Vitamin A
Although vitamin A, and especially its cousin betacarotene, is currently a nutrient of popular interest, it's had a long history. Vitamin A was the first of the fat soluble vitamins to be discovered. In 1913, two groups of American researchers unveiled the mystery simultaneously. Both research teams found that animals became sick when fed fat-free diets. The animals failed to grow and suffered a high rate of infection and eye problems, which were relieved by feeding cod liver oil or butterfat.
Ten years later, a Danish researcher reported that a condition in children, xerophthalmia, which can lead to permanent blindness, could be prevented by adding butterfat or oil to their diets. And long before that, history records that the ancient Egyptians were the first to cure night blindness by applying juice squeezed from liver into the eyes of those afflicted. The Greeks subsequently advocated eating liver, in addition to its topical application, for the cure.
The related compound, betacarotene, arrived on the scene in 1932 when researchers discovered that vegetable foods also possessed vitamin A activity. Closer analysis of vegetables uncovered the class of compounds known as carotenoids, which includes carotene and other yellow pigments. These pigments give the brilliant colors to red and yellow vegetables. Dark green vegetables also contain carotenoids, but the color is masked by the chlorophyl pigment. The same principle keeps leaves from changing color until fall.
Names for vitamin A can be confusing unless you look at the chemical formula. The chemical formula for biologically active vitamin A is an alcohol since it contains a hydroxyl group (oxygen and hydrogen). The name retinol is derived from this alcohol component and vitamins function in the retina of the eye. Vitamin A is a generic term for all compounds that have the biological activity of retinol, excluding the carotenoids.
The biological activity of a nutrient tells you about its ability to carry out important functions in the body. Other names and forms include retinyl esters, retinal, retinaldehyde, and retinoic acid. Retinoic acid can carry out some, but not all, of the functions of retinol. The term retinoid refers to all forms of retinol both in natural and synthetic forms.
Carotene compounds are called provitamins since they serve as precursors to vitamin A. Humans and animals can't synthesize carotene, but both can convert it to vitamin A in the liver. Of all the carotenoids, beta-carotene gives you the highest amount of active vitamin A, which is why scientists consider it the most important of the carotenoids. In addition, in the average American's diet, betacarotene supplies almost two-thirds of the vitamin A requirement. More recently, scientists are finding that other carotenoids, like lycopene in tomatoes, offer other health advantages.
Vitamin A in the Body
It's Every Place You Want It to Be
Vitamin A is absorbed into the body in the same way as dietary fat, and, as mentioned, absorption requires some fat. The vitamin enters the bloodstream and goes to the liver for either storage or immediate use. The liver stores 90 percent of the body's vitamin A,and this supply can sustain most people for six months to a year. When cells need vitamin A, the liver makes a carrier called retinol-binding protein (RBP) to transport the vitamin in blood, since vitamin A is fat soluble. And it's a good thing that vitamin A gets around because the jobs it has to do are many and varied: from your toes to your eyes, this nutrient is a must.
One of vitamin A's most important and well-known roles is in maintaining vision. It achieves this through a compound called rhodopsin, also known as visual purple, which enables the eye to adapt to changes in light. When the retina receives light, it splits the rhodopsin molecule into its two constituents, opsin and retinal, or active vitamin A. In the dark, they recombine to form rhodopsin. Every time this reaction occurs, you use up some retinol. This is the reason why one of the first signs of deficiency is night blindness.
Another vital function is in the formation and maintenance of epithelial tissue. These tissues form the body's first line of defense against infections and carcinogens (cancer-causing agents) by serving as barriers to invaders. Epithelial tissues include the skin and membranes lining the eye and mouth cavities, stomach and intestines, lungs, and other organs. Some animal studies show that both retinoids and carotenoids can prevent cancers originating in epithelial tissue.
Vitamin A also plays a critical role in growth and reproduction. Even the earliest studies showed the vitamin's importance by causing growth failure in deficient rats, but scientists have yet to discover how it works. Most think that vitamin A is essential in the growth of soft tissues and bone by affecting protein synthesis, cell division, or cell membrane stability. Female animals who are vitamin A-deficient abort or produce malformed offspring. However it works, normal reproductive function in both sexes depends on adequate vitamin A. Results from human studies show that vitamin A deficiency causes degeneration of the sex glands and eventual sterility-now there's a scary thought!
How Much Is Enough or Too Much?
