Vitamin D Interactions & IU Dosing Conversions

Today I was asked to analyze the clinical relevance of a drug interaction (severe rating, Lexicomp) between sucralfate and vitamin D3 with specific regards to patients with CKD. The concern was that vitamin D enhances the absorption of the aluminum from sucralfate, which is already a concern in renal failure patients. However, the trials referenced in the interaction report listed the vitamin D dosages in micrograms rather than IU, which is how it is typically dispensed in the community setting. To determine the relevance of this, I had to convert this dose back to IU. We don't have heavy coverage on nutrition in pharmacy school, but I found the following list of conversions to calculate dose in mcg from international units that I thought others may need or find useful.

• To convert Vitamin A as retinol: 
      From IU to mcg:  IU * 0.3 = mcg
      For example: 5000 IU * 0.3 = 1500 mcg
      From mcg to IU: mcg / 0.3 = IU 

• To convert Vitamin A as beta-carotene: 
      From IU to mcg:  IU * 0.6 = mcg
      For example: 5000 IU * 0.6 = 3000 mcg
      From mcg to IU: mcg / 0.6 = IU 

• To convert Vitamin D: 
      From IU to mcg: IU * 0.025 = mcg 
      For example: 400 IU * 0.025 = 10 mcg
      From mcg to IU: mcg / 0.025 =IU 

• To convert Vitamin E if the product label has DL-Alpha-tocopherol as the ingredient:
      From IU to mg: IU * 0.9 = mg 
      For example:  30 IU * 0.9 = 27 mg 
      From mg to IU: mg / 0.9 = IU

• To convert Vitamin E if the product label has D-Alpha-tocopherol as the ingredient:
      From IU to mg: IU * 0.67 = mg. 
      For example: 30 IU * 0.67 = 20.1 mg
      From mg to IU: mg / 0.67 = IU
For those not keen on math, here is a link to an automatic calculator:

http://www.robert-forbes.com/vitamin-converter

Returning to the topic of the drug interaction, the trial referenced in Lexicomp (Fournier et al, available here from PubMed: D3 & sucralfate) administered 6 micrograms per week (240 IU) to patients on hemodialysis and found this to significantly increase the patients' plasma aluminum concentrations. This increased concentration was maintained for up to 6 weeks after D3 was discontinued. Therefore, it can be inferred that this would be a clinically relevant interaction for a majority of patients. 

Presentation of Aluminum Toxicity

• Osteomalacia
• Premature osteoporosis  
• Mental status changes
• Proximal muscle weakness
• Bone pain
• Nonhealing fractures
• Anemia

References:

1. Yuan B, Klein MH, Contiguglia RS, et al. The role of aluminum in the pathogenesis of anemia in an outpatient hemodialysis population. Ren Fail. 1989;11:(2-3)91-6. [pubmed]

2. "Aluminum Toxicity." : Background, Pathophysiology, Epidemiology. Web. 7 Aug. 2015. <http://emedicine.medscape.com/article/165315-clinical)>.

3. Fournier A, Demontis R, Tahiri Y, et al. 1 alpha OH vitamin D3 increases plasma aluminium in haemodialysed patients taking Al (OH)3. Proc Eur Dial Transplant Assoc Eur Ren Assoc. 1985;21:390-4. [pubmed]