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1. Vitamin A and food: The current situation
2. The complexities of understanding Vitamin A in food and diets: The problem
Harriet V. Kuhnlein and Gretel H. Pelto
Overview: What this book is about
The Vitamin A situation
Proposed solutions to the problem
Focused ethnography to understand local culture and environment for Vitamin A programs
The structure of this book
The purpose of this book is to contribute to understanding the sociocultural and environmental factors that affect vitamin A intake and responses to vitamin A deficiency. The enterprise described here is based on the assumption that knowledge about the sociocultural and environmental contexts of vitamin A is essential for instituting and sustaining food-based prevention of vitamin A deficiency. It describes how a group of nutritionists and anthropologists worked together to create a protocol to evaluate the natural food sources of vitamin A in areas at risk for vitamin A deficiency. A manual describing the creation of a locally contextual protocol is a companion to this volume, and is titled Community Assessment of Natural Food Sources of Vitamin A: Guidelines for an Ethnographic Protocol. The protocol combines nutrition and anthropological tools, and is called focused ethnographic study (FES). FES is within the realm of rapid assessment procedures (RAP) of anthropologically-based methods for relatively rapid evaluation of health problems and prevention programs. In this case, FES is applied to understand how culture, environment, and food can prevent vitamin A deficiency.
this book for development planners in health, agriculture,
education, or other: areas. It is also suitable for scholars and
students of nutrition, public health, agriculture, anthropology
and human cultural ecology to describe and discuss the issues
surrounding the use of natural food sources for the prevention of
vitamin A deficiency. Ethnographic research tools and their
testing in a broad range of cultures and environments in five
developing countries are outlined, as are the findings from this
work. Chapters contributed by the investigators in these
countries describe the suitability and generalizability of the
research tools, the data generated, practical applications, and
directions for policy.
Vitamin A deficiency is a major global problem, affecting populations in developing areas of more than seventy-five countries where clinical and subclinical conditions have been observed (McLaren, 1986; WHO, 1994). Worldwide, this public health problem involves 2.8 to 3 million children with clinical deficiency and 251 million with subclinical deficiency. Vitamin A affects many physiological systems; it plays an essential role in vision and eye health, and it affects growth and susceptibility to infection (particularly diarrhea and measles) and anemia in children (Sommer et al., 1984; Campos et al., 1987; Chandra and Vyas, 1989). The consequences of vitamin A deficiency include blindness, poor growth, severe infection, and death; its control and prevention are central in child health and survival programs (Wasantwisut and Attig, 1995). The International Conference on Nutrition (WHO/FAO, 1993) pledged the elimination of vitamin A deficiency by the year 2000.
prevention of vitamin A deficiency at the community and household
levels depends on the availability and consumption of vitamin
A-rich food from either plant or animal sources, and on the
presence of other dietary factors needed for bioavailability,
absorption, and metabolism of vitamin A, such as sufficient fat,
protein, zinc, and other essential nutrients (Booth et al.,
1992). Inadequate intake of the appropriate quantity and quality
of food to meet vitamin A requirements affects all members of
populations with deficiency, but is most common in infants, young
children, and pregnant/lactating women. Extensive reviews of the
variety of foods containing vitamin A and the effects of vitamin
A deficiency are presented elsewhere, and will not be covered
here (see, for example, Sommer, 1982, 1995; Bauerfeind, 1986;
West, 1991; Booth et al., 1992; Underwood, 1994).
Actions aimed at preventing vitamin A deficiency may draw on several potential types of solutions. Solutions at the community level can be diverse and may involve a variety of multisectoral community and development programs. Public health programs in breastfeeding, immunizations, family planning, health education, and maternal and child care are relevant, as are agricultural extension, agricultural education, horticultural promotion, etc. Education sector involvement might include food education, school gardens, and hygiene promotion for the prevention of infection (WHO/UNICEF, 1994). The training of professionals delivering services and programs is key to appropriate activities that will effect positive change.
