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BIOACTIVE FOODS IN PROMOTING HEALTH: FRUITS AND VEGETABLES
Academic Press
Copyright © 2010 Elsevier Inc.
All right reserved. ISBN: 978-0-08-092220-1
Chapter One
Botanical Diversity in Vegetable and Fruit Intake: Potential Health Benefits Matthew D. Thompson and Henry J. Thompson Crops for Health Research Program and the Cancer Prevention Laboratory, Colorado State University, Fort Collins, CO
1. OVERVIEW
Dietary guidelines are evolving from a primary focus on providing adequate intake of essential nutrients in order to prevent nutritional deficiency to an emphasis on reducing the prevalence of chronic diseases including cardiovascular disease, cancer, type II diabetes, and obesity. During this transition, there has been a movement to broaden nutritional terminology such that nutrients are divided into two categories: essential and nonessential. Essential nutrients are those substances that cannot be made in the human body but that are required for normal cellular function. The absence of essential dietary nutrients results in defined disease syndromes. Nonessential nutrients are not required for life, but they promote health. Many chemical constituents of plant-based foods, i.e. foods which are plants or are derived from plants, are termed nonessential nutrients since they positively impact health; such phytochemicals are also referred to as phytonutrients. Current dietary recommendations attempt to meet these nutrient requirements and are based on grouping foods using culinary definitions and knowledge of essential nutrient content. Despite the recognition that literally thousands of chemicals exist in plant-based foods and that they are likely to exert a wide range of bioactivities in living systems, dietary guidelines continue to provide limited direction about the specific types of plant-based foods that should be combined to render maximal health benefits. This situation exists for many reasons including: 1) the lack of a systematic approach by which plant-based foods are nutritionally classified; 2) the limited information regarding the chemical profile of each type of plant-based food; 3) the lack of data on the biological activities of plant chemicals; and 4) the paucity of information about the impact of plant-based food combinations on health outcomes. However, technological advances in chromatographic separation and chemical identification of phytochemicals are occurring at a rapid rate [6] and this progress is providing a large amount of information regarding the chemical composition of plant-based foods. This situation has created an unprecedented opportunity to expand our approach to dietary guidelines and menu planning.
The objective of this chapter is to raise the awareness of health care professionals about opportunities to extend dietary guidance about plant-based food intake beyond meeting the recommended servings/day of cereal grains, vegetables, and fruit by incorporating information about the botanical families from which the plant-based foods are selected for menu planning. The approach also has the potential to identify food combinations that reduce chronic disease risk. The remainder of this chapter addresses three topics: Section 2 details the rationale underlying the proposed use of botanical families, Section 3 provides evidence of the potential usefulness of this approach in an effort to reduce chronic disease risk, and Section 4 considers how botanical families can be applied to meal planning.
2. RATIONALE FOR USING BOTANICAL FAMILIES
2.1 Categorizing Vegetables and Fruit
The focus of this chapter is on vegetables and fruit, yet a careful inspection of how these terms are defined and the manner in which they are used reveals a surprising amount of ambiguity about the foods placed in each category. While the term 'vegetable' generally refers to the edible parts of plants, the categorization of foods as vegetables is traditional rather than scientific, varying by cultural customs of food selection and preparation. Moreover, in the biomedical literature, some foods are not classified as vegetables because of their content of starch, e.g. potatoes, without consideration that these foods, as well as other staple food crops, are vehicles for the delivery of a wide array of small molecular weight compounds in addition to carbohydrate. The categorization of foods as fruits is no less ambiguous. Strictly, a fruit is the ripened ovary of a plant and its contents. More loosely, the term is extended to the ripened ovary and seeds together with any structure with which they are combined. The botanical definitions for fruit are not uniformly applied in nutrition and dietetics; rather, cultural customs tend to determine what differentiates a fruit from vegetables and grains. Examples include: 1) the apple, a pome, in which the true fruit (core) is surrounded by flesh derived from the floral receptacle; 2) wheat, a fertilized ovule is comprised of an outer coat (testa) that encloses a food store and embryo; seeds of wheat, rice, and oats, which are botanically the fruits of the plant, are classified in food terms as cereal grains, i.e. they are neither vegetable nor fruit; 3) tomato is classified as a vegetable, though it is the ovary of the plant; and 4) legumes, which could be botanically classified as fruits, are sometimes considered vegetables, but if they are consumed as a staple crop, they are categorized in the meat food group. Together, these examples demonstrate the need to acknowledge how we classify plant foods and in categorizing them, may bias ourselves to thinking certain foods are either more or less related, more or less diverse, or more or less likely to provide health benefit. To overcome this bias, we need to acknowledge the different ways we categorize plant-based foods, e.g. scientific and cultural perspectives. The remainder of this discussion is as inclusive as possible, as almost all the plant-based foods we eat could be classified as a fruit or vegetable depending on the organizational scheme. Inclusion allows us to consider what advantages might be gained from using a scientifically-derived botanical and taxonomic scheme as an additional filter through which plant-based foods are categorized.
