Current guidelines for Vitamin D evaluation, treatment and prevention of D deficiency have recently been published by leading Vitamin D expert, Professor Michael Holick and colleagues. This paper is critical reading for those involved in treating Vitamin D deficiency and optimizing D levels for health and well being.

Take homes:
* The ideal test to evaluate Vitamin D status is the level of serum circulating 25-hydroxyvitaminD [25(OH)D];
* Vitamin D deficiency is defined as a 25(OH)D below 50 nmol/L;
* Vitamin D insufficiency is defined as a 25(OH)D of 51–74 nmol/L;
* Vitamin D sufficiency is defined as a 25(OH)D of 75–250 nmol/L;
* Vitamin D toxicity begins at 25(OH)D levels >375 to 500 nmol/L;
* It is suggested that adults aged 19–50 yr require at least 600 IU/d of vitamin D to maximize bone health and muscle function. However, to raise the blood level of 25(OH)D consistently above 75 nmol/L may require at least
1500–2000 IU/d of vitamin D.
* It is suggested that all adults aged 50–70 and 70+ yr require at least 600 and 800 IU/d, respectively, of vitamin D. However, to raise the blood level of 25(OH)D above 75 nmol/L may require at least 1500–2000IU/d of supplemental vitamin D;
* It is suggested that all adults who are vitamin D deficient be treated with 50,000 IU of vitamin D2 or vitamin D3 once a week for 8 wk or its equivalent of 6000 IU of vitamin D2 or vitamin D3 daily to achieve a blood level of 25(OH)D above 75 nmol/L, followed by maintenance therapy of 1500–2000 IU/d;
* In obese patients, patients with malabsorption syndromes, and patients on medications affecting vitamin D metabolism, it is suggested that a higher dose (two to three times higher; at least 6000–10,000 IU/d) of vitamin D
to treat vitamin D deficiency to maintain a 25(OH)D level above 75 nmol/L, followed by maintenance therapy of 3000–6000IU/d.

 Source: Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. 
Holick MF, Binkley NC, Bischoff-Ferrari HA, Gordon CM, Hanley DA, Heaney RP, Murad MH, Weaver CM. J Clin Endocrinol Metab. 2011 Jun 6 http://www.ncbi.nlm.nih.gov/pubmed/21646368