A large proportion of Australians are vitamin D deficient, which may be in some part due to the success of the Sun Smart message.
What does vitamin D do?
Vitamin D is required to regulate blood calcium levels by promoting intestinal and renal calcium absorption. Vitamin D requirements are mainly fulfilled by sunlight exposure which converts 7-dehydrocholesterol to cholecalciferol (vitamin D3) in the skin (Figure 1). Small amounts of vitamin D3 can be derived from diet. Once formed, vitamin D is hydroxylated to 25-hydroxyvitamin D (25OHD) in the liver. However, 25OHD is biologically inactive and must be renally converted to 1,25 dihydroxyvitamin D (1,25[OH]2D) to exert its biological actions.
Impact of vitamin D deficiency?
Inadequate sunlight exposure is the major cause. This triggers parathyroid hormone (PTH) secretion, which in turn increases bone resorption. PTH also stimulates renal excretion of phosphate causing phosphate deficiency. Consequently, these combinations result in impaired bone mineralisation leading to bone diseases including rickets in children and osteomalacia in adults and may also contribute to osteoporosis.
Who is at risk of vitamin D deficiency?
- people who are institution/house/office bound,
- dark skinned women, particularly if veiled,
- people with osteoporosis or hip fracture,
- people with symptoms suggestive of malabsorption, and
- people taking certain medications, including anticonvulsants and glucocorticoids.
It is recommended that pregnant women at risk, have 25OHD tested during first trimester, however, the Royal College of Pathologists of Australasia recommend all pregnant women be tested.
Biochemical Tests for Diagnosis and Monitoring of Vitamin D Deficiency
Vitamin D
25OHD is a major circulating and storage form of vitamin D, used to assess vitamin D status. Measurement of 1,25 [OH]2D is unnecessary. To monitor treatment, 25OHD can be measured 2-3 months after commencing supplementation, as vitamin D has a long half-life (2-3 weeks).
What cut off should we use to indicate vitamin D deficiency?
The recommended decision limit for 25OHD is 50 nmol/L. However, the Australian guidelines recognise that this limit should be higher in summer than winter. The typical summer limit may be 60 nmol/L, but may be less in northern Queensland, but higher in Victoria and Tasmania. Deficiency can also be graded as mild, moderate or severe.
25OHD (nmol/L) Vitamin D status:
- <12.5 – Severe deficiency
- 5 -29 Moderate deficiency
- 30 – 49 Mild deficiency
- 50 + sufficiency
PTH
PTH may be used sometimes to evaluate a borderline low 25OHD. An elevated PTH with a low 25OHD confirms vitamin D deficiency. However, a PTH level within reference interval does not exclude vitamin D deficiency.
Serum Calcium, Phosphate and Magnesium
Hypocalcaemia and hypophosphataemia may occur in severe vitamin D deficiency although serum calcium and phosphate are usually normal in mild to moderate deficiency. In people on calcitriol (1,25[OH]2D) supplementation, serum calcium and phosphate are used to monitor treatment as toxicity could result in hypercalcaemia. Measurement of serum magnesium is sometimes necessary as hypomagnesaemia may blunt the PTH rise in response to vitamin D deficiency.
LFT and UEC
These tests are important to ensure the active form vitamin D (1,25[OH]2D) can be produced. Occasionally, vitamin D deficiency is detected as a result of isolated mildly raised ALP.
Treatment for Vitamin D Deficiency
Treatment strategies for moderate to severe vitamin D deficiency usually require vitamin D supplementation coupled with advice to increase sun light exposure. Dietary modification alone (even with vitamin – D fortified foods) will not provide adequate amounts of vitamin D. For people who have 25OHD levels in the equivocal range (50-75nmol/L) but are not in the high risk group, it may be advisable to increase sunlight exposure, then to measure 25OHD again in three months.
Vitamin D supplementation and toxicity
Currently most supplements are vitamin D3 (cholecalciferol) in Australia. For adults with moderate to severe deficiency, a recommended start is vitamin D3 such as Ostelin or OsteVit – with D at 3000-5000 IU per day for at least 6-12 weeks, then 1000 IU for ongoing treatment. This is also applicable to women during pregnancy.
Vitamin D toxicity due to supplementation is rare. One report said 10,000 IU per day orally for 90 days in postmenopausal women did not result in adverse effect and monthly doses of 50,000 IU are not uncommon in clinical practice, particularly in nursing homes.
Further Reading
- ANZBMS updated Position Statement, Vitamin D and health in adults in Australia and New Zealand. MJA 196 (11) 2012.
- Adapted from – Dr Ken Sikaris, Vitamin D Insight – November 2018.
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