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Vitamin A
Vitamin A is important for a lot more than whether our eyes adjust to the darkness. It plays a crucial role in protecting at least three of our senses; it is absolutely necessary if we're to fight off infections; and it promises to be of great benefit in battling problems, abnormally heavy menstrual bleeding, and cancer.
Vitamin A's chemical name, retinol, is derived from its function in eyesight. A healthy retina must contain a mini amount of vitamin A because the vitamin actively participates in the chemical process by which light stimulates the rod: cones into sending sight messages to the brain.
More than the eyes depend on vitamin A. Epithelial tissues, the layers of tissue covering an organ or an entire organism (such as the skin or mucous membranes), depend on vitamin A for healthy structure and function. Vitamin A is also requires for the proper growth and maintenance of the bones and teeth. Some current research is attempting to find out more about vitamin A's suspected role in the structure and function of all biologic membranes, the 'border' membranes between cells.
Vitamin A is also necessary for the proper structure function of the adrenal glands, which control our response to stress. When the body is under stress, the adrenal glands secrete hormones which orchestrate the various organs necessary to deal with this stress. Keep in mind that our stress response evolved when stress invariably meant physical danger. So even if few contemporary stresses can be dealt with by force, the body still prepares for a fight or a flight. The heart rate speeds up, blood pressure rises, and the conversion of fat and proteins to energy increases in order to make more energy available for the muscles.
It's quite reasonable to assume that human requirements for vitamin A vary greatly, too. There are too many factors that can get in the way of a person's getting an adequate supply. For example, heat is not destructive of vitamin A, but oxidation is. And heat increases the rate of oxidation. So cooking at high temperatures for a long time, with subsequent exposure to the air during storage, can substantially reduce vitamin A content of foods. Furthermore, vitamin A in plants is not as available to human digestion unless a small amount of cooking has softened the cell walls that make up the vegetable. Smashing, blending, or liquefying vegetables can help make more vitamin A available by breaking down many of the cell walls. Vitamin A content of vegetables will vary greatly, too, according to the condition of the soil, the amount of sunlight, and other factors.
Vitamin A is a fat-soluble vitamin. That doesn't necessarily mean you must eat a lot of fatty foods to get vitamin A. Provitamin A, or carotene, exists in abundance in the yellow and orange pigments of many fruits and vegetables and is converted to vitamin A in the intestine. It does help, however, if a little fat is present in the gut when the vitamin is ingested. And if there are any conditions which interfere with fat absorption, absorption of the vitamin will also be inhibited. Such conditions include chronic diarrhea, biliary and pancreatic dysfunction, celiac disease, or the consumption of mineral oil. Once vitamin is absorbed, it's stored in the liver and other fatty portions of the body. Where is it found?
Preformed vitamin A is available only from animal sources, the principal one being liver. Egg yolk and dairy products are also good sources. Vegetable sources of carotene include dark- green leafy vegetables, deep yellow vegetables, and tomatoes. Vitamin A supplements are available in a wide range dosages in both natural and synthetic forms. Water-soluble forms are available for people with digestive problems which interfere with fat absorption. Natural vitamin A from fish Iiver oils is usually priced competitively with synthetic forms.
Who is likely to be deficient?
Individuals who limit their consumption of liver, dairy foods, and beta-carotene-containing vegetables can develop a vitamin A deficiency. The earliest deficiency sign is poor night vision. Deficiency symptoms can also include dry skin, increased risk of infections, and metaplasia (a precancerous condition). Severe deficiencies causing blindness are extremely rare in Western societies. How much to take
The RDA (recommended daily dietary allowance) for vitamin A is 5000 IU for men, 4000 IU for women, 1400 for infants, 2000-3300 for children, 5000 for pregnant women, and 6000 for lactating mothers. Individual requirements for vitamins and minerals vary greatly. In one experiment in which the vitamin A requirement for rats was to be determined, the range was varied that the researchers could not arrive at a definite figure. There were healthy, vigorous animals in all groups, including the group that got no vitamin A at all for the length of the study. Side effects
Since a 1995 report from the New England Journal of Medicine, women who are or could become pregnant have been told by doctors to take less than 10,000 IU (3,000 mcg) per day of vitamin A to avoid the risk of birth defects. A recent report studied several hundred women exposed to 10,000 to 300,000 IU (median exposure of 50,000 ill) per day. Three major malformations occurred in this study, but all could have happened in the absence of vitamin A supplementation. Surprisingly, no congenital malformations happened in any of the 120 infants exposed to maternal intakes of vitamin A that exceeded 50,000 ill per day. In fact, the high-exposure group had a 50% decreased risk for malformations compared with infants not exposed to vitamin A. The authors note that previous trials based the link to birth defects on very few cases, didn't measure vitamin A intake, or found no link to birth defects whatsoever. A closer look at the recent study reveals a 32 % higher than expected risk of birth defects in infants exposed to 10,000 to 40,000 IU of vitamin A per day, but paradoxically a 37% decreased risk for those exposed to even higher levels; this suggests that both 'higher' and 'lower' risks may have been due to chance. At present, the level at which birth defects might be caused by vitamin A supplementation is not known, though it may well be higher than 10,000 IU per day. Women who are or who could be pregnant should talk with a nutritionally oriented doctor before supplementing with more than 10,000 IU per day.
For other adults, intake above 25,000 UI (7,500 mcg) per day can-in rare cases-cause headaches, dry skin, hair loss, fatigue, bone problems, and liver damage. At higher levels (for example 100,000 UI per day), these problems become more common.
Taking vitamin A and iron together helps overcome iron deficiency more effectively than iron supplementation alone.
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