Despite the recent focus on vitamin D, and what it can or cannot do for human health, the fat-soluble vitamin with the longest history is vitamin A—the first vitamin ever identified (thus the decision to name it with the first letter of the alphabet). Scientists originally called it “fat-soluble A,” since it was initially identified in animal fats.

Stored primarily in the liver, vitamin A encompasses a group of fat-soluble compounds composed of retinoids and several carotenoids. Retinoid compounds are retinal, retinol, and retinoic acid. Retinol is also referred to as preformed vitamin A because it is already in an active form in foods. Though retinol is the only retinoid present in significant amounts in our diet, once it is absorbed we can readily (and reversibly) convert it to retinal, and a small amount of retinal is irreversibly converted to retinoic acid. In contrast, compounds that have vitamin activity only after conversion to active forms are provitamins. For example, provitamin A carotenoids are compounds (primarily beta-carotene) that have vitamin A activity once they are converted in the body into one of the active forms of the vitamin (retinol, retinal, or retinoic acid).

In the United States, approximately two-thirds of the vitamin A is consumed as preformed vitamin A (retinol) from fortified foods, and supplements, and foods that naturally contain retinol. For example, preformed vitamin A occurs in animal products; it is highest in liver, but fish, eggs, and dairy foods (containing fat) are also good sources. In the United States and Canada, reduced-fat milk and yogurt must be fortified with vitamin A to make up for the loss of this vitamin during the removal of the fat.

The rest of our vitamin A comes from provitamin A carotenoids, which must be converted into an active form in the body before they can fulfill biological functions. These compounds are the yellow, orange, and red pigments of fruits and vegetables—think sweet potatoes, carrots, cantaloupe, and apricots, as well as dark, leafy greens (which, despite their color, contain a lot of beta-carotene). Although more than 700 carotenoids have been identified, about 50 are present in commonly consumed foods, and of the approximately 12 carotenoids that have been identified in blood and tissues, only three are converted in significant amounts to active vitamin A—with beta-carotene being by far the most abundant.

The bioavailability of vitamin A differs upon the food source. Preformed vitamin A (retinol), found in foods from animal sources, is more easily absorbed than the carotenoids found in foods from plant sources. To account for these differences in bioavailability, the Dietary Reference Intake (DRI) values are expressed as micrograms (mcg; one millionth of a gram) of retinol activity equivalents, or RAE. The RDA for vitamin A for men aged 19 to 50 years is thus 900 mcg RAE; for women of the same age, it is 700 mcg RAE. Of the provitamin A carotenoids, beta-carotene is best converted into retinol, but this conversion is never complete. Even when beta-carotene is consumed as a supplement in oil (to improve absorption), it takes two micrograms of beta-carotene to provide the equivalent of one microgram of retinol. Because the absorption of carotenoids from foods is even poorer than from supplements, you need to eat 12 micrograms of dietary beta-carotene to reach the equivalent RAE of only one microgram of retinol. Because the release of carotenoids from food is difficult, processing the food by slicing, chopping, and even juicing can improve their bioavailability. The smaller food particles are more completely broken down by mechanical digestion than are larger particles, allowing nutrients to disperse more readily into digestive fluids. Cooking can also often increase nutrient bioavailability because it ruptures plant cells, releasing the nutrients that otherwise might be trapped within those cells and subsequently excreted. Thus, the potential vitamin A is actually higher from sliced and cooked carrots than from raw carrots. (INFOGRAPHIC 10.5)

218

Question 10.4

Vitamin A serves many critical biochemical and physiological functions in the body related to vision, cell development, immune function, and growth. Along with these important functions, vitamin A also plays key roles in bone health and reproduction, such as sperm and fetal development. Except for its role in vision, vitamin A almost always functions as a hormone that exerts its effects by controlling the synthesis of numerous proteins encoded in our genes.

The hormonal actions of vitamin A play a key role in normal, healthy cell development, such as in cell differentiation (the process by which cells become progressively more specialized). Vitamin A is particularly important to epithelial cells, which form the skin and mucous membranes inside the body, such as those present in our eyes, lungs, and intestines. It also plays a role in the development of immune cells, which affects how well our immune system functions and, therefore, influences our susceptibility to disease. In developing countries, a child’s vitamin A deficiency significantly increases his or her risk of death from an infectious disease.

In addition, vitamin A is required by our eyes to convert light into nerve impulses that bring messages to the brain, telling us what we’re seeing. Specifically, vitamin A is a key component of rhodopsin, the visual pigment that is formed when retinal binds to the protein opsin. Rhodopsin is found in light-sensing cells within the retina at the back of the eye. When rhodopsin absorbs light, retinal changes its shape and is then released, triggering a chain of events that generate a nerve impulse that transmits the visual signal to the brain. When the vitamin A level is low, these light-sensing cells are unable to quickly regenerate rhodopsin, which can make it difficult to see in low light. We also need vitamin A to maintain the health of the cornea, the clear outer covering at the front of the eye. (INFOGRAPHIC 10.6)

Question 10.5