2.2 Linnaean Taxonomy
Plant taxonomy classifies plants in a hierarchical manner. Table 1.1 shows an example of the taxonomic classification scheme for the apple (Malus domestica, Borkh.) using the Linnaean system, which is the most common method of classification for living organisms. Ascertaining groupings of plant-based foods by this taxonomic classification at the level of the botanical family, as shown in Table 1.2, is useful in promoting an understanding of general relationships among food crops which often go unrecognized. This classification scheme has been used: 1) to gain insight regarding specific chemical components of foods that may account for health benefits; and 2) to develop functional foods and nutraceutical supplements that emphasize a particular class of chemicals. However, little attention has been given to using this information to identify health-promoting combinations of plant-based foods that, when eaten as a regular component of the diet, result in a reduced risk for chronic diseases.
2.3 Chemotaxonomy
To better understand how botanical family classification informs understanding of the phytochemical composition of various plant-based foods, an additional approach to taxonomy is needed. Chemotaxonomy, also called chemosystematics, is the attempt to identify and classify plants according to differences and similarities in their biochemical components [16]. The products of plant biosynthesis are generally divided into primary and secondary metabolites as shown in Figure 1.1. Primary plant metabolites, e.g. carbohydrate, protein, and fat, are considered as the essential building blocks for plant growth and development. The production of these macromolecules is under stringent genetic control and while variation among plants in the content of primary metabolites does exist and is of interest to nutritionists, those differences are of limited value in chemotaxonomy. On the other hand, plants have evolved the capacity for the combinatorial chemical synthesis of a vast array of secondary metabolites. The synthesis of secondary metabolites by plants has two main purposes: 1) signaling (e.g. plant hormones); and 2) defense against abiotic (e.g. ultraviolet light) and biotic stressors (e.g. microbes). In terms of defense, secondary metabolites protect plants against microbes, pests, and other plants as indicated in Figure 1.1. In a broad sense, all of these chemicals function as semiochemicals, i.e. 'message carriers', a term often used in chemical ecology. The chemicals that have evolved over the millennia span at least 14 defined chemical classes of compounds (Table 1.3) and in excess of 200,000 chemical structures. Available evidence indicates that all chemical classes have a biological activity that was selected for during evolution and that these biosynthetic strategies sustained the survival of the plant species. When classical taxonomic information and chemotaxonomic data are overlaid, relationships become apparent; plants within a botanical family tend to have greater chemical similarity than plants in different families, i.e. plants within a botanical family emphasize biosynthetic pathways for specific classes of chemicals, and plants within botanical subfamilies emphasize particular chemical compounds within those subclasses in comparison to plants in other subfamilies. The further apart the botanical families are from one another in the evolutionary tree (Figure 1.2), the more likely they are to differ in the composition of secondary metabolites. In fields such as pharmocognosy, where medicinal plants, crude herbs or extracts, pure natural compounds, and foods are being evaluated for health benefits, the goal is to identify chemicals with specific molecular targets. The success of efforts to identify natural products will be based on successfully targeting mammalian proteins involved in cell signaling during the pathogenesis of chronic diseases. The fact that compounds from all chemical classes listed in Table 1.3 have activity in mammalian systems provides considerable support for developing recommendations for using phytochemically diverse food combinations (recipes and menus) as a method to maximize the potential to enhance health and to prevent chronic diseases in the context of promoting variety and moderation. As more research reveals the level of conservation among plant and animal signaling pathways, the relationships among botanical taxonomy, chemotaxonomy, and the phytochemical composition of plant-based foods will provide a framework for predicting the bioactivity of the foods that are consumed as part of a diverse plant-based diet (Figure 1.1).
