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In a recent review of the literature about food beliefs and practices pertaining to vitamin A intake, a multidisciplinary approach was taken using literature ranging from clinical trials to anthropological studies of dietary practices among different societies (Johns et al., 1992). The limitations in the available literature highlighted the difficulties in estimating dietary vitamin A intakes in communities with vitamin A deficiency.
Nutritional data are often lacking, so assumptions have to be made about the potential vitamin A activity of the food items being discussed. This is exacerbated when authors do not include scientific names, or even common names of foods, in their discussion of dietary practices. It is impossible to differentiate poor from excellent sources of vitamin A within the commonly used categories of vegetables and meat. Results of any dietary survey method are dependent on the quality of the food composition tables used to calculate nutrient intakes (Bingham, 1991). As discussed earlier, there are many gaps in the current food composition data for this nutrient, particularly for carotenoids. West and Poortvliet (1993) noted that national food composition tables generally reported vitamin A values that were overestimates when compared with data generated from individual studies. Moreover, the significance of the variability in the nutrient content of natural food sources is not always given adequate attention in dietary surveys. There are many stages during which a change in nutrient composition may occur, including storage of raw food at home, preparation of food prior to cooking, cooking, and finally storage of prepared food (Any and Livingston, 1974). Calloway et al. (1993), for example, expressed concern about the quality of food composition data in their crosscultural study on the vitamin intake of toddlers in Kenya, Mexico, and Egypt. Most of their food composition data was derived from U.S. Department of Agriculture food composition tables, with little data available on the impact that local cooking and preparatory methods had on nutrient values.
Questionnaire Techniques
Despite extensive research on diet survey research methodology, an ideal technique for estimating individual food consumption has yet to be developed (Bingham, 1991). More intrusive methods, such as direct observation, weighed diet records, or diet histories, are considered more accurate in terms of the nutrient intake estimates generated (Barrett-Connor, 1991). However, these methods are too expensive and time-consuming to use at the community level in developing areas at risk for vitamin A deficiency.
Moreover, they only reflect the actual intake of those days recorded and they are not often representative of the usual diet. This is an important consideration when evaluating vitamin A intake that has seasonal fluctuations dependent on food availability. The 24-hour recall method costs less in time and manpower but it only reflects recent diet and not usual intake, unless the interview is repeated many times for the same individual (Block, 1982). Intra-individual variation in nutrient intake is particularly problematic with vitamin A, so repeated measurements are required for a representative index of consumption (Beaton et al., 1983; Basiotis et al., 1987). However, this is not feasible for most field studies due to limitations on resources and the risk of low subject participation (Tangney et al., 1987).
Food frequency questionnaires are considered valuable epidemiological tools because of their simplicity; however, they are not statistically comparable in nutrient estimates to those obtained from other dietary survey methods (Willett, 1990). However, this method is not without its limitations. The order and the actual food items listed may influence responses, so the questionnaires need to be culture-specific as the respondent is prompted by the food items listed (Barrett-Connor, 1991). To determine which foods should be included on the frequency list, descriptive dietary data are required for which recall methods are recommended (Hankin, 1987). This approach was successfully used by Abdullah and Ahmed (1993) in the initial screening of areas and population groups at risk of vitamin A deficiency in Bangladesh. These authors confirmed that 24-hour recalls were inadequate indicators of usual intake if administered in isolation of other dietary survey methods. Simplified food frequency methods have been developed and tested for assessing dietary vitamin A deficiency (IVACG, 1989; Abdullah and Ahmed, 1993; Rosen et al., 1993).
In addition to food frequency questionnaires and 24-hour recalls, other research tools have been used to ascertain food available and consumed in communities at risk for vitamin A deficiency. Market surveys can be simple or complex, depending on their size and the range of imported and local food, as well as the access the population under study has to them. Market surveys, garden surveys, and other information can be used to generate a seasonal calendar of food items under consideration. Seasonal calendars are useful to understand the availability of particular items during the year. Information from public health records or questionnaires can provide data on the extent of breastfeeding and patterns of infant feeding and weaning, all of which are important for periods when a population may be vulnerable to vitamin A deficiency (Rosen, 1992; Rosen et al., 1993).
