(OH)D) and then in the kidney to form the active hormone. It’s the concentration of 25(OH)D that’s measured in blood when you have a vitamin D test, and this is expressed in terms of nanomoles per litre (nmol/L).

In the new position paper, the two bodies conclude that the target blood level for adequate vitamin D is a 25(OH)D concentration of 50nmol/L or greater. Nevertheless, many experts around the world advocate for higher minimum targets, such as 75 or 80nmol/L. Opinions differ according to how the research evidence is interpreted, and it is worth thinking about how decisions are made on what constitutes the appropriate target levels.

The link between sun exposure and healthy bones was first noted in 1822, based on observations of rickets in urban Polish children. Rickets is a bone disease of young children, seldom seen in developed countries today, that causes weakening of the bones and typically results in bowing of the legs

A Mississippi sharecropper’s child suffering from rickets and malnutrition, photographed in 1935. Children's Bureau Centennial

The link between sun exposure and rickets was later shown to be vitamin D. And so the first vitamin D target proposed was at a level sufficient to avoid rickets. We now know that rickets is seldom seen before vitamin D levels are very low, what we would call severe deficiency of levels well below 20nmol/L.

In 1997, the Standing Committee on the Scientific Evaluation of Dietary Reference Intakes described the normal range of vitamin D levels in terms of the average in the population for a group of healthy individuals. These were “usual” levels, rather than a definition based on health or disease.

One problem with this definition is that, because vitamin D levels reflect how much sun exposure people have had, “usual” levels vary according to season and to location. Higher latitude regions typically have lower sun exposure and therefore lower vitamin D levels than lower latitude regions, and levels are lower in winter than in summer. Target levels would differ in Australia depending whether you lived in Hobart or Brisbane!

The primary role for vitamin D in the body is to maintain serum calcium levels. Calcium is critically important in the physiology and biochemistry of cells and blood levels must be maintained within a narrow range.

When calcium levels fall, parathyroid hormone is released and this stimulates conversion of inactive vitamin D into the active hormone. This increases absorption of calcium from the diet in the gut and draws the mineral out of the bones to bring its level back to normal. The latter can weaken the bones and that eventually results in diseases such as rickets.

More recently, the target level for vitamin D has been focused on the levels required to minimise withdrawal of calcium from bones. Many studies have looked at the vitamin D levels required to minimise levels of parathyroid hormone – but the results have varied from optimal level being 30nmol/L to more than 100nmol/L.

This wide variation may be due to high variability in the tests (assays) for both vitamin D and parathyroid hormone; different methods of statistical analysis of the data; or different population groups being tested.

The most recent report from the US Institute of Medicine (IOM) reviewed a wide range of other markers of bone health, including measures of calcium absorption in the gut, bone density measurements and risk of fracture. But none of these provided a clear answer to the desirable target for vitamin D.

The report also raised questions about whether the target level was the same for all people, at all times of the year, and in relation to all health outcomes. After their comprehensive review, the authors of the IOM report concluded that, for the moment, in the absence of any better evidence to the contrary, a level of 50nmol/L or greater, should be the target for vitamin D.

Today’s position statement reflects what the IOM also settled at, but clearly, many questions remain.

Challenges to redefining an optimal level for vitamin D – if a universal target is even possible – include better understanding individual and ethnic variability in vitamin D needs in relation to bone health, whether there are disease-specific optimal target levels, whether seasonal variability is healthy or unhealthy, and, particularly relevant here, whether infants, children, adolescents, adults and the elderly all have the same target levels.

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