|Culture, Environment, and Food to Prevent Vitamin A Deficiency (INFDC, 1997, 208 p.)|
|Part I. Vitamin A in food and diets|
|2. The complexities of understanding Vitamin A in food and diets: The problem|
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.
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).