Human biomarkers of population vitamin E status
Serum concentrations of α-tocopherol depend on the action of the liver, and the hepatic α-tocopherol transfer protein (α-TTP) is considered as the major regulator of vitamin E status in humans (3). α-tocopherol in serum is therefore the biomarker of choice of vitamin E status.
The majority α-tocopherol in the circulation is associated with low-density lipid fractions and α-tocopherol concentrations may need to be considered in the context of circulating lipid concentrations and may be adjusted for cholesterol concentration (7, 8).
The vitamin E content of foods and dietary supplements is often listed on labels in international units (IUs), which is a measure of biological activity rather than quantity. Naturally sourced vitamin E is called RRR-alpha-tocopherol (commonly labelled as d-alpha-tocopherol); the synthetically produced forms RSR-, RRS- and RSS- are together known as all rac-alpha-tocopherol (commonly labelled as dl-alpha-tocopherol).
Conversion rules are as follows:
To convert from mg to IU:
To convert from IU to mg:
- 1 IU of the natural form is equivalent to 0.67 mg of alpha-tocopherol
- 1 IU of the synthetic form is equivalent to 0.45 mg of alpha-tocopherol
For more information, see the NIH factsheet.
Samples are commonly analysed for Vitamin E and Vitamin A in the same preparation. This is performed by the addition of an internal standard (tocopherol acetate for Vitamin E), a protein crash to disrupt protein binding and solvent extraction followed by chromatographic separation using reversed phase HPLC on a C18 column and detection with a UV/Vis or PDA detector. Vitamin E, serum retinol and carotenoids can be measured simultaneously by HPLC: see this SOP from CDC.
Note: There is no primary reference method listed on the Joint Committee for Traceability in Laboratory Medicine (JCTLM) database at the present time (November 2018). This information will be updated as applicable.
Collection and storage of samples:
Recommendations from the Australasian Association of Clinical Biochemists (AACB) vitamins working party in 2014 for the collection and storage of samples for vitamin E analysis (9):
- Patient preparation: No recommendation for or against the patient to be fasting.
- Sample collection: Recommend collection of blood into plain, gel separator, heparin or EDTA tubes for analysis of serum/plasma
No recommendation for or against specific transport conditions if delivered to laboratory within 24 hours of collection.
Recommend that samples are chilled to 4°C during transport to the laboratory if transport takes between 24 hours and four days.
Recommend against delaying sample transport and receipt by laboratory beyond four days.
No recommendation for or against light protection of the sample during short term receipt and processing by the laboratory (up to 24 hours under laboratory lighting).
Recommend for short term storage up to one week that vitamin A and E samples are stored at ≤4°C.
Strongly recommend for vitamin E samples to be stored at ≤-20°C for up to six weeks and at ≤-70°C for longer term storage i.e. up to one year.
The stability of α-tocopherol seems to be intermediate compared to retinol and β-carotene with losses evident at higher temperatures and very long term storage. Vitamin E has demonstrated stability at -20°C for at least one year with deterioration evident past this time point.