Retinol vitamin dietary sources

Vitamin Ai (retinol) is derived in mammals by oxidative metabolism of plant-derived dietary carotenoids in the liver, especially -carotene. Green vegetables and rich plant sources such as carrots help to provide us with adequate levels. Oxidative cleavage of the central double bond of -carotene provides two molecules of the aldehyde retinal, which is subsequently reduced to the alcohol retinol. Vitamin Ai is also found in a number of foodstuffs of animal origin, especially eggs and dairy products. Some structurally related compounds, including retinal, are also included in the A group of vitamins. 

The major dietary sources of retinol are dairy products, eggs and liver, while important sources of /3-carotene are spinach and other dark-green leafy vegetables, deep orange fruits (apricots, cantaloupe) and vegetables (squash, carrots, sweet potatoes, pumpkin). The richest natural sources of vitamin A are fish liver oils, particularly halibut and shark. 

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 A activity is expressed as USP units, international units (lU). retinol equivalents (RE), and arotene equivalents. The USP units and lU are equivalent. Each unit ex-pre.sses the activity of 0.3 /ug of all-/rans-retinol. Thus. I mg of all-/rufis-retinol has the activity of 3.333 unit.s. Other equivalents are li.sted in Table 26-3. One RE represents the biological activity of I /7g of all-rnin.t-relinol, 6 /ug of carotene. and 12 /rg of mixed dietary carotenoids. The RE i.s u.sed to convert all dietary sources of vitamin A into a single unit for easy comparison."... 

Dietary sources of vitamin A also provide the metabolites that are necessary for vision. The ingestion and processing of )3-carotene a retinyl esters to retinol and the transport of RBP-retinol complexes to target tissues was discussed earlier in this chapter, and the ret-... 

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]. 

Vitamins A, D, E, and K are lipids (Sections 25.9 and 29.6). Vitamin A is the only water-insoluble vitamin we have not already discussed. 8-Carotene, which is cleaved to form two molecules of vitamin A, is the major dietary source of the vitamin. Vitamin A, also called retinol, plays an important role in vision. 

The dietary source of vitamin A-active substances for herbivores and carnivores is provided, respectively, by carotenoids and by retinol and its esters. But for omnivores such as man, the proportionate contribution of each to habitual diets varies widely, as does the absolute quantity. In the FAO/WHO report of 1967, the lowest intakes of foods with vitamin A activity were said to be in the Near East, Asia, and the Far East, intermediate intakes in Latin America and... 

Two approaches have been used to evaluate the vitamin A value of diets a biological assay and a chemical assay. Biological assays in animals measure the total physiologically available vitamin A activity of the diet, i.e., not only retinol but also its active isomers. Biological assays, therefore, account for variable efficiencies in absorption, conversion, and utilization of preformed and precursor vitamin A sources. These assays were the standard technique for dietary evaluation in the older literature and continue to be used as the ultimate test even today. However, biological assays are time-consuming and expensive. 

Serum levels of total carotenoids in the absence of retinol levels are not useful for assessing vitamin A status of individuals or populations because they reflect the level of immediate dietary intake (Clausen and McCoord, 1938 WHO, 1976, 1982). Furthermore, as shown in Table IX, as much as two-thirds or more of the total carotenoids may be non-vitamin A-active compounds (Buzina etaL, 1971). The seasonal effect of carotenoid intake on plasma vitamin A and carotenoid levels is illustrated in Fig. 4. An exception may be among populations, especially children, where malnutrition is prevalent and essentially all the dietary vitamin A throughout the year comes from carotenoid sources (Le Francois et aL, 1980). In this special circumstance when dietary sources of both vitamin A and carotenoids are low, a low carotene value in blood is evidence for inadequate vitamin A status. But in the absence of a serum vitamin A level, a low carotene level may reflect either an inadequate amount in the diet or malabsorption, and without additional supportive evidence cannot be interpreted to reflect poor vitamin A status (Arroyave et al., 1982). 

Effect of Dietary Source of Retinol Equivalents (RE), Preformed versus Precursor, on Plasma Levels of Vitamin A and Carotenoids in Senegal" and the United States ... 

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. 

Generally, vitamin A serves three classes of functions (1) support of the differentiation of epithelial cells, (2) support of the viability of the reproductive system (fetal growth and vitality of the testes), and (3) utilization in the visual cycle. Dietary retinoic acid can support only the first function. Animals raised on diets containing retinoic acid as the only source of vitamin lose their ability to see in dim light and become sterile. In males, sperm production ceases. In females, fetuses are resorbed. Retinoic acid cannot be stored in the liver, as it lacks the hydroxyl group needed for attachment of the fatty acid. Retinyl esters, retinol, and retinal are interconvertible. Retinal can be oxidized to form retinoic acid. All three functions of vitamin A can be supported by dietary retinyl esters, retinol, or retinal. Although these forms can be converted to retinoic acid, retinoic acid apparently cannot be reduced to form retinal. These relationships are summarized in Figure 9.44. 

At the other end of the spectrum of serum values, populations with high mean serum carotenoids often are reported to have lower or no different mean retinol levels than found among populations with normal or low carotenoid values. This is illustrated in Table X by comparing data from surveys conducted in Senegal where 98% of the dietary vitamin A activity comes from carotenoids with surveys from the United States where about 50% is from preformed sources. It is difficult to determine if the high blood carotenoids are causally related to a real shift downward in the retinol distribution curve or if this is a methodologic artifact. Several of the colorimetric and fluorometric analytical methods for... 

Carotene is the primary source of retinol in most underdeveloped countries and is an important source of vitamin A in many of the following reports. It is therefore difficult to discern whether the observed anticancer effects ate due to retinol, 3-carotene itself, or some other dietary constituent. Nevertheless, increased consumption of foods rich in vitamin A content appears to be inversely related to the incidence of several types of carcinoma. In the following discussion, the term vitamin A will be used to include both retinoids and carotenoids since most epidermiological reports unfortunately do not distinguish between these two classes of substances. 

Some 50 or more dietary carotenoids are potential sources of vitamin A a-, P- and y-carotenes and cryptoxanthin are quantitatively the most important. Although it would appear from its structure that one molecule of P-carotene will yield two of retinol, this is not so in practice (section 11.2.2.1) 6 ig of P-carotene is equivalent to 1 Ig of preformed retinol. For other carotenes with vitamin A activity, 12 J,g is equivalent to 1 Ig of preformed retinol. 

The study of carotenoid bioefficacy from foods is important in international health as the most frequently consumed sources of vitamin A are fruit and vegetables. A 100% bioefficacy means that 1 pmol of dietary /3-carotene provides 2 pmol of retinol in the body however, 100% bioefficacy does... 

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