All dietary assessment techniques for vitamin A intake require estimations of the amount of food consumed, the vitamin A content of the food, and frequency with which it is consumed. From this information it is possible to calculate intake and potential risk of deficiency in percentages of the population in various age and gender groups (IVACG, 1989; Rosen et al., 1993; Sungpuag et al., 1994). Simple frequency forms for summarizing community data and estimating percentages of risk for those who consume food groups (for example, dark green, leafy vegetables or foods of animal origin) in frequency categories (for example, greater than or less than 3 times/week) can be prepared from brief household surveys (WHO/UNICEF, 1994). The IVACG simplified approach to dietary assessment of vitamin A intake of preschool children was validated in Bangladesh using weighed dietary evaluation for three consecutive days (Abdullah and Ahmed, 1993).
As reviewed by Cassidy (1994), crosscultural differences can also create errors in a diet survey method. Differential dietary practices are observed among different ethnic groups within the same region, or within the same ethnic group in different communities. When authors do not give details on a specific ethnic group and location of the study, generalizations may erroneously be made. This is of particular importance for a study that is devised by an individual who is of a different cultural background than that of the study population. In a study in Liberia, direct translation of English terminology was misinterpreted when administering dietary questionnaires and yielded inaccurate conclusions with respect to the ages at which solid foods were introduced Jarosz, 1990). Dietary studies often cover reported behavior and statements of beliefs and attitudes, but often do not include reports of direct observations of behavior. There are important differences between reported practice, which tends to fit the ideal or norm, and real practice. Furthermore, although statements of belief may be true, there is not always a direct relation between belief and practice. This is why it is so important not to assume that beliefs and attitudes always dictate the way people act, especially with regard to food.
Unusual Food Sources
Wild foods, particularly local leafy green vegetables and fruits that are generally rich sources of provitamin A, are often overlooked in dietary surveys. In one study examining the dietary habits of preschoolers in Guatemala, children did not include edible greens when asked to recall food consumed, yet were observed to gather and consume at least two different species (Pigott and Kolasa, 1979). The authors concluded that these plants had a low status, hence were excluded from the dietary recall. In another study, the authors queried the low intakes of vitamin A recorded in the dietary surveys given the low prevalence of nutritional blindness in the same population (Flores et al., 1964). It was suggested that provitamin A-containing foods, such as fruits and vegetables, were not purchased and therefore were not considered part of the diet. It may be that foods not actively cultivated by a population engaged in agriculture (wild greens, insects) would not be classified as food, even though they are consumed. The effectiveness of a recent vitamin A intervention trial was confounded by the onset of mango season (Carrier et al., 1992). These authors argued that it was difficult to quantify the carotenoid intake of mangoes by children because the fruit was not eaten at meal time. As a consequence the intake of foods rich in vitamin A activity is often underestimated. The same dilemma occurs when foods are consumed outside of the home (e.g., at the kill site for game or at the market).
Recommended Intakes
The current recommended nutrient intakes (RNI) of vitamin A published by the FAO/WHO are two-tiered (FAO, 1988), 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 (NRC, 1989) which corresponds to an intake that provides for adequate liver storage of this fat-soluble vitamin (Olson, 1987). The basal level for adults ranges from 270 to 400 retinol equivalents (RE.), whereas the recommended safe level corresponds to 500 to 600 RE, with additional recommended intakes of 100 RE during pregnancy and 350 RE during lactation (FAO, 1988).
Forms of Vitamin A
It is estimated that the median intake of vitamin A in the United States is composed of approximately 25% provitamin A (carotene) and 75% preformed vitamin A (retinol), with dairy products and fortified foods being the major contributing dietary items (Olson, 1987). Of the proformed vitamin A, b -carotene is the most abundant carotenoid in the diet, with carrots accounting for more than 30% of the total population intake (Block, 1994). In contrast, studies from developing regions suggest that Up to 80% of the dietary intake of vitamin A comes from provitamin A food sources, predominantly in the form of b -carotene (Simpson and Tsou, 1986).