2.4 From Botanical Family to Chronic Disease Prevention
A reason for developing dietary guidelines is to reduce the risk of four related chronic diseases. Specifically, cardiovascular disease, cancer, type II diabetes, and obesity are metabolic disorders with shared impairments in both cellular processes and metabolism, although each disease also retains unique characteristics. A better understanding of their interrelationships has come as a result of proteomic investigations providing evidence of a common pathogenic basis for their occurrence. At the cellular level, the pathologies associated with each disease display alterations in cell proliferation, blood vessel formation, and cell death. Also common to these diseases are alterations in glucose metabolism, chronic inflammation, and cellular oxidation attributed to a common network of cell signaling events that are perturbed in each of these disease states. In addition, emerging evidence indicates that modulation of gut microflora predisposes an individual to each of the disease processes. Microflora appear to be able to exert effects through either biosynthesis of new compounds or chemical transformations of ingested ones, and as a consequence, influence exposure of the host to gut microflora-associated endotoxins. By overlaying the chemotaxonomic and bioactivity relationships, i.e. matching of plant-based foods with dysfunctional signaling pathways associated with chronic disease, a basis for identification and implementation of patterns of plant-based food consumption that inhibit the pathogenesis of chronic diseases will be developed.
2.5 Advantages and Limitations of Using Botanical Families
The use of botanical families to investigate food combinations provides a broader framework upon which to construct diets, as opposed to the reductionist approach that does not take into account the full spectrum of plant chemicals available and essentially violates the variety and moderation axiom. Perpetuation of the quest for a single phytochemical solution, whether it is at the level of a specific chemical (e.g. beta-carotene), a class of compounds (e.g. carotenoids), or a particular food (e.g. carrots), will fail to provide the diversity of the botanical family approach. Use of botanical families requires the consideration, identification, and recommendation of dietary patterns of plant-based food intake for enhanced health benefit that encourages variation and, as a corollary, moderation.
Nonetheless, there are limitations to this approach that have been identified by efforts associated with using chemical composition for taxonomic classification of plants. One limitation of the botanical family approach is that in some cases, plant-based foods from different botanical families may have more chemical similarities than foods from within a botanical family. This would result from the convergent use of a certain class of chemicals for a specific function (e.g. soil-microbe interactions), environmental factors (e.g. exposed to light versus underground), and plant anatomical origin (e.g. fruit versus stem versus root). Potato versus tomato is a good example of plant-based foods which are chemically quite different from each other, though they reside within the same botanical family. Tomatoes are exposed to sunlight, do not have to contend with soil fauna and microbes, and are of fundamentally different plant anatomical origin (i.e. tomatoes are the ovaries of the plant while potatoes are underground stems). In considering these within-family disparities, examples of within-family divergent function of chemical classes may also be found. In general, one should always exercise caution in exploring botanical families beyond the commonly consumed foods, i.e. wild versus cultivated plants. As an example, the nightshade family (Solanaceae), which has the well-known members potato and tomato, also has plants known for toxic effects such as belladonna. In general, plant-based foods could be further categorized based on plant anatomical origin (i.e. leaves, roots, stems, fruits) or location of plant-based food components relative to the surroundings (i.e. underground/on ground/above ground or soil/ microbes/pests/fauna/light