In general, providing more vitamin A to vulnerable populations has been undertaken in three major intervention activity categories: distribution of large doses of vitamin A supplements, fortification of selected food items, and dietary modification to include more vitamin A-rich food. These have been accomplished in context with public health, agriculture, and/or education sector programs as noted above. A combination of these activities together with various public health measures and economic improvements is considered appropriate and effective (Subcommittee on Nutrition, 1994).
It is recognized that distribution of supplements provides a quick-acting intervention, best accomplished with sufficient health infrastructure to targeted populations, in particular to children at risk for protein-energy malnutrition and various infections. Improving vitamin A status in this way resulted in reduced mortality and morbidity (Beaton et al., 1993).
Fortification of food with vitamin A and its distribution is most feasible where the processed food industry is well-developed and supported, which may not be the case in resource-poor areas where vitamin A is lacking in the diet, deficiency is most extreme, and various barriers exist for the most vulnerable to access fortified food (Trowbridge et al., 1993). Examples of vitamin A fortification programs have been reported (McKigney, 1983), and include cod liver oil in margarine, and vitamin A in milk, sugar, and monosodium glutamate.
Lack of vitamin A in the diet is the root cause of vitamin A deficiency and dietary modification is generally regarded as the ultimate goal for the prevention of vitamin A deficiency in all members of households and communities. This is a long-term approach and requires improvements in food availability and education of those most vulnerable to take advantage of improved food supplies. One important dietary modification is the sustained breastfeeding of infants when mothers have sufficient dietary vitamin A. It is thought that the most effective dietary modification programs target improvement of dietary intake of vitamin A for women in the child-bearing years, during pregnancy and lactation, and for young children at weaning and during rapid growth and development (Subcommittee on Nutrition, 1994; Underwood, 1994; Wasantwisut and Attig, 1995).
Considerations for Sustainability
To solve the root cause of vitamin A deficiency, more vitamin A must be present in the diets of vulnerable people. Program planners and development leaders in health, agriculture, education, and other sectors must understand the culture and ecology of food availability and consumption at the local level. This understanding will lead to improvements in the dietary quality and quantity of vitamin A, through dietary modification and food fortification programs. With respect to food supplies, it requires understanding the species of vitamin A-rich foods that are culturally acceptable and available, their seasonality, methods of preservation and preparation, and barriers to their use due to cost, health beliefs, or other reasons of accessibility are also important. Only when these factors are known will agricultural, food processing, social marketing, and public health education programs have a sustained impact on behavior change and in improving dietary modification for vitamin A (Wasantwisut and Attig, 1995).
elements of understanding the culture and ecology of food
availability and consumption at the local level are addressed in
several avenues of scientific communications including
agriculture, food science, nutrition and social science journals,
and other publications. The use of food sources to solve the
vitamin A problem has recently been reported by Gopalan et al.,
1992; IVACG, 1992; Smitasiri et al., 1993; Wasantwisut et al.,
1994; and Wasantwisut and Attig, 1995. These publications give
examples of successful programs to improve dietary vitamin A.
The methodology that was used in the studies reported in this book can be described as focused ethnography. We drew on research techniques from anthropology and nutrition to create a manual that facilitates the collection and interpretation of data on cultural and environmental aspects of food use and vitamin A deficiency.
Focused ethnography evolved as an approach based on principles of contemporary method and theory in cultural anthropology, modified by the requirements and constraints of program development in public health, agriculture/horticulture, and other public service sectors (Gove and Pelto, 1994). These methods are akin to the pioneering developments widely known as rapid assessment procedures (RAPs) (Scrimshaw and Hurtado, 1987; Scrimshaw and Gleason, 1992).
FES shares many fundamental characteristics with general ethnography:
Data-gathering is carried out in a specific locality (community or regional cluster of communities).
In-depth key-informant interviewing is a primary data-gathering.