Differential intake of provitamin A and preformed vitamin A can be explained by the natural variation in the nutrient content of individual food items. As reviewed by Johns et al. (1992), inclusion and exclusion of food items can also be determined by dietary beliefs and practices, as well as by seasonal and economic factors. Dietary prescriptions and proscriptions are structured both in relation to normal physiological status and to illness prevention and management. Exclusion from the diet of an item rich in vitamin A activity can relate to dietary beliefs, although these are usually limited to certain sectors of the population. Certain food items may be proscribed from the diet in response to alterations in physiological status, e.g., menstruation, pregnancy and lactation, and illness. Time availability is also a determinant of food consumption, particularly time to gather and prepare food. Crosscultural differences in parental control and dietary beliefs influence the timing of the introduction of foods rich in vitamin A activity during the weaning period, and the quantity ingested. This has important implications for the vitamin A status of infants and children whose liver stores of this nutrient are more rapidly depleted than those of adults, who eat relatively small quantities of food.
Of the vitamin A-rich food sources, the literature makes ample reference to the dietary use of green leafy vegetables, particularly gathered, wild species, with few examples of limitations on consumption due to dietary beliefs Johns et al., 1992). However, green leafy vegetables are under-utilized relative to their availability and potential nutrient contribution as documented in Liberia (May and McLellan, 1970), India (Pereira and Begum, 1976), Papua New Guinea (Bailey and Whiteman, 1963), among the Quechua in Peru (Mazess and Baker, 1964), and the Haustec in Mexico (Alcorn, 1981). Although Rahman et al. (1993) demonstrated that children in a clinical setting could eat a sufficient amount of leafy green vegetables to fulfill their daily vitamin A requirements, it was not known if this was a feasible expectation on a daily basis in the home environment.
Vitamin A-rich fruits, such as mangoes and papayas, are also referred to in numerous anthropological and nutritional studies. Wilson (1985) suggested that there is a global promotion of intake of those foods for which there are cravings during pregnancy, fruits being one of the more common food groups. There are also numerous references to the popularity of fruits among children because of their sweet taste and soft texture Johns et al., 1992).
In contrast, there is very little literature available on dietary beliefs and practices surrounding the use of plant oils other than the documented use of red palm oil in certain regions of Africa Johns et al., 1992). Given the small quantities that are used in the cooking process, the frequency and amounts of oils consumed are difficult to quantify in a dietary survey (Sommer, 1982). As previously stated, red palm and buriti palm oil are the richest known sources of provitamin A. While other plant oils have little if any vitamin A activity, their contribution to vitamin A intake is important for the absorption of this fat-soluble vitamin (Mejia, 1986). Therefore, promotion of dietary fat should not be limited to those oils known to be rich in vitamin A activity, although the latter arguably would have the greatest impact on improving dietary vitamin A intake.
In terms of natural food sources of retinol, Abrams (1987) stated that all human cultures include some form of animal protein and fat in the diet, with chicken meat and eggs consumed most frequently, followed by cattle meat and milk. Although organ meats are the richest known sources of retinol, there are few data on their inclusion or exclusion in the diet Johns et al., 1992). There are few reports of egg consumption in the literature compared with the numerous dietary restrictions for them, especially for women of childbearing age Johns et al., 1992). Milk and its byproducts, such as yogurt and ghee, have important dietary roles for nomadic groups (Casimir, 1988; Nestel, 1989), but other regions and cultures have emerged as nonmilk users. Simoons (1973) offered three possible explanations for exclusion of milk in the diet: lactose intolerance; milk is considered an unpleasant secretion milk is only considered suitable for feeding calves. Other authors report that milk consumption is not possible in regions of Zambia due to the presence of the tsetse fly (Awdry et al., 1967). Similarly, endemic liver flukes limit beef liver consumption among the Masai in Kenya (T. Johns, personal observation).
Including and Excluding Vitamin A Food
The data strongly reinforce the significance of both intercultural and intracultural diversity with respect to dietary inclusions and exclusions (Cassidy, 1994). The same foods are subject to very different interpretations in different cultural settings. For example, the green leaves of root crops are consumed in the northern regions of India but avoided in the southern regions of the country (Pereira and Begum, 1976). Not all edible species of fruits are consumed, as demonstrated by different species preference among different ethnic groups within the same ecological region in the Taita Hills of Kenya (Fleuret, 1986). Differential preference for ripeness in mangoes among Gambian women affects the nutrient intake since provitamin A activity varies with the stage of maturation (Villard and Bates, 1987).
