5 Determination of Vitamin A and Zinc levels: Serum samples will be evaluated for Vitamin A level by ELISA (e.g., Human Vitamin A, EIAAB) according to the manufacturer's instructions. Briefly, the microtitre plate provided in this kit has been pre-coated with an antibody specific to Vitamin A. Standards or samples are then added to the appropriate microtitre plate wells with a biotin-conjugated polyclonal antibody preparation specific for VA and Avidin conjugated to Horseradish Peroxidase (HRP) is added to each microplate well and incubated. Then a tetramethylbenzidine substrate solution is added to each well. Only those wells that contain Vitamin A, biotin-conjugated antibody and enzyme-conjugated Avidin will exhibit a change in colour. The enzyme-substrate reaction is terminated by the addition of a H2SO4 and the color change is measured spectrophotometrically at a wavelength of 450 nm. The concentration of VA in the samples is then determined by comparing the O.D. of the samples to the standard curve.