The research design produces a qualitative description of cultural and behavioral patterns, that is, the models or systems of relationships among elements in a sociocultural domain.
Data-gathering places an emphasis on describing the perspective of the client (their own language, concepts, and cultural beliefs) the emic perspective.
The theoretical approach of cultural ecology directs data-gathering, that is, attention is given to the description of culture, behavior/practices, and to the physical and social environment.
In contrast to general ethnography, program requirements determine several special features of the FES approach:
Data-collection is focused on a specific set of predetermined questions. In our work the questions relate to: (i) identifying key foods, particularly those important for vitamin A; (ii) cultural beliefs regarding these foods; (iii) food acquisition, preparation and storage; (iv) patterns of food use and the vitamin A content of diets; and (v) community perceptions about the signs and symptoms of vitamin A deficiency.
To be feasible with respect to cost, time, and organizational/political logistics, the Protocol is designed to be completed in a short period of time - six to eight weeks.
Standardized methods are applied in which data-collection is very clearly specified, and forms for data-recording and analysis are provided. As a result, the investigator and field assistants have a dear picture of the expected products of the dare. It is therefore possible to have interviewers without university training.
A manual of the procedures in the protocol provides a framework for training the field team. A pre-study training workshop, with step-by-step instruction in data-collection, insures that interviewers fully understand the purposes and procedures, and that they record data accurately and completely.
approach is intended to demystify the processes of qualitative
data-collection, in this case to understand how culture,
environment, and food can prevent vitamin A deficiency. FES
approaches have been developed for acute respiratory illness,
malaria, and diarrhea (Herman and Bentley, 1993; WHO, 1993a,
1993b, 1994). This approach has also been applied to situation
analysis of high risk behaviors in relation to HIV/AIDS (Pelto,
1993; NACO, 1994). Similar manuals, some of which include methods
for assessing child labor situations in countries such as
Bangladesh, are in various stages of preparation (personal
communication, P. J. Pelto, 1994). Further discussion of the FES
approach is given in Chapter 3.
This book is structured to give the reader the logical flow of our research process to create the FES protocol, the final result which is a manual presented in the companion volume. The book contains four parts: 1) the background of knowledge on vitamin A in food and diets; 2) creating the protocol; 3) the community assessments of natural food sources of vitamin A that tested the protocol; and 4) the final section that contributes new understanding about community deficiency of vitamin A. Following this introduction, we discuss the factors involved in understanding vitamin A in food and diets with emphasis on populations at risk for vitamin A deficiency (Chapter 2). Chapter 3 describes how the process evolved through the International Union of Nutritional Sciences, Committee II/6 in Nutrition and Anthropology, with funding from the International Development Research Centre of Canada. We also describe the FES methods developed for testing in five diverse cultural and environmental areas where vitamin A is at risk.
The manual was tested with the Aetas of Canawan in the Philippines during wet and dry seasons, with the Hausas of Filingué in Niger, with the people of Doumen of Kai Feng Municipality in China, with the Comunidad Campesino of Chamis and the Barrio San Vicente of Cajamarca in Peru, and with the people of Sheriguda Village of the Ranga Reddy District of Andhra Pradesh in India. In Chapters 4 through 8, reports from the research teams in these five areas relate their experiences with the FES protocol, and important findings that resulted from using the protocol in their countries.
Chapter 9 summarizes key points of the field tests from a methodological perspective. It also includes general observations concerning culture, environment, and vitamin A deficiency. As future studies are conducted with the manual, we hope the methodology provided by the FES structure will serve as the framework for systematic cross-cultural comparisons on a broader database. The chapter concludes with a discussion of utilization of knowledge about culture and environment in developing interventions to prevent vitamin A deficiency.
are given at the end of the book. These are followed by an
Appendix that gives the Table of Contents of the manual, entitled
Community Assessment of Natural Food Sources of Vitamin A:
Guidelines for an Ethnographic Protocol.
