Fat-soluble vitamins, vitamin A

Vitamin A

Commonly known as retinol, vitamin A is present in two form in our diet i.e. retinol and carotenoids .Human body can convert them into retinal and retinoic acid which are biologically active forms of vitamin A .

However, during conversion and absorption some losses may occur, hence one micro g of trans B- carotene is equivalent to about 1/6 micro g of retinol. Some carotenoids such as lycopene do not have vitamin . A activity, but they impart red color to food items.

Vitamin A activity is measured in the international unit (IU) but is also expressed in micrograms of retinol equivalent. One I.U. of vitamin A is equivalent to 0.3 micro g retinol equivalent.

Vitamin A helps to prevent infections and is essential for the growth and metabolism of cells. It participates in the formation of rhodopsin which is found in retinal and helps to adjust vision in dim or reduced light. Vitamin A also helps to fight against the infection of mouth lining, nose, and digestive tract.

Deficiency of vitamin A;

Vitamin A deficiency may occur because of liver diseases and bile duct obstruction since bile salt are required for its absorption. The deficiency of vitamin A causes loss of sight and skin keratinization . In children , its deficiency lead to retarded growth and development .

Measurement of vitamin A status;

Vitamin A status is grouped in five categories i.e. adequate, marginal, deficient, excessive and toxic. Vitamin A assessment includes measurement of plasma retinol concentration, dose-response test and retinol isotope dilution test.

Less frequently used test are dark adaptation, conjunctiva epithelial cell examination and liver biopsy.

a)Plasma level

Plasma retinol concentration in is the most used measure to assess the status of vitamin A . Under normal conditions, 95r%of Vitamin A is present in the form of retinol and bound to retinol-binding proteins, whereas, 5% is unbound in the form of retinal ester.

Low plasma value indicate deficiency or depletion of body reserves of vitamin A since slight change does not affect its concentration. However, plasma concentration help to diagnose clinical deficiency and dietary adequacy. Plasma levels <10 micro g /dL (0,35 micro mol/L)are indicative of severe deficiency, and level of <20 micro g /dL (0.70micro mol/L)are categorized as deficient.

Plasma values >20 micro gram/ dl are consider adequate and safe while values >300 microgram/dl are indicative of hypervitaminosis.

b) Relative dose response test:

The relative dose response test and modified dose response test are based on the principle that when retinol stores are high , plasma values are slightly affected by oral intake of vitamin A. But in case of low level of plasma retinol, a marked peak is observed after 5 hours of oral dose.

With the depletion of vitamin A stores unbound form retinol binding protein accumulate in the liver. Once vitamin A is absorbed from small intestine it is taken up by the liver where it is bound to the apo-RBP and form complex known as holo-RBP. In the RDR test a sample is taken before and after administration of vitamin A separated by an interval of 5 hours.

A comparison of post dosing holo-RBP and fasting Apo-RBP is made to find the status of vitamin A.

RDR= Vitamin A after 5h-Vitamin A at fasting/ Vitamin A after 5h

multiply by 100.

The plasma RDR value of >50% between 20–50% and <20% are indicative of acute deficiency, marginal status and adequate intake. One limitation of this test is that subject must wait for 5h to get their sample drawn.

MDRD is also based on the same principle but only one sample is drawn after 5h of administrating the dehydro-retinol ( naturally occurring form of vitamin A ) but rarely present in most diets the response is measured in molar ratio of dehydro-retinol to rational serum A ratio <0.03 represent the adequate intake, whereas >0.06 indicate poor vitamin A status.

The limitation of this test is that dehydro-retinol is not available commonly, chromatography is performed to separate the two forms of vitamin A.

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