Antioxidant Activity
Vitamin A has several forms that are used for vital functions. Provitamin A, betacarotene, performs antioxidant functions that none of the other forms of vitamin A can achieve. In addition to its vital antioxidant functions, beta-carotene can be split apart into retinal and converted to all other forms of preformed vitamin A, as previously seen in Figures 3-1 and 3-2.
Beta-carotene is one of the most powerful antioxidants in food. Antioxidants neutralize free radicals to reduce the risk of macular degeneration, cancer, heart disease, and stroke. Some of the beta-carotene in foods and supplements can be converted into the retinal form of vitamin A. About 10 percent of the carotenoids (beta-carotene is one of the carotenoids) in plant foods can be converted into retinal. The remaining carotenoids may be used as antioxidants.
Beta-carotene is abundant in yellow and orange vegetables and fruit.
The other forms of vitamin A do not exhibit antioxidant activity. The forms of vitamin A found in meat (retinyl esters), dairy products, and eggs do not possess antioxidant activity. Vitamin A supplements made without beta-carotene or other sources of antioxidants also do not possess antioxidant activity. Many supplements are made with retinyl palmitate and retinyl acetate; these forms of vitamin A are not antioxidants.
Beta-carotene is plentiful in yellow and orange vegetables and fruit. Green vegetables also are rich in beta-carotene; the colorful pigments are masked by the green chlorophyll. Some of the other carotenoids that can be converted into retinal include alpha-carotene and beta-cryptoxanthin. Some carotenoids that cannot be converted into retinal are lycopene (from tomatoes) and lutein. All carotenoids have antioxidant activity.
Vitamin A and Night Vision
Vitamin A is needed by the retina of the eye for vision. The retina is located at the back of the eye. Light passes through the lens of the eye and hits the retina. The retina converts the light into nerve impulses for interpretation by the brain. Retinol is transported by the bloodstream to the retina. In the retina, the retinol is used by the epithelial cells on the inside surface of the retina.
Retinol is stored in the retina in the form of retinyl ester until it is needed. When it is needed, the retinyl ester is hydrolyzed into retinol, as shown in Figure 3-3. The retinol is changed to a special form of retinal that is a “cis” isomer. Isomers are mirror images. This cis-retinal goes to the rod cells of the retina. Rod cells in the retina are responsible for vision in dim light. In the rod cells, the retinal binds to a protein called opsin. The retinal and the opsin together form the compound
Figure 3-3 How vitamin A helps night vision.
rhodopsin, also known as visual purple. Rod cells with rhodopsin are able to detect tiny amounts of light; this is important for night vision. When a photon of light hits a rod cell, cis-retinal is released and transformed to trans-retinal. This release of retinal generates an electrical signal to the optic nerve. The optic nerve sends a signal to the brain. Some of the trans-retinal becomes available for another cycle of vision. Some of the trans-retinol is converted to retinoic acid and is no longer available to bind to opsin to form rhodopsin.
These losses must be replaced with vitamin A either from the diet or from vitamin A stored in the liver. Each cell in the retina contains about 30 million molecules containing forms of vitamin A—and there are over 100 million cells in the retina. If enough retinol is not available to the retina, this can result in night blindness.
Vitamin A Deficiency and Blindness
Vitamin A has several forms that are used for vital functions. Provitamin A, betacarotene, performs antioxidant functions that none of the other forms of vitamin A can achieve. In addition to its vital antioxidant functions, beta-carotene can be split apart into retinal and converted to all other forms of preformed vitamin A, as previously seen in Figures 3-1 and 3-2.
Beta-carotene is one of the most powerful antioxidants in food. Antioxidants neutralize free radicals to reduce the risk of macular degeneration, cancer, heart disease, and stroke. Some of the beta-carotene in foods and supplements can be converted into the retinal form of vitamin A. About 10 percent of the carotenoids (beta-carotene is one of the carotenoids) in plant foods can be converted into retinal. The remaining carotenoids may be used as antioxidants.
Beta-carotene is abundant in yellow and orange vegetables and fruit.
The other forms of vitamin A do not exhibit antioxidant activity. The forms of vitamin A found in meat (retinyl esters), dairy products, and eggs do not possess antioxidant activity. Vitamin A supplements made without beta-carotene or other sources of antioxidants also do not possess antioxidant activity. Many supplements are made with retinyl palmitate and retinyl acetate; these forms of vitamin A are not antioxidants.
Beta-carotene is plentiful in yellow and orange vegetables and fruit. Green vegetables also are rich in beta-carotene; the colorful pigments are masked by the green chlorophyll. Some of the other carotenoids that can be converted into retinal include alpha-carotene and beta-cryptoxanthin. Some carotenoids that cannot be converted into retinal are lycopene (from tomatoes) and lutein. All carotenoids have antioxidant activity.
Vitamin A and Night Vision
Vitamin A is needed by the retina of the eye for vision. The retina is located at the back of the eye. Light passes through the lens of the eye and hits the retina. The retina converts the light into nerve impulses for interpretation by the brain. Retinol is transported by the bloodstream to the retina. In the retina, the retinol is used by the epithelial cells on the inside surface of the retina.
Retinol is stored in the retina in the form of retinyl ester until it is needed. When it is needed, the retinyl ester is hydrolyzed into retinol, as shown in Figure 3-3. The retinol is changed to a special form of retinal that is a “cis” isomer. Isomers are mirror images. This cis-retinal goes to the rod cells of the retina. Rod cells in the retina are responsible for vision in dim light. In the rod cells, the retinal binds to a protein called opsin. The retinal and the opsin together form the compound
rhodopsin, also known as visual purple. Rod cells with rhodopsin are able to detect tiny amounts of light; this is important for night vision. When a photon of light hits a rod cell, cis-retinal is released and transformed to trans-retinal. This release of retinal generates an electrical signal to the optic nerve. The optic nerve sends a signal to the brain. Some of the trans-retinal becomes available for another cycle of vision. Some of the trans-retinol is converted to retinoic acid and is no longer available to bind to opsin to form rhodopsin.
These losses must be replaced with vitamin A either from the diet or from vitamin A stored in the liver. Each cell in the retina contains about 30 million molecules containing forms of vitamin A—and there are over 100 million cells in the retina. If enough retinol is not available to the retina, this can result in night blindness.
Vitamin A Deficiency and Blindness
Severe vitamin A deficiency is one of the leading causes of blindness in children. This type of blindness is not related to night vision. Over half of a million children lose their sight each year from severe vitamin A deficiency. This preventablem childhood blindness results from a lack of vitamin A in the cornea of the eye. Thecornea is the transparent outer layer of the eye. This type of childhood blindness is known as xerophthalmia. In the first stageof xeropthalmia, the cornea becomes hard and dry, a condition known as xerosis. Xerosis can progress to a softening of the cornea that can lead to irreversible blindness.
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