Retinol (Vitamin A)

Foreword

Vitamins are minor but essential constituents of food. They are required for the normal growth, maintenance and functioning of the human body. Hence, their preservation during storage and processing of food is of far-reaching importance.

The vitamin requirement of the body is adequately supplied by a balanced diet. A deficiency can result in hypovitaminosis and, if more severe, in avitaminosis. Both can occur not only because insufficient supply of vitamins by food intake, but can be caused by disturbances in resorption, by stress and by disease.

An assessment of the extent of vitamin supply can be made by determination of vitamin content in blood plasma, or by measuring a biological activity which is dependent on the presence of a vitamin, as are many enzyme activities.

Vitamins are usually divided into two general classes: the fat-soluble vitamins, such as A, D, E, and K, and the water-soluble vitamins B₁, B₂, B₆, B₁₂, nicotinamide, pantothenic acid, biotin, folic acid, and C.

Biological Role

Retinol is of importance in protein metabolism of cells, which develop from the ectoderm (such as skin or mucous-coated linings of the respiratory or digestive systems). Lack of retinol in someway negatively affects epithelial tissue (thickening of skin, hyperkeratosis) and causes night blindness.

Requirement and Occurrence

The daily adult requirement of vitamin A is 1.5-1.8 mg. Approx. 75% is provided by retinol intake (as fatty acid esters), while the remaining 25% is through beta-carotene and other provitamin active carotenoids. Due to the limited extent of carotenoid cleavage, at least 6 g of beta-carotene are required to yield 1 g retinol.  

Vitamin A resorption and its storage in the liver occur essentially in the form of fatty acid esters. Its content in liver is 250 micro grams/grams fresh tissue, i.e. a total of about 240-540 mg is stored. The liver supplies the blood with free retinol, which then binds to protein in blood.  Vitamin A concentration is 45-84 micro grams/100 milliliters plasma in adults; values below 15-24 micro g/100 ml indicate a deficiency.

A hypervitaminosis is known, but the symptoms disappear if the intake of retinal is decreased.

Vitamin A occurs only in animal tissue, above all in fish liver oil, in livers of mammals, in milk fat and in egg yolk. Plants are devoid of vitamin A but do contain carotenoids, which yield vitamin A by cleavage of the centrally located double bond (provitamins A) 

Carotenoids are present in almost all vegetables but primarily in green, yellow, and leafy vegetables (carrots, spinach, cress, kale, bell peppers, paprika peppers, tomatoes) and in fruit, with outstanding sources being rose hips, pumpkin, apricots, oranges and palm oil, which is often used for yellow coloring. Animal carotenoids are always of plant origin, derived from feed.

Stability and Degradation

Food processing and storage can lead to 5-40% destruction of vitamin A and carotenoids. In the absence of oxygen and at higher temperatures, as experienced in cooking or food sterilization, the preferential reactions are isomerization and fragmentation. In the present of oxygen, oxidative degradation leads to a series of products, some of which are volatile. This oxidation often parallels lipid oxidation. The rate of oxidation is influenced by oxygen partial pressure, water activity, temperature, etc. Dehydrated foods are particularly sensitive to oxidative degradation.

published 1 year ago

infotech25

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