The amount of vitamin A you need depends on two factors: the foods you eat that contain either active vitamin A, retinol, or its precursors, the carotenoids, and normal functioning of all digestive organs including the liver. For most people, the RDA with its margin of safety, 1,000 RE for men and 800 RE for women, covers these bases. Historically, vitamin A was measured in international units (IU), with one IU being equivalent to the biological activity of 0.6 micrograms of beta-carotene or 0.3 micrograms of retinol.
Retinol Equivalents (RE) were becoming the standard unit for research internationally a few years back. Scientists preferred the RE which is more accurate because it takes into consideration carotenoid conversion. Manufacturers often use IUs, so you still see these units on vitamin labels with one RE the equivalent of 3.33 IUs. But with the increasing awareness of the health importance of various carotenoids independent of their conversion to vitamin A, the use of milligrams (mg) is gaining favor.
Food sources of vitamin A consist of either preformed retinol, which is found only in animal products, or the plant sources of carotene found in dark green, orange, and deep yellow vegetables and some fruits. Food sources offer high concentrations of the vitamin, but some people can still become deficient because the intestinal tract isn't absorbing the nutrient. Poor absorption can be the result of problems in the nutrient-absorbing cells of the intestines themselves or diseases that affect the intestines. Diseases such as Crohn's and celiac disease cause absorption problems because the absorbing cells become damaged when the disease flares up.
Any disease that affects the digestion or absorption of fat will also reduce the absorption of fat soluble vitamins such as vitamin A. A common example is liver disease, because this organ makes bile, which emulsifies dietary fat so that fat-busting enzymes can digest it. In a similar way, diseases of the pancreas can cause problems with fat and fat soluble nutrients because the pancreas makes the enzymes that digest fats.
Another problem that leads to vitamin A deficiency is when the liver can't convert carotene to active vitamin A, which can occur in liver diseases such as hepatitis. And new research shows that people with diabetes who must use insulin have significantly lower blood levels of both vitamin A and RBP. This might be a concern for those who have low to marginal intakes of vitamin A.
Aside from diseases, researchers know there are some people who just can't absorb beta-carotene because of an unknown genetic defect. These people need to make sure they eat enough foods containing active vitamin A. At the beginning of this section, the problem of very low-fat diets introduced the fat soluble vitamins. The milliondollar question is, "What is too low fat?" Although no one knows the exact answer, and individual variation is important, too, some researchers think that 20 percent fat might be a problem for many people.
Vitamin A deficiency is an international problem, especially in less developed countries. Worldwide, vitamin A deficiency ranks as the second most prevalent nutritional disease behind protein calorie malnutrition. Experts estimate that 1 to 5 million people, mostly infants and children, suffer from the deficiency, and as many as 250,000 become permanently blind as a result. In developing countries, studies have shown that even a marginal vitamin A deficiency leads to higher death rates from respiratory disease and diarrhea, and vitamin A supplementation lowers death and illness rates associated with measles in children.
As you can only foods from animal sources contain active vitamin A. Many people who don't eat dairy products, or take a daily spoonful of cod liver oil, would have a hard time getting enough vitamin A if it weren't for the carotenoids in fruits and vegetables.
And remember, too much active vitamin A can be harmful, especially for pregnant women. The arctic explorers who overdosed on polar bear livers died of vitamin A toxicity because of the high content of the vitamin even in chicken livers. You can't get toxic amounts from plant sources because the liver won't convert carotenoids into active vitamin A unless it's needed. Excess amounts of carotenoids will be stored in fat cells accumulating under the skin, and since the compounds are yellow pigments, you might look like you have a serious case of hepatitis!
You can, however, get toxic amounts if dietary intake from animal sources is high. This usually is a result of eating lots of liver, combined with taking vitamin supplements. The toxic level of vitamin A depends on your body size-a ISO-pound person would be in trouble if he or she took in about 7,000 RE. Another toxicity concern is for people using a prescription drug, Accutane, for acne. The drug is related to vitamin A, and, like its relative, it is highly toxic.
This is the reason why physicians make sure a person is not pregnant when they prescribe the drug and caution against the patient becoming pregnant while taking it. Another vitamin A relative, Retin-A, also cashes in on the vitamin's role in epithelial tissue by helping to reduce wrinkles.
Interestingly, toxic levels of vitamin A affect the same body areas or functions as does a deficiency. In the digestive tract, deficiency causes diarrhea and so does toxicity, along with other digestive problems. Vitamin A deficiency depresses the immune system, while toxicity stimulates it. And in skin, deficiency plugs up hair follicles with an opaque protein called keratin, and toxicity causes dryness, itching, and the peeling of skin.