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Natural food sources of vitamin A and provitamin A

S. L. Booth, T. Johns, and H. V. Kuhnlein

Overview of natural sources

The new recommended nutrient intakes (RNI) of vitamin A published by FAO/WHO [1] are two-tiered (table 1), with a basal level, corresponding to a recommended intake to prevent deficiency, and a safe level, similar to the recommended dietary allowance (RDA) set for the United States [2], which corresponds to an intake recommended for adequate liver storage of the vitamin [1, 3, 4] The dietary sources of vitamin A are of two categories: vitamin A. or retinol, also known as preformed vitamin A; and provitamin A, which refers to those carotenoid precursors that are biologically active as retinol.

TABLE 1. Recommended dietary intakes of vitamin A (RE)

Source: Ref .1 .

The parent compound of vitamin A is all-trans retinol, which is an isoprenoid compound found in animal tissue [4-6]. Vitamin A is the generic term for all ß-ionine derivatives, excluding the carotenoids. The major storage form' retinyl palmitate, is an ester of a fatty acid chain, 90% of which is stored in the liver. Carotenoids are a class of more than 400 known naturally occurring pigments found in certain fruits. vegetables. oils. and animal foods such as egg yolk and shrimp, of which approximately 50 are known to be biologically active as vitamin A [7, 8]. These carotenoids, of which ß-carotene has the highest known vitamin A activity, are converted to vitamin A by oxidative cleavage [9]. Some carotenoids are absorbed intact and then deposited in various body tissues, including fat deposits, skin, shell, milk, and eggs.

Retinol is found exclusively in animal foods, whereas carotenoids are found primarily in plant foods. Their occurrence in animal foods is dietary in origin [6]. It is estimated that the median dietary intake of vitamin A in the United States is composed of approximately 25% provitamin A and 75% preformed vitamin A [3], with dairy products and fortified foods being the major contributing dietary items. In contrast, studies from developing regions suggest that up to 80% of the dietary intake of vitamin A comes from provitamin A food sources [6]. A common observation in the literature is the prohibitive cost of preformed vitamin A food sources for most regions where xerophthalmia is a documented health problem.

The richest known sources of provitamin A are the palm oils. Red palm oil, a common cooking product in west Africa, is usually cited as having the highest concentration of provitamin A activity [10]. However, recent studies indicate that the oil of the buriti palm tree has a tenfold greater concentration of vitamin A activity than red palm oil [11]. The highest levels of preformed vitamin A are found in animal and fish livers and fish oils [12]. Other food sources of the vitamin include the following:

» provitamin A

• dark leafy vegetables,
• algae,
• red and yellow vegetables and tubers.
• red and orange fruits, flowers, and juices.
• red palm oil;

» preformed vitamin A

• milk and milk products,
• eggs,
• fish and associated oils,
• shellfish,
• liver and organ meats,
• chicken.

These categories are generalizations, because variations between and within species in both provitamin A and preformed vitamin A are very large. as will be discussed in greater detail later.

White roots and tubers and whole grains are considered very low in provitamin A content, and it is often observed that xerophthalmia is more prevalent in rice-consuming areas when vitamin A-rich foods are not introduced early in the weaning stage [13].

Colour intensity, however, is not necessarily a reliable indicator of biologically active carotenoids. For example, the chlorophyll of green leafy vegetables masks the carotenoid pigmentation, yet as a group these vegetables are considered excellent sources of provitamin A [6]. Moreover, as mechanisms of carotenoid physiology and biochemistry are being clarified, and with the introduction of high performance liquid chromatography (HPLC) for the analysis of the carotenoid profile of these foods, many previous assumptions about natural food sources of provitamin A need to be revised.

Storage of retinol in animal species is not evenly distributed. The richest stores are found in the kidneys and liver [12]. Retinol is also stored in the intestinal walls of fish, in the body fat of eels, and in the eyes of certain species of shrimp. With the exception of fowl, meat products, including beef and pork, do not contain significant quantities of preformed vitamin A.

Published values

The values in tables 2-8 have been compiled to present a sample of the reported vitamin A activity of selected foods from the current food composition literature. In a number of cases, several species are aggregated to demonstrate differences in nutrient content. Foods devoid of or low in vitamin A activity have been included to demonstrate trends in food categories and the use of colour as a general indicator of activity.

TABLE 2. Published values for vitamin A activity in green vegetables, algae, and flowers

TABLE 3. Published values for vitamin A activity in fruits

TABLE 4. Published values for vitamin A activity in plant storage organs and seeds

TABLE 5. Published values for vitamin A activity in plant oils

TABLE 6. Published values for vitamin A activity in milk, milk products, and eggs

TABLE 7. Published values for vitamin A activity in fish

TABLE 8. Published values for vitamin A activity in organ meat and other animal products

For simplicity and consistency, all food items are identified in the tables by their English common names as some of the references do not include scientific names. Because there is wide species-specific variation, the reader is referred to the original references for the scientific names when these are given. A range presented for more than one species is denoted by "diverse species."

The column headed "Description" specifies the part sampled when this information is available. gives other relevant identification of the food items such as colour, and indicates the method of sample preparation.

The units for vitamin A activity have been presented as retinol equivalents (RE) or, ß-carotene equivalents (CE) per 100 grams of edible portion. Conversion of international units (IU) to RE and calculation of total vitamin A activity (TA) in retinol equivalents will be discussed later. As these nutrient values include original data using various analytical techniques, values converted from IU or RE, and calculation of total vitamin A activity, they should be interpreted with caution.

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