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Harriet V. Kuhnlein
Hypovitaminosis A is considered one of the major nutritional deficiency diseases affecting populations in developing regions, with 73 countries having been identified as potential endemic areas . Vitamin A deficiency affects multiple physiological systems, but most of the prevalence data are on documented xerophthalmia, the series of ocular diseases associated with a severe deficiency of vitamin A . problems of deficiency have been documented in Bangladesh [3-5], India [6, 7], Indonesia [8, 9], Nepal , the Philippines , and Thailand . Data on the prevalence of hypovitaminosis A have only recently become available for other regions of the world, including the African continent [12-15]. Latin America [16, 17] and the Mediterranean , and suboptimal intakes of the nutrient have been reported among indigenous populations in northern Canada .
The prevalence of hypovitaminosis A is not uniformly distributed across regions or within populations. Physiological factors influencing the vitamin A status of an individual have been documented, but the mechanisms are still not clarified [19, 20]. Less emphasis has been given to ecological, economic, and cultural factors that influence the intake of natural food sources of moderate and high vitamin A activity. Consideration of these is essential in the identification of high-risk groups and of temporal factors that create periods of greater risk of deficiency in order to implement successful long-term prevention programmes.
Three major programme strategies have been designed to prevent and control the problem of vitamin A deficiency: periodic oral dosing, fortification, and dietary modification . Prophylaxis programmes evolved with an expanding literature that revealed widespread prevalence of vitamin A deficiency . Whereas initial efforts using high-dose vitamin A supplements were designed to reduce and control xerophthalmia, there is now concern for populations at risk in whom there are not yet the overt clinical signs of the disease . Once clinical signs appear in the form of night blindness and dryness of the conjunctiva, the course of the deficiency is rapid .
Poor growth, anaemia, and increased susceptibility to infection are now associated with hypovitaminosis A in preschool-aged children . Vitamin A has a well documented role in the human immune responses , which has implications for treatment of diarrhoeal diseases and measles [26, 27]. A Guatemalan study also suggests that there is a positive association between vitamin A levels in umbilical cord serum at delivery and birth weight .
Administering periodic oral doses of 200,000 international units (IU) of vitamin A to preschool-aged children (with half doses for infants 6-12 months old) and a single dose within one month after delivery to lactating mothers is the most straightforward approach to treating, as well as preventing, xerophthalmia in endemic areas . The underlying principle of this approach is to boost liver stores of retinol. Breast milk provides adequate vitamin A intake in infants exclusively breast-fed for at least the first six months of life . UNICEF-distributed capsules are currently incorporated in prophylaxis programmes, including those in Bangladesh, Brazil, India, the Philippines, and Indonesia [3, 21, 31-33]. A low rate of coverage, failure to target those at risk, failure to sustain coverage, and poor nutritional status and socioeconomic conditions are cited as the major limitations of this strategy. Likewise, while fortification of a food vehicle with vitamin A has demonstrated success in clinical trials [32, 34], unstable economic and political systems have interfered with the sustained implementation of this strategy at the national level .
Although periodic oral dosing and food fortification have had documented success and have established their merit in preventing nutritional blindness, they are also considered expensive, temporary solutions. Long-term intervention through dietary modification of the intake of foods rich in provitamin A and vitamin A |21, 22, 35] is more effective. Among the benefits of promoting such natural food sources is that they provide concurrent intake of other nutritive and non-nutritive substances that contribute to the prevention of illness, an example being fruits and vegetables that are currently inferred to provide protection against certain types of cancer . Another benefit is the avoidance of the potential toxicity associated with the overconsumption of vitamin A supplements . Thorough cost-benefit analyses of the three strategies to prevent and control vitamin A deficiency have yet to be made.
Historical accounts, as reviewed by Sharman  and Wolf , document that night blindness has been successfully treated with the intake of vitamin A-rich foods. Positive dietary change is a gradual process, initiated by education and choice, which has been given low priority in most vitamin A intervention programmes [4, 33]. Promoting dietary change requires identifying and quantifying natural foods that are rich in vitamin A and provitamin A, in conjunction with foods rich in nutrients that interact with vitamin A uptake and bioavailability . However, as demonstrated by the prevalence of xerophthalmia in regions with abundant food sources of vitamin A [1, 2], the emphasis cannot be limited to identifying and producing the food item. Physiological factors such as protein and zinc status also affect the absorption and metabolism of vitamin A . Dietary fat, fibre intake, and the amount of carotenoid in the diet also influence the utilization of vitamin A. Food availability, cost, and consumption patterns as well as attitudes and beliefs about food and feeding behaviour all need to be defined and incorporated into programmes, as these are often the underlying causes of the deficiency .
The two papers that follow are reviews of natural foods with moderate to high vitamin A activity and the factors and programmes that influence improvement of vitamin A status. The first presents published values for the vitamin A activity of a selection of these foods and discusses the current state of food composition data: with the development of new analytical techniques, much of the older data for vitamin A activity needs to be re-evaluated. The second paper discusses factors that influence the dietary intake of foods rich in vitamin A activity. Dietary beliefs and practices are described, with anecdotal information to highlight the differential influence of these beliefs and practices on the intake of vitamin A. This is followed by a discussion of several programmes designed to increase the intake of natural foods rich in vitamin A activity, chosen to highlight strengths and weaknesses of the gardening, nutrition-education, and social-marketing approaches to improving the vitamin A status of the target population.
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