Vitamin A also stimulates the production and activity of white blood cells, takes part in remodeling bone, helps maintain healthy endothelial cells (those lining the body's interior surfaces), and regulates cell growth and division such as needed for reproduction.
The two main forms of vitamin A in the human diet are preformed vitamin A (retinol, retinyl esters), and provitamin A carotenoids such as beta-carotene that are converted to retinol. Preformed vitamin A comes from animal products, fortified foods, and vitamin supplements. Carotenoids are found naturally in plant foods. There are other types of carotenoids found in food that are not converted to vitamin A but have health-promoting properties; these include lycopene, lutein, and zeaxanthin.
Vitamin A and Health
The evidence suggests that eating a variety of foods rich in vitamin A, especially fruits and vegetables, is protective from certain diseases, though the health benefit of vitamin A supplements is less clear.
Lung Cancer: Observational studies following nonsmokers and current or former smokers have found that higher intakes of carotenoids from fruits and vegetables are associated with a lower risk of lung cancer. However, three large clinical trials did not find that supplements of beta-carotene and vitamin A helped to prevent or reduce lung cancer risk. In fact, two of those three trials actually found a significant increase in lung cancer risk among study participants taking supplements with beta-carotene or retinyl palmitate (a form of vitamin A). Therefore, it is recommended that current or former smokers and workers exposed to asbestos do not use high-dose beta-carotene and retinyl palmitate supplements. Additionally, based on current evidence the U.S. Preventive Services Task Force does not support the use of beta-carotene supplements for the prevention of any cancer.
Age-related macular degeneration (AMD) is a common painless eye condition but a leading cause of vision loss among people age 50 and older. It distorts the sharp, central vision needed to see fine details such as for reading and driving. The exact cause is unclear but oxidative stress is believed to play a role. Smokers and those with poor diets lacking fruits and vegetables have a higher risk of developing AMD. Lutein and zeaxanthin are two carotenoids with protective antioxidant effects that are found in the retina, the eye tissue that is damaged by AMD. Studies have looked to see if supplements containing lutein and zeaxanthin, as well as beta-carotene, might be useful for preventing or treating this condition. The NIH-funded Age-Related Eye Disease Studies (AREDS, AREDS2) found that daily intakes of high-dose vitamins including vitamins C and E and lutein and zeaxanthin slowed the progression of intermediate and late-stage AMD, particularly in participants who ate the lowest amounts of carotenoids. Beta-carotene was not found to be protective.
Many breakfast cereals, juices, dairy products, and other foods are fortified with retinol (preformed vitamin A). Many fruits and vegetables and some supplements contain beta-carotene, lycopene, lutein, or zeaxanthin.
- Leafy green vegetables (kale, spinach, broccoli), orange and yellow vegetables (carrots, sweet potatoes, pumpkin and other winter squash, summer squash)
- Red bell pepper
- Cantaloupe, mango
- Beef liver
- Fish oils Milk Eggs
- Fortified foods
Signs of Deficiency and Toxicity
DeficiencyVitamin A deficiency is rare in Western countries but may occur. Conditions that interfere with normal digestion can lead to vitamin A malabsorption such as celiac disease, Crohn's disease, cirrhosis, alcoholism, and cystic fibrosis. Also at risk are adults and children who eat a very limited diet due to poverty or self-restriction. Mild vitamin A deficiency may cause fatigue, susceptibility to infections, and infertility. The following are signs of a more serious deficiency.
- Xerophthalmia, a severe dryness of the eye that if untreated can lead to blindness
- Nyctalopia or night blindness
- Irregular patches on the white of the eyes
- Dry skin or hair
Vitamin A toxicity may be more common in the U.S. than a deficiency, due to high doses of preformed vitamin A (retinol) found in some supplements. Vitamin A is also fat-soluble, meaning that any amount not immediately needed by the body is absorbed and stored in fat tissue or the liver. If too much is stored, it can become toxic. The tolerable upper intake of 3,000 mcg of preformed vitamin A, more than three times the current recommended daily level, is thought to be safe. However, there is some evidence that this much preformed vitamin A might increase the risk of bone loss, hip fracture, or some birth defects.
Another reason to avoid too much preformed vitamin A is that it may interfere with the beneficial actions of vitamin D. Signs of toxicity include the following.
- Vision changes such as blurry sight
- Bone pain
- Nausea and vomiting
- Dry skin
- Sensitivity to bright light like sunlight
In contrast to preformed vitamin A, beta-carotene is not toxic even at high levels of intake. The body can form vitamin A from beta-carotene as needed, and there is no need to monitor intake levels as with preformed vitamin A. Therefore, it is preferable to choose a multivitamin supplement that has all or the vast majority of its vitamin A in the form of beta-carotene; many multivitamin manufacturers have already reduced the amount of preformed vitamin A in their products. However, there is no strong reason for most people to take individual high-dose beta-carotene supplements. Smokers in particular should avoid these, since some randomized trials in smokers have linked high-dose supplements with increased lung cancer risk.
Did You Know?
There have been claims that vitamin A (in the form of retinol or retinyl palmitate) added to some sunscreens, moisturizers, and lip balms can cause vitamin A toxicity or cancer if used excessively. However, there has not been evidence to date to support this. Vitamin A in topical creams is not absorbed into the bloodstream and therefore would not contribute to toxic levels.
The concern with cancer stemmed from studies in mice conducted by the FDA. The results showed increased oxidative stress (a potential precursor to cancer) in cancer cells exposed to retinyl palmitate and ultraviolet light. After review of these and other studies, a statement from the American Academy of Dermatology asserted, Based on the current available data from in vitro, animal and human studies, there is no convincing evidence to support the notion that retinyl palmitate in sunscreens causes cancer. They cited the high susceptibility of mice to skin cancer after ultraviolet exposure, even in the absence of retinyl palmitate, and therefore the results of these animal studies should not be applied to humans.
Retinoids in skin creams can cause skin to become highly sensitive to bright light, so it is advised to apply vitamin A creams at night and to avoid strong sun after their use.
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