Serum retinol levels remain constant over a wide range of intakes and liver retinyl ester stores. RBP levels are usually constant and do not reflect vitamin A intake except at extremes of intake. RBP is bound to transthyretin which limits glomerular loss. Only apoRBP which is ~4% of total RBP is filtered. It is usually not necessary to measure RBP when serum retinol is determined.
To my knowledge the relationship between vitamin A, RBP and Zinc is vague. Zinc is concerned with protein synthesis and RBP is a protein. Possibly in a zinc deficiency the gut epithelium is impaired, resulting in malabsorption of vitamin A. I have no references to this.
Winklhofer et al Clin Chem 43 146 1997 gives the alpha-tocopherol : cholesterol ratio 4.3 - 6.8 mmol/mol.
Serum beta carotene would also be a useful index of absorption in CF and should be included.
Jeffrey Barron
Dr Jeffrey Barron
Consultant Chemical Pathologist
St Helier Hospital
Carshalton
Surrey SM5 1AA
UK
Tel UK 20 8296 2660
Fax UK 20 8641 2633
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