Sarah L. Booth, Timothy A. Johns, and Harriet V. Kuhnlein
Overview of natural Food sources of Vitamin A
Vitamin A food composition data
Effect of food processing on Vitamin A
Food composition tables for Vitamin A
Assessment of dietary Vitamin A intake
Dietary Vitamin A intake patterns
Other dietary and health factors influencing Vitamin A status
Programs that improve intake of Vitamin A-rich food
We began our work with the premise that to prevent vitamin A deficiency, more vitamin A needs to be present in diets of those vulnerable to deficiency. Where deficiency exists the solution rests in getting more vitamin A into diets on a regular basis. The problem, therefore, is to find out how to do this - and this requires knowing the following:
How much vitamin A is already in the diet: What and how much food is eaten, what is the vitamin A content of that food?
Why people eat what they do: what are the food beliefs and behaviors that are practiced?
What would bring about positive dietary change to prevent vitamin A deficiency: What vitamin A-rich food is available in the local environment that can be used to better advantage, perhaps by better processing? What other dietary and health factors would make dietary vitamin A more physiologically active? How and why would those who are vulnerable to deficiency change their behavior to improve their diet?
chapter we review current knowledge of food sources of vitamin A
present in nature (natural food sources), and discuss the current
status of food composition data. This is followed by a discussion
of factors that influence dietary intake of foods rich in vitamin
A activity (that is, both retinol and carotene), and the impact
these have on assessment of diets for this nutrient. We also
include brief reviews of methods that have been used to assess
diets for vitamin A in populations vulnerable to deficiency, and
of food programs that have improved dietary vitamin A.
Vitamin A in food is found as retinol or as carotenes. Retinol is found exclusively in animal foods including eggs, milk, and milk products (Heinonen, 1991; Booth et al., 1992). Storage of retinol in animal species is not evenly distributed among tissues, with the highest levels of preformed vitamin A found in animal and fish livers and fish oils (Leth and Jacobsen, 1993; Morrison and Kuhnlein, 1993). 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.
Carotenoids are found primarily in plant foods (Simpson and Tsou, 1986), whereas meats, fats, and dairy products are reportedly low in carotenoid content (Heinonen, 1991). 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 (Cottrell, 1991).
recent studies indicate that the oil of the buriti palm tree has
a tenfold greater concentration of vitamin A activity when
compared with red palm oil (Rains-Mariath et al., 1989). Other
food categories rich in provitamin A activity include dark green,
leafy vegetables; algae; red/yellow vegetables and tubers; and
red/orange fruits, flowers, and juices (Booth et al., 1992).
White roots and tubers and whole grains are considered very low
in provitamin A content. Color 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 excellent sources of provitamin A (Simpson
and Tsou, 1986).
This section and the following two sections ("Effect of Food Processing on Vitamin A" and "Food Composition Tables for Vitamin A") give details of vitamin A chemistry and analysis.
Vitamin A can be obtained in two forms from the diet: preformed vitamin A, also referred to as retinol, and provitamin A, also known as the carotenoid precursors that are biologically active as retinol. The term vitamin A is used in two contexts; a generic term for all b-ionone derivatives, excluding the carotenoids; and as a generic term for all those compounds, including carotenoids, that are precursors to retinol and can reverse symptoms of deficiency associated with this fat-soluble vitamin (Davison et al., 1993). For the purpose of this chapter, the latter definition will be used, with discussion of carotenoids limited to those that are retinol precursors.
The parent compound of vitamin A is all-trans retinol, which is an isoprenoid compound found in animal tissue (Simpson and Tsou, 1986; Bendich and Langseth, 1989). 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 600 known naturally occurring pigments found in certain fruits, vegetables, and oils, and animal foods, such as egg yolk and shrimp (Daun, 1988; Erdman, 1988). The nutritional functions of the carotenoids have recently been reviewed in response to the interest in the role of carotenoids as chemoprotective agents (Davison et al., 1993; Thurnham, 1994). Only fifty of these carotenoids, of which b-carotene comprises 10% to 15% of total serum carotenoids in humans, are known to be converted to retinol by oxidative cleavage (Thurnham, 1994).