Traditional beliefs and practices in many societies are also being modified by the influence of the media and government programs such that both historical and contemporary factors contribute to actual dietary intake Johns et al., 1992). For example, in Java the majority of women increased consumption of leafy green vegetables during lactation to increase vitamin A intake, a reflection of beliefs introduced by the media and government programs (Hull, 1986). Conversely, Doolan (1991) argued that the current shift to marketed foods among northern native populations results in an inadequate intake of vitamin A. When this extends to methods of food preparation, the vitamin A activity is modified from the estimated nutrient content in the raw form. Therefore, selection and consumption of vitamin A-rich foods appear to be highly situation-specific, which points to the need for locally relevant data.
Season and Economic Constraints
Seasonality in the utilization of vitamin A source foods appears to be significant in many, perhaps most, geographical-cultural settings. Ecological factors such as climate, soil, water, and genetic integrity all determine the availability of a food item, particularly in regions where transport and storage facilities are not well developed (Willett, 1990). Variation in preference for different species of wild greens in some regions of Mexico may reflect differences in availability according to ecological conditions and agricultural practices (Messer, 1972). In contrast, selective weeding encourages growth of preferred species of wild greens in other regions of Mexico (Altieri and Trujillo, 1987). Most descriptions of fruit intake make reference to the seasonal availability that creates periods of variable risk of hypovitaminosis A (Fleuret, 1986; Villard and Bates, 1987; Dei, 1991). There are also many accounts of the seasonal availability and consumption of green leafy vegetables, with an abundance of wild, leafy greens noted during the rainy season in some regions (Newman, 1980; Bates, 1983; Malaisse and Parent, 1985). Booth et al. (1993) noted that a seasonal variation in the consumption of indigenous greens in a region in Guatemala was related to agricultural practices, with the highest intake recorded during the months of clearing land for the maize crop. When liver retinol stores are low or vitamin A status is compromised by disease, seasonal fluctuations can lead to periods of greater risk of hypovitaminosis A. With the rapid loss of micronutrients associated with light, high temperatures, and oxidation (Harris, 1988), storage needs to be a priority if food sources rich in vitamin A activity are to be fully realized for their potential. This is particularly true in regions that have distinct fluctuations in seasonal availability of vitamin A-rich foods.
Economic constraints may be significant not only in relation to the relatively more costly preformed vitamin A, but also with respect to provitamin A from plant sources. Dietary intake of vitamin A has been positively correlated with household income Jyothi et al., 1963; May and McLellan, 1970; Villard and Bates, 1987). This correlation is most evident when the provitamin and preformed vitamin A food sources are not readily available. In vitamin A-deficient regions eggs, milk, and fish are sold often, instead of being consumed at the local level Jyothi et al., 1963; May and McLellan, 1970; Villard and Bates, 1987; Dettwyler and Fishman, 1990). Fruit intake has also been associated with higher income (Hull, 1986). However, Kennedy and Oniang'o (1993) argued that while household income has a positive impact on vitamin A intake, community-based health, sanitation, and nutrition interventions are needed also to improve the dietary needs of preschool-aged children.
Market value is also linked to issues of social status associated with a given food item, and vice versa. Developing commercial markets have created a shift in the species of fish caught by- the Miskito of Nicaragua, with the perception of what is valuable determined by what can be sold (Neitschmann, 1972). Conversely, there is a stigma of poverty associated with wild greens that has been documented in various regions, including Mexico (Wilken, 1969), Guatemala (Booth et al., 1993), and India Jyothi et al., 1963). This negative association is particularly strong among urban populations (Messer, 1972). However, chronic poverty has maintained the knowledge and consumption of wild greens among many populations (Caballero and Mapes, 1985).
