Vitamin A stress test

Vitamin A (axerophthol) is a fat-soluble vitamin with a molecular weight of 286.44, existing in two natural forms - retinol (A1) and 3-dehydroretinol (A2). The precursor of vitamin A is β-carotene.

Chemical formulas of ß-carotene and vitamin A

In the stress test, vitamin A is administered in the form of a fat-soluble ester, which is hydrolyzed by pancreatic enzymes and then re-esterifies in the enterocyte most often with palmitic acid; in plasma, it binds to a specific lipoprotein, retinol-binding protein (RBP).

Execution of stress test

The patient comes on an empty stomach, a blood sample is taken and then a load of 5000 IU vitamin A / kg body weight and fluid (tea) is given. A blood sample is taken 3 and 5 hours after vitamin A administration for analysis. Laboratory determination of vitamin A in serum can be performed by extraction method with spectrophotometric measurement. The basic methodology according to Carr-Price is based on the reaction with antimony trichloride, correction for β-carotene is necessary. Modern methods of vitamin A determination use HPLC separation options.

Serum vitamin A reference values are in the range of 1.8–2.3 µmol / l, and clinically simple determination of serum levels is rarely used. The vitamin A stress tolerance test is evaluated according to the increase in serum levels at 3 and 5 hours after the test load. Normal values in 3 hours are in the range 3.6–12.6 µmol / l, in 5 hours 7.2–24.6 µmol / l. The pathological result of the test is at values <3.6 µmol / l in 3 hours, resp. <7.2 µmol / l in 5 hours.

Clinical significance

The vitamin A tolerance test is of clinical importance in the differential diagnosis of malabsorption syndrome. The positivity of the test correlates with the excretion of fats in the stool.

Resources

Related article

• with permission of the author taken from KOCNA, Petr. GastroLab : MiniEncyklopedie laboratorních metod v gastroenterologii [online]. ©2002. The last revision 2011-01-08, [cit. 2011-03-04]. <http://www1.lf1.cuni.cz/~kocna/glab/glency1.htm>.

Used literature

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• ANDREOLI, R. , et al. Development of a simplified method for the simultaneous determination of retinol, alpha-tocopherol, and beta-carotene in serum by liquid chromatography-tandem mass spectrometry with atmospheric pressure chemical ionization. Anal Bioanal Chem. 2004, y. 378, no. 4, p. 987-94, ISSN 1618-2642 (Print), 1618-2650 (Electronic). PMID: 14598012.

• TANG, G. , et al. Short-term (intestinal) and long-term (postintestinal) conversion of beta-carotene to retinol in adults as assessed by a stable-isotope reference method. Am J Clin Nutr. 2003, y. 78, no. 2, p. 259-66, ISSN 0002-9165 (Print), 1938-3207 (Electronic). PMID: 12885706.

• JOHNSON, EJ. , et al. Evaluation of vitamin A absorption by using oil-soluble and water-miscible vitamin A preparations in normal adults and in patients with gastrointestinal disease. Am J Clin Nutr. 1992, y. 55, no. 4, p. 857-64, ISSN 0002-9165 (Print), 1938-3207 (Electronic). PMID: 1550069.

• SCHMIDT, G. , et al. Vitamin A absorption test. I. Pancreatic insufficiency and sprue (author's transl). Med Klin. 1975, y. 70, no. 43, p. 1745-50, ISSN 0025-8458. PMID: 1186578.