Vitamin A is obtained from colourful fruits and vegetables in the form of "provitamin A" carotenoids, which are converted into retinol in the body after food is consumed. Good dietary sources of provitamin A carotenoids include carrots, sweet potatoes, spinach, kale, and cantaloupes.
Beta-carotene is one of the most common and effective provitamin A carotenoids.
Benefits of Vitamin A and Beta-Carotene for Vision
Because vitamin A helps protect the surface of the eye (the cornea, the eye's clear front layer), it is important for good vision.
Research shows that vitamin A eye drops are effective for treating dry eyes. In fact, one study found that over-the-counter lubricating eye drops containing vitamin A were just as effective in treating dry eye syndrome as more expensive prescription eye drops formulated to relieve dry eye.
Vitamin A eye drops have also proven effective in treating a certain type of eye inflammation known as limbal keratoconjunctivitis.
Vitamin A and the Prevention of Eye Disease
Vitamin A, at least in combination with other antioxidant vitamins, also appears to play a role in reducing the risk of macular degeneration (AMD). In people who were at high risk of developing the condition and who took a daily multivitamin supplement containing vitamin A (as beta-carotene), vitamin C, vitamin E, zinc, and copper, the risk of advanced AMD was reduced by 25 percent over six years.
The combination of vitamin A and lutein also appears to extend vision in people with retinitis pigmentosa (RP). A four-year study conducted by researchers at Harvard Medical School and other prominent universities showed that people with retinitis pigmentosa who took daily supplements of vitamin A (15,000 IU) and lutein (12 mg) lost peripheral vision more slowly than those who did not take the combined supplements.
Because beta-carotene is converted into vitamin A in the body, this beta-carotene (provitamin A) likely offers similar benefits to the eyes as preformed retinol-type vitamin A, although more research is needed to confirm this.
In early 2011, researchers at Columbia University Medical Center (New York) found that a synthetic modified form of vitamin A may slow the progression of Stargardt's disease, an inherited eye condition that causes severe vision loss in young people.
When the modified vitamin A was administered to mice that had the same genetic defect as people with Stargardt's disease (also known as juvenile macular degeneration), the modified vitamin A inhibited the growth of clump-like deposits in the retina known as "vitamin A dimers," which are associated with degenerative changes and vision loss.
Vitamin A Deficiency and Its Consequences
Vitamin A deficiency causes the cornea to become very dry, leading to clouding of the front of the eye, corneal ulcers, and vision loss. Vitamin A deficiency also damages the retina, which can likewise result in blindness.
Because vitamin A is also important for resistance to infection and a healthy immune system, vitamin A deficiency can cause death from respiratory and other infections.
Common Superfoods for Vitamin A
In most cases, the best approach is to obtain vitamins and minerals from a healthy and balanced diet. Just one cup of plant-based foods such as carrots, pumpkin, or sweet potato provides a dose that exceeds the recommended daily intake by 5 to 9 times.
Beta-carotene and other provitamin A carotenoids found in fruits and vegetables do not pose a risk of vitamin A toxicity. These compounds are water-soluble and easily eliminated from the body, which is why vitamin A toxicity from plant-based food sources is rare.
However, beta-carotene supplements may carry a serious risk for smokers. Two studies have shown that smokers who took daily supplements containing 20–30 mg of beta-carotene had a higher risk of developing lung cancer compared with smokers who did not take these supplements.
Potential toxic reactions resulting from long-term daily intake of vitamin A from supplements or animal-based foods include birth defects, liver abnormalities, reduced bone mineral density which may lead to osteoporosis, and central nervous system disorders. It is therefore advisable to keep to plant-based foods in order to avoid any such risks.
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