As discussed earlier, the carotenoids with vitamin A activity are less bioavailable compared with retinol because of the former's specific requirement for bile salts for absorption (West and Sommer, 1987). The absorption of vitamin A is dependent on several factors, including the amount of carotenoid consumed in the diet, the interactions of the carotenoids, and digestibility (Erdman, 1988). With increased protein in take, there is an increased oxygenase available to convert b -carotene to retinol. Adequate protein also improves activity of lipolytic enzymes needed to hydrolyze fat and produce monoglycerides and lipolytic products to facilitate micellar solubilization of carotenoids before cell uptake. The quantity of fat in the diet is also important for its role in the transport of vitamin A and for the stimulation of bile flow. Physiological conditions, including hormone status, gastrointestinal diseases, and weight changes also affect utilization of carotenoids (Erdman, 1988). Solomons and Bulux (1993; 1994) argued that the assumption that b -carotene has one-sixth the vitamin A activity of an equivalent amount of retinol has not been substantiated in vivo in humans. Therefore, absolute nutrient values must be interpreted with caution, preferably in conjunction with other complete food intake data. Promotion of food sources rich in other nutrients, notably fat, protein, and zinc, may optimize the utilization of preformed and provitamin A in natural food sources. These observations have important ramifications in public health policy for communities with documented vitamin A deficiency, as overestimation of the contribution of carotenoids to total vitamin A activity would reduce the effectiveness of dietary interventions (Calloway et al., 1993; Solomons and Bulux, 1994).
Strategies and programs to prevent vitamin A deficiency at the community level by providing vitamin A in food consumed by those vulnerable to deficiency is recognized as the best long-term solution to the problem Fawzi et al. (1993; 1994) and Nestel et al. (1993) reported that total dietary vitamin A intake was inversely associated with the risk of xerophthalmia and mortality among Sudanese children. Vitamin A supplementation programs have been shown to improve vitamin A status, and reviews of child mortality demonstrated that improved status reduced mortality (Beaton et al., 1993; Glasziou and Mackerras, 1993). However, the logistics for sustaining the delivery of supplements to all those in need on a regular basis has not been demonstrated. Fortification of sugar in Guatemala was reported to improve national vitamin A status, but the fortification of food vehicles in other areas has not been as effective (Arroyave, 1986). While prophylactic use of high dose vitamin A supplements and the use of fortification of food staples are proposed to be effective if coverage is consistent among those most at risk, it is the improvement in dietary intake of vitamin A-rich food that can have the greatest effect for the most people in the community. Combined approaches, using a range of intervention techniques to make best use of local resources, including local food resources, are expected to be needed in most areas (Underwood, 1994).
Several successful programs to improve vitamin A status with food interventions have been reported. Improvement in the condition of nightblindness in Bangladesh was accomplished by conducting nutrition education among parents, so that more vegetables and fruits rich in vitamin A and oil were incorporated in children's diets (Yusuf and Islam, 1994). A demonstration project in Thailand using participatory action and social marketing of vitamin A-rich food resulted in improved dietary intake and general health (Smitasiri et al., 1992,1993). Several examples of programs to identify and promote vitamin A-rich foods in Asia and the Pacific Region have been described (Wasantwisut and Attig, 1995). Dietary and anthropological studies in Myanmar revealed several cultural principles important for social marketing of vitamin A-rich foods for interventions to prevent deficiency (Thwin et al., 1995). There has been skepticism that carotene in vegetables can improve vitamin A status because stir-fried vegetables in Indonesia did not improve women's serum retinol compared with controls (de Pee et al., 1995). However, in India, plant food production and processing is recognized as the best way to combat vitamin A deficiency, and several programs for producing and promoting vitamin A-rich plant foods are in effect (Gopalan et al., 1992; Seshadri, 1992).
Communication techniques are important for the success of all intervention programs. Several strategies for successful communications in vitamin A programs have been described recently (IVACG, 1992; Wasantwisut and Attig, 1995). It is clear that for food intervention programs to be successful, strategies that are suitable to the cultural patterns and environmental constraints of the target population must be developed.
To improve vitamin A status and prevent deficiency, an increase in consumption of vitamin A is needed by those at risk of poor intake. Public health programs that aim to prevent vitamin A deficiency need to proceed with clear ideas on how to accomplish this within the context of the culture and environment of the target population. Understanding the current intake of vitamin A depends on knowing the kinds of food available that are consumed by those vulnerable (young children and women), and the carotene and retinol content. There are various dietary assessment techniques to accomplish this. In order to develop food intervention programs it is essential to understand why people eat what they do, why they feed their children as they do, and what belief and behavior patterns will support dietary change to use food that is accessible. Successful food intervention programs that improve vitamin A status depend on careful evaluation of the local culture and environment, and communications incorporating this information.