Provitamin dietary sources

Vitamin A is an essential nutrient for humans and other vertebrates. Dietary sources of vitamin A are provided either by retinol esters, which are present in foods of animal origin and are hydrolyzed in the intestine to form retinol, or by plant carotenoids. More than 600 carotenoids have been identified in nature, of which 50 to 60 possess provitamin A properties and 10 have nutritional relevance (De Flora et al., 1999). 

Vitamin D-active substances are required in the diets of growing children and pregnant women, but normal adults receiving sufficient doses of sunshine can manufacture sufficient amounts of these compounds to meet their needs. Active vitamin D compounds can by synthesized in such individuals from 7-dehydro-cholesterol (see Table 6.2), an intermediate in cholesterol biosynthesis. Dietary sources also include cholecalciferol, which is produced from 7-dehydrocholesterol and ergosterol (Table 6.2). 7-Dehydrocholesterol and ergosterol are often referred to as provitamins. 

DIETARY REFERENCE INTAKE FOR VITAMIN A ILaHI The current DRI is 700 (women) to 900 (men) meg, which is the equivalent of 2,333 to 3,000 international units (IU). For packaged goods, look on the Nutrition Facts panel for vitamin A content higher than 10 percent DV (daily value), and look on the ingredients list as well for a carotenoid provitamin A source—such as one of the superfruits. 

Vitamin Ai (retinol, 13) is essential for the visual process and for normal growth and development of human beings. Carotenoids are the main dietary source of vitamin Aj. As provitamins Ai serve particularly 13-carotene (1) and also carotenoids with one molecular half of (3-carotene (1) intact [24,25]. 

In animals, the major function of carotenoids is as a precursor to the formation of vitamin A. Carotenoids with provitamin A activity are essential components of the human diet, and there is considerable evidence that they are absorbed through the diet and often metabohzed into other compounds. Beyond their important role as a source of vitamin A for humans, dietary carotenoids, including those that are not provitamin A carotenoids, have been implicated as protecting against certain forms of cancer and cardiovascular disease. ... 

Vitamin A deficiency affects more than 100 million children around the world (Miller and others 2002) and thus remains an important public health problem in many countries. Vitamin A is essential for vision, reproduction, growth, immune function, and general health of humans (van Lieshout and others 2001). The major sources of vitamin A in the human diet are retinyl esters (preformed vitamin A) found in foods of animal origin and provitamin A carotenoids from fruits and vegetables. Unfortunately, foods containing preformed vitamin A (meat, milk, eggs, etc.) are frequently too expensive for some economically deprived developing countries, and therefore dietary carotenoids are the main source of vitamin A in these countries. 

Plants are the major source for dietary provitamin A. As mammals and humans cannot synthesize carotenoids, dietary provitamin A is obtained from plant sources that contain carotenoids having 2,6,6-trimethyl-l-cyclohexen-l-yl rings, such as P-carotene. More than 600 carotenoids have been identified in plants and algae, which together biosynthesize about 0.1 billion tons of carotenoids each year. However, only about ten carotenoids, including P-carotene, are nutritionally significant members of the provitamin A class that can be oxidatively metabolized to retinal in mammals and humans by such organs as the intestine, liver, and kidney and then reduced to retinol. 

The vitamin A content of foods is often given in terms of the international unit (IU). One IU of vitamin A is defined as 0.3 tg of all-trans-retinol. The term retinol equivalent (RE) is used to convert all sources of vitamin A and carotenoids in the diet to a single unit. One RE is by definition 1 pg of all-trans retinol, 12 pg of P-carotene, or 24 pg of other (mixed) provitamin A carotenoids. The recommended dietary allowance for vitamin A ranges from 375 pg RE/day for infants to 1,000 RE/day for adults. 

One practical outcome of research on a fruit is its widespread application as a food for alleviating diseases associated with malnutrition. Particularly in impoverished western Africa, mango is a practical food source of nutrients, especially provitamin A carotenoids, dietary fiber,... 

Molecular sieves arc used in various applications in nuclear medicine. For example, small beads of zeolites were soaked in a solution of radioactive ions. These zeolite beads are employed as point source markers for the identification of anatomical landmarks and for gamma camera uniformity. Due to their small size and relatively high uptake they provide excellent devices for measuring spatial resolution, detector unifonnity and energy resolution.[54] Zeolites are also utilized as binding agents for toxic compounds and antioxidant for selenium, vitamins, and provitamins, and arc also used as mineral additive in various dietary strategies.[55]... 

In the food industry, vitamins are used to enrich many products, by the processes of restitution and fortification (enrichment). Restitution means the return of the vitamin content to the original level found in the raw material fortification is enrichment to a higher level needed for physiological or other reasons. Some vitamins have also found use as natural dyes (riboflavin and provitamins A, in particular -carotene) and as antioxidants (provitamins A, vitamin E and vitamin C). Intake of vitamins currently significantly influences the consumption of various concentrated sources of vitamins such as dietary supplements and multivitamin preparations. Intake of vitamins in these concentrated forms may lead, in extreme cases, to hypervitaminosis. 

The addition of carotenoids to the diet of different animals is commonly used as a method to incorporate certain carotenoids into products that wUl be obtained from such animals. For instance, (3-carotene is added to cattle foodstuff to increase the concentration of provitamin A in milk. In poultry, alfalfa and maize respectively are used as lutein- and zeaxanthin-rich sources, pigments that are incorporated for the pigmentation of the skin and, in particular, egg yolk. The red and yellow coloring of the feathers of certain birds is due to the presence of dietary carotenoids. In the case of salmon, the red color of the flesh is due to pigmentation with dietary astaxanthin and canthaxanthin, which can be introduced artificially in animals bred on fish farms. 

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