Units of Expression
Nutrient values of preformed vitamin A and provitamin A can be combined into a single numerical value of vitamin A activity (Thompson, 1986). Originally, the internationally accepted values were international units (IU). One IU was defined as 0.30 mg of all-trans retinol, or 0.60 mg of all-trans b-carotene. These units are still found in many food composition tables.
In theory, one mmol of all-trans b-carotene should cleave to form 2 mmols of all-trans retinol (Olson, 1989). However, the absorption rate for carotene is 20% to 50% compared with that of retinol, which is estimated at 70% to 90%, and the absorption of the former becomes less efficient with increasing levels of intake (Olson, 1990). Discrepancies in the conversion of carotenoids to retinol have been attributed to factors influencing bioavailability and absorption, including the amount of carotenoid in the diet, interactions with other carotenoids, dietary fat and fibre, nutritional deficiencies of zinc and/or protein, and the substrate requirements for absorption (Olson, 1986; Erdman, 1988).
Given the strong evidence for lower bioavailability, the biological activity of all-trans b-carotene and other carotenoids were revised. This gave rise to units of expression for vitamin A activity called retinol equivalents (RE). These are now the internationally accepted units for vitamin A activity (Simpson and Tsou, 1986), and can be summarized as follows:
= 1 mg all-trans retinol
= 6 mg all-trans b-carotene
= 12 mg other biologically active
= 3.33 IU retinol
= 10.0 IU carotene
However, there is still confusion between IU and RE given the differences in equivalency when converting b-carotene to retinol. Use of RE reduces the contribution of provitamin A to total vitamin A activity compared to the system of IU as described in greater detail by Olson (1987).
Other vitamin A-related compounds exist in dietary sources of preformed A, particularly in fish liver and oils. All-trans dehydroretinol, referred to as vitamin A2 in the older literature (Moore, 1957), is a vitamin A-related compound found in freshwater fish flesh and liver, and to a lesser extent, in some marine fish (Olson, 1986; Ball, 1988). This compound is estimated to have 40% to 50% of the vitamin A activity of all-trans retinol (Parrish et al., 1985). Likewise, cis isomers of retinol, which can account for up to 35% of preformed vitamin A measured in fish liver oils, have up to 75% relative activity of all-trans retinol. These discrepancies in vitamin A activity have been overlooked frequently in food composition literature, although recent studies have adjusted retinol activity values according to differential biological activity (Pepping et al., 1988; Morrison and Kuhnlein, 1993).
Isomerization is also an important issue in quantifying provitamin A activity in processed forms of plant products, but has not yet been given adequate attention in the calculation of units for expressing vitamin A activity. Likewise, new analytical techniques available for carotenoid analysis have facilitated the analysis of individual carotenoids within a single food item (Rizzolo and Polesello, 1992). This allows for the direct calculation of individual carotenoid intakes, instead of the conventional estimate based on conversion of vitamin A values (Forman et al., 1993).
Analysis of Vitamin A
With recent interest in the possible link between cancer and the intake of carotenoids (Krinsky, 1988; Knekt et al., 1990; Zeigler, 1991), an extensive literature has emerged describing the available methods for analyzing carotenoids, particularly those by high-pressure liquid chromatography (HPLC). Several thorough reviews exist, with descriptions of the theoretical and practical applications of each method (Davies, 1976; Rodriguez-Amaya, 1989; Rizzolo and Polesello, 1992).
Carotenoid analysis is accomplished by extraction, followed by partial purification, separation according to hydroxyl groups, isolation by chromatography, and then measurement by spectral absorption (Lee et al., 1989). The Association of Official Analytical Chemists (AOAC) method for carotene analysis is an open-column chromatography method using a magnesium oxide column, which separates carotenoids from xanthophylls on the basis of polarity, followed by visible absorption spectrophotometry (AOAC, 1984). The first fraction eluted is assumed to be, b-carotene.
