My Study Stuff

03/12/2013

Vitamin D deficiency

Filed under: Health — anlactunay @ 8:35 AM

Vitamin D deficiency is common, even in Australia. Most experts would recommend that serum 25-hydroxy vitamin D3 be at least 70 nmol/L. Concentrations less than 50 nmol/L are common and in certain groups (the elderly, those with dark skin, those who cover up habitually or avoid sunlight) may be less than 30 nmol/L.
Vitamin D nutrition is dependent primarily on sun exposure. Twenty to thirty minutes in the sun is equivalent to ingesting approximately 10,000 IU, but this depends on several factors, especially skin pigmentation. Vitamin D may be acquired from some dietary sources (including oily fish, milk, margarine, cheese and eggs), but large quantities need to be consumed to meet requirements. Cod-liver oil is a good source of vitamin D but it also contains vitamin A and there are reports of vitamin A toxicity when it is consumed in large quantities.
Given the relationship between sun and serum vitamin D levels, vitamin D status may be a surrogate for outdoor physical activity. Furthermore, there is an inverse relationship between serum vitamin D and adiposity. These potential confounders make it difficult to make conclusions and recommendations from observational studies.
The pancreatic beta cell has the receptor for vitamin D and also the 1-alpha-hydroxylase enzyme that converts vitamin D to its most active form, 1,25(OH)2D3. The exact mechanism of vitamin D’s potential benefit in diabetes (outlined below) is not known, but has been proposed to be a direct effect on the pancreatic islet, or indirectly, via calcium-mediated insulin secretion or a reduction of inflammatory cytokines.

TYPE 1 DIABETES
Studies in mice have shown that vitamin D supplementation in non-toxic doses can prevent type 1 diabetes and reduce established autoimmune inflammation of the islets. In humans, a number of cross-sectional studies of vitamin D supplementation in pregnant women and infants have shown a reduced incidence of type 1 diabetes in the babies of mothers given the vitamin. A meta-analysis of these observational studies concluded that vitamin D supplementation in infancy may be protective against the development of type 1 diabetes (odds ratio 0.71, 95% CI 0.60- 0.84).[1]

TYPE 2 DIABETES
Numerous cross-sectional and prospective trials report beneficial outcomes of vitamin D supplementation in patients with type 2 diabetes. Of only two randomised controlled trials, both poorly designed and of very short duration (four days -three weeks), one showed some benefit, the other none. In subjects with impaired glucose tolerance, a post-hoc analysis of a randomised control trial designed for osteoporosis outcomes found that vitamin D and calcium reduced insulin resistance and fasting blood glucose levels relative to the subjects receiving a placebo.
Currently, recommendations for target serum vitamin D levels in people with diabetes remains problematical as there is an absence of well-designed, randomised controlled trials. However, for people who are clearly vitamin D deficient, sub-optimal doses of vitamin D3 replacement are often prescribed due to an unwarranted fear of toxicity, which is extremely rare with oral vitamin D3 supplementation. For example, in our recently completed trial, a dose of 6000 IU per day of vitamin D3 was used for six months with no evidence of toxicity. One sun exposure recommendation is to expose the face, arms and hands to the sun before 10am and after 2pm for six to eight minutes in summer and fifteen to forty minutes in winter, depending on latitude.[2] This recommendation is for people of moderately fair skin; exposure for three to four times longer is recommended for people with highly pigmented skin. For the many people who find it difficult or are not willing to meet these guidelines, oral vitamin D is a safe and effective alternative.

References
[1]. Zipitis CS et al, Vitamin D supplementation in early childhood and meta-analysis and risk of type 1 diabetes: a systematic review. Arch Dis Child 2008; 93 (6): 512-517
[2]. Position Statement, MJA 2005; 182: 281-28

Dr Shirley Elkassaby
MBBS, FRACP
Endocrinologist,
Royal Melbourne Hospital
Researcher,
Walter & Eliza Hall
Institute of Medical
Research,
Melbourne, Victoria
09:34 11/10/2013
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