Vitamin A is vital for a variety of bodily functions. These include:
- Maintaining healthy teeth.
- Building strong bones.
- Strengthening the optical system.
- Growth and maintenance of healthy skin and tissue.
- Protection from cardiovascular disease and cancer.
- Promoting good vision, particularly in low light.
- Reproduction and breastfeeding.
- Cell division and cell differentiation (a process in which a cell becomes part of the brain, muscle, lungs, blood, etc.).
- Regulation of the immune system.
Vitamin A occurs in 2 forms: preformed Vitamin A, also known as active Vitamin A, and proVitamin A. Preformed Vitamin A is better known as retinol. This is considered the active form because it most immediately usable form. Retinol is fat-soluble, meaning it is stored in the liver and requires dietary fats for absorption. Large amounts of this form can be toxic, as the body does not rid itself of the excess.
Retinol is found only in animal foods. Sources that are especially rich in retinol are fish liver oils from cod, halibut, salmon, and shark. Other good sources include eggs and dairy products.
ProVitamin A is actually a group of carotenoids. Carotenoids are dark colored dyes found in plant foods that the body turns into Vitamin A as needed. Scientists have identified 563 carotenoids. Few of these can be used by the body to produce Vitamin A. The best known and researched carotenoid as a Vitamin A precursor is beta-carotene.
Beta-carotene is an antioxidant, which functions to protect cells from damage caused by free radicals. Free radicals are unstable by-products of oxygen metabolism that contribute to illness and the degenerative process of aging.
Beta-carotene is considered non-toxic. The body will only convert this to Vitamin A when needed. Excessive beta-carotene intake can result in a yellow / orange cast to the skin, which is sometimes seen in those who drink extreme amounts of carrot juice. This condition is harmless and will correct itself as soon as the beta-carotene intake is lowered.
Beta-carotene is only found in plant-based foods. Sources include carrots, pumpkin, sweet potatoes, winter squash, cantaloupe, broccoli, kale, and spinach. As a general rule, the more intense the color of the fruit of vegetable, the higher the beta-carotene content will be.
Many healthcare practitioners find that beta-carotene works remarkably well in the treatment of acne, as well as eczema and psoriasis. Beta-carotene is completely nontoxic, allowing high doses to be used safely.
Human studies have shown that Vitamin A, particularly in the form of beta-carotene, aids in the protection and treatment of cardiovascular disease. Harvard researchers followed a total of 22,000 male physicians in the Physician’s Health Study. Ten years into the study, researchers evaluated a subgroup of 333 men with histories of cardiovascular disease. The physicians in the subgroup who were taking approximately 85,000 IUs of beta-carotene every other day experienced half the heart attacks, strokes, and deaths due to heart disease as did those not taking the supplement.
One study of women with breast cancer who were being treated with chemotherapy found that those with higher blood levels of Vitamin A responded twice as well to their treatment as did those with lower levels of Vitamin A. In a separate study, Scottish researchers found that patients with breast cancer, bowel cancer, and melanoma all responded better to chemotherapy when their Vitamin A levels were high.
Two separate clinical trials raised concerns about the association between beta-carotene supplements and a greater incidence of lung cancer and death in smokers. However, other conflicting studies show opposite results. Differences in these studies, as well as their methods, make this information difficult to interpret.
In the Alpha-Tocopherol Beta-Carotene (ATBC) Cancer Prevention Study, more than 29,000 male smokers were randomized to receive 20 mg of beta-carotene alone, 50 mg of alpha-tocopherol (vitamin E) alone, supplements of both, or a placebo for 5 to 8 years. The incidence of lung cancer was 18 percent higher among the men taking the beta-carotene supplement. Eight percent more men in this group died, as compared to those receiving other treatments or placebo.
Similar results were seen in the Carotene and Retinol Efficacy Trial (CARET). This lung cancer chemo prevention study provided subjects with supplements of 30 mg of beta-carotene and 25,000 IU of retinyl palmitate (a form of Vitamin A), or a placebo. Researchers stopped this study once they that the subjects receiving beta-carotene had a 46 percent higher risk of dying from lung cancer.
On the opposite side of the spectrum, in a study of more than 22,000 male physicians, the Physicians Health Study compared the effects of taking 50 mg of beta-carotene every other day to taking a placebo. No adverse health effects were discovered.
A clinical trial in China tested the ability of four different nutrient combinations to help prevent the development of esophageal and gastric cancers in 30,000 men and women. This study showed that after five years those who had taken a combination of beta-carotene, selenium, and vitamin E had a 13 percent reduction in cancer deaths.
Swiss researchers studied the association of Vitamin A and beta-carotene with lung cancer in an “at-risk” population of smokers and workers exposed to asbestos. Researchers reported a more than 50 percent reduction in the development of lung cancer in the supplemented group when compared to the control group.
One lung cancer research trial may help explain the discrepancy between these studies. This research found that men who consumed more than 11 grams per day of alcohol (approximately one drink per day) were more likely to show an adverse response to beta-carotene supplements. These results suggest a potential negative relationship between alcohol and beta-carotene.
Studies are currently being run on the relationship between Vitamin A and osteoporosis. In Sweden, a small study of nine healthy people found that the amount of Vitamin A in one serving of liver may impair the ability of vitamin D to promote calcium absorption.
In the U.S., the Nurses Health Study looked at the association between Vitamin A intake and hip fractures in over 72,000 postmenopausal women. Those women who consumed the most Vitamin A in foods and supplements (3,000 mcg or more per day as retinol equivalents, which is over three times the recommended intake) had a marked increase in their risk of hip fracture as compared to those consuming the least amount (less than 1,250 mcg/day) of the same form of Vitamin A.
No association has been found between beta-carotene intake and increased risk of osteoporosis. Research shows only the possible link between osteoporosis and excess retinol intake.
As with all supplements and medications, for information on specific use be sure to discuss your particular situation with your doctor.