Vitamin A - Benefits, Deficiency Symptoms And Food Sources

Alternative names :: Beta carotene, Retinol

Vitamin A was the first fat-soluble vitamin to be discovered by McCollum and Davis in 1913. Three different forms of vitamin A are active in the body: retinol, retinal and retinoic acid. Collectively, these are known as retinoid. In the natural form, retinoids are found only in animal foods, usually associated with lipids.

The original source of retinoids is the class of plant pigments known as carotenoids, which includes alpha, beta and gamma carotenes. These pigments were so called because one of their main source was carrot. Since carotenes can be converted to vitamin A in the body, they are often referred to as precursors of vitamin A or provitamin A. The main source of carotenes are dark green, leafy vegetables and yellow, orange and red fruits and vegetables.

Of all the carotenoids, betacarotene is the most plentiful in human food. Carotenes are split in the intestine to form retinal which is then converted to retinol. Retinol is absorbed from the intestine and transported to the liver for storage. About 90% of body stores of vitamin A are found in the liver, the remaining stores are in the kidney, lungs, adrenal glands and adipose tissue. From the liver, vitamin A is transported to other tissues to be used for various body functions. A healthy adult has reserves of vitamin A that can last him for months.

Since the efficiency of conversion of betacarotene to retinol is not complete and absorption of carotenes is only about 50%, betacarotene is considered only one-fourth as effective as retinol on a weight-for-weight basis.

Vitamin A is soluble in fat and fat solvents but it is insoluble in water. Also, it is relatively stable to heat. Because of these reasons, vitamin A is fairly stable in general cooking.

Functions and benefits of vitamin A

Vitamin A is a versatile vitamin. Its major roles include:

• Promoting vision
• Participating in protein synthesis and cell differentiation (and thus it maintains the health of epithelial tissues and skin)
• Supporting reproduction and growth

Each form of vitamin A performs specific tasks. Retinol supports reproduction and is the major form in which the vitamin is transported and stored. Retinal actively helps in the betterment of the vision. Retinoic acid acts like a hormone, regulating cell differentiation, growth, and embryonic development.

Vitamin A in vision :- Vitamin A plays an indispensable role. It participates in the conversion of light energy into nerve impulses at the retina of the eye, enabling us to see. Light passes through the cornea of the eye and strikes the cells of the retina. Inside the cells, pigment molecules called rhodopsin absorb light. Each rhodopsin molecule is composed of a protein called opsin a,nd a molecule of retinal. When light energy strikes the retina, the configuration of the retinal changes and gets released from the rhodopsin molecule. This results in the generation of an electrical impulse, which is transmitted to the nerve cells which, in turn, convey the message to the brain. The released retinal has a different configuration and hence, it is inactive. Much of the retinal is converted back to its active form and combined with opsin to regenerate rhodopsin. A lot of retinal is destroyed at night and needs to be replenished through a good diet.

Vitamin A in protein synthesis and cell differentiation :- Vitamin A participates in protein synthesis and cell differentiation. All body surfaces, both inside and outside, are covered by layers of epithelial cells. The epithelial tissue on the outside of the body is in the form of skin and those that line the inside of the body are the mucous membranes - the linings of the mouth, stomach, intestines, lungs, urinary bladder, urethra, uterus, vagina and so on. The epithelial tissue forms the first line of defence against invading microorganisms and infections. Vitamin A helps to maintain a healthy epithelium. It is also required for cell differentiation (development of specific functions) of bone, nerve and epithelial tissues.

Vitamin A in reproduction and growth :- Vitamin A supports reproduction and growth. Retinol helps in sperm development in men and Vitamin A supports normal foetus development in women during pregnancy. Deficiency of vitamin A results in growth failure in children.

Betacarotene as an antioxidant : - The provitamin A betacarotene has been shown to have antioxidant capacity, that is, it has the ability to protect a person against cell damage caused by free radicals. Free radicals are either parts of by-products of normal metabolism, or they may be created by environmental exposure to sunlight, tobacco smoke, car exhaust fumes, ozone, or x-rays. Free radicals damage the DNA, cell membranes, and cell compounds, or even kill the cell. Antioxidant substances help neutralise free radicals in the cell, protecting the individual from possible damages.

Daily allowances of vitamin A

The Recommended Dosage for vitamin A are :-

• 5,000 IU (or 3 mg beta carotene) daily for men.
• 4,000 IU (or 2.4 mg beta carotene) daily for women.

Rich sources of vitamin A

Vitamin A, in the form of retinyl palmitate, is found in beef, calf, chicken liver; eggs, and fish liver oils as well as dairy products including whole milk, whole milk yogurt, whole milk cottage cheese, butter, and cheese.

Vitamin A can also be produced in the body from beta-carotene and other carotenoids (fat-soluble nutrients found in fruits and vegetables). Most dark-green leafy vegetables and deep yellow/orange vegetables and fruits (sweet potatoes, carrots, pumpkin and other winter squashes, cantaloupe, apricots, peaches,and mangoes) contain substantial amounts of beta-carotene. By eating these beta-carotene rich foods, a person can increase their supply of vitamin A.

Deficiency Symptoms of Vitamin A

Because vitamin A is fat-soluble, it is stored in the body's fat for a long time, making deficiency uncommon. However, deficiency symptoms of vitamin A includes :-

• Dry hair or skin.
• Dryness of the conjunctiva and cornea.
• Poor growth.
• Night blindness.

Other possible results of vitamin A deficiency include abscesses in the ears; insomnia; fatigue; reproductive difficulties; sinusitis, pneumonia, and frequent colds and other respiratory infections; skin disorders, including acne; and weight loss.