Recent studies in carotenoid analyses revealed that assumptions inherent in the AOAC method are incorrect, so much of the published provitamin A nutrient data are overestimates of the true carotenoid value of certain foods (Simpson and Tsou, 1986). This is most critical for food items with mixed carotenoid activity, particularly those carotenoids that do not have vitamin A activity but elute out with the b-carotene fraction.
HPLC is rapidly becoming the preferred method for carotenoid
analysis, given its flexibility in the identification and
quantification of the numerous carotenoids (Rizzolo and
Polesello, 1992). The use of HPLC is also becoming the preferred
method of retinol analysis. However, the complexity of
carotenoids, their isomers, and other chemical substances in
foods prevented the development of a single HPLC method for
carotenoid analysis (Lee et al., 1989) until recently. Also,
while the methodologies using HPLC for carotenoid and retinol
analyses are evolving, standardization among and within different
laboratories is difficult to attain. In a recent study on the
intercomparison of methods used for vitamin A determination of
foods, the results for retinol analyses in milk agreed very well
(Hollman et al., 1993). However, comparison of b-carotene contents in green beans analyzed
by different laboratories showed poor agreement. Another limiting
factor for all analytical methods, particularly HPLC, is the cost
of equipment and solvents, which is prohibitive in most
developing regions (Rodriguez-Amaya, 1989).
Carotenoids and retinol are affected by pH, enzymatic activity, light, and oxidation associated with the conjugated double bond system (Elkins and Dudek, 1985). The chemical changes occurring in carotenoids during processing has been reviewed by Simpson (1986). Fresh plant tissue may contain enzymes that are only activated during and following processing. Therefore, the preformed and provitamin A content of the raw form of a food item may be reduced as a consequence of food preparation. The most dramatic example of this is found in red palm oil, which in its raw form is considered one of the richest sources of provitamin A (Cottrell, 1991). After heating to 200°C for thirty minutes, the b-carotene content becomes negligible.
Numerous reports document changes in carotenoid content attributed to various cooking methods (Park, 1987; Chandler and Schwartz, 1988; Nagra and Khan, 1988; Micozzi et al., 1990). As a general rule, foods boiled in an open container show the greatest losses. Regardless of the method used, most report that dehydration significantly reduces the carotene content in vegetables, which has implications for storage of seasonally available foods (Renquist et al., 1978; Park, 1987). However, in a study that controlled for complete extraction of carotenoids in raw samples, Khachik et al., (1992) reported no significant changes in the b-carotene content in several green vegetables, after microwaving, steaming, or boiling. Likewise, the carotenoid content of tomatoes did not change when they were dehydrated. It should be noted though that dehydration was performed in a laboratory environment and by sundrying.
Sweeney and Marsh (1971) reported that processing of fruits and vegetables induced isomerization of carotenoids, resulting in an estimated 15% to 20% reduction in vitamin A potency in green leafy vegetables, and 30% to 35% in yellow vegetables. Traditional processing methods, including preservation, induce formation of the cis-isomer of carotenoids from the all-trans form (O'Neil and Schwartz, 1992). With increased temperature, the presence of light, and catalysts such as acid, isomerization from the bans form to the cis form of carotenoids increases (Chandler and Schwartz, 1988).
documentation of processing effects on retinol is less abundant.
Losses of up to 40% in fish sources rich in vitamin A have been
reported following boiling (Burt, 1988). In a study on the
traditional food system of the Sahtú Dene/Métis, there were no
consistent trends in retinol levels between raw and cooked forms
of various food samples probably due to biological variation
(Morrison and Kuhnlein, 1993). Smoking fish and mammal meat did
not appear to reduce retinol levels.
Reasonably accurate food composition data is needed to calculate the vitamin A intake of a population from dietary surveys, or to select food items rich in this nutrient, for education programs. However, food tables contain nutrient values from chemical analyses of foods, with no allowance for the biological utilization of the item (Ferrando, 1987), so these values are estimates, at best, of active vitamin A. The limitations of vitamin A nutrient values in food composition tables have been reviewed (Simpson and Tsou, 1986; Booth et al., 1992); in their current state, most contain inconsistencies in preformed and provitamin A values. Differential use of units and conversion rates and reliance on outdated analytical techniques limit their use in the identification of vitamin A-rich foods and the calculation of dietary intake of vitamin A, particularly from carotenoid sources.
With the strong evidence that an increased intake of fruits and vegetables is associated with a reduced risk of certain types of cancers, the current food composition database of carotenoid values for foods consumed the United States was recently re-evaluated (Mangers et al., 1993). An artificial intelligence system was developed to evaluate existing carotenoid data, including indicators of data quality, and to prioritize future laboratory analyses. Only HPLC-generated data were incorporated into the database to eliminate the problem of overestimation associated with analytical methods that quantify total carotenoids instead of individual carotenoids. A modified version of this artificial intelligence system was subsequently used by West and Poortvliet (1993) to evaluate existing carotenoid data for developing countries. Most carotenoid values reported are for vegetables and fruits, although there were limited data for meat, fish, fats, eggs, cereals, and dairy products. Given the paucity of carotenoid data in many geographical regions, particularly African, these authors used less stringent criteria for including food composition data into their data base. In particular, carotenoid data generated from methods other than HPLC were included. This decision was made by West and Poortvliet (1993) in recognition of the limited number of resources available to laboratories in many developing countries.
A common criticism of food composition data is the inadequate amount of information available on sampling methods. Regardless of the analytical method selected for carotenoid and retinol determination, error introduced during the collection and preparation of samples can create large discrepancies in the final nutrient values. Sources of sampling and preparation error have been reviewed (Elkins and Dudek, 1985; Kuhnlein, 1986), and show that much of the variation is attributable to the nature of the item being analyzed (Thompson, 1986). West and Poortvliet (1993) reported multiple problems in compiling carotenoid data: use of different languages and nomenclature for identification of food items; inadequate data on the sample size and handling; limited data on the time between sampling and analyses, and sample treatment in the interim; exposure to light and air; details on the analytical methods used; and absence of information on the use of quality control procedures. In the absence of information on sampling, it is not known whether discrepancies in the published literature on vitamin A content reflect natural and/or analytical variation.
Heterogeneity in nutrient content is a consequence of numerous factors, including soil pH, amount of rainfall, seasonality, genetic diversity, and the stage of maturation. Vitamin A is not uniformly distributed within the animal or plant tissue, so the accuracy of the nutrient value is determined in part by the portion size and the number of individual units selected for a representative sample. Retinol concentrations in liver oils among fish and mammalian species can differ by more than a thousandfold, and mammalian liver retinol concentrations within species can vary in a range of more than 200-fold (Moore, 1957; Pepping et al., 1988; Morrison and Kuhnlein, 1993). Bureau and Bushway (1986) found a very large range in provitamin A values for a sample of twenty-two fruits and vegetables, but this was not consistent across seasons or location. Nutrient data of green leaves analyzed in two different seasons also showed inconsistent variation among seasons (Tagaki, 1985). Variation attributable to different cultivars and handling conditions, including the time of marketing, have been confirmed Johnson et al., 1985; Bushway et al., 1986).
compiling carotenoid data from multiple sources, Forman et al.
(1993) grouped similar foods into a single general food
description. Variability was indicated by the range of individual
carotenoid values. Unfortunately, there were insufficient data to
desegregate carotenoid values for a single food item based on
factors such as season, that may influence carotenoid levels. In
contrast, West and Poortvliet (1993) limited the amount of
aggregation of carotenoid data due to the wide variation in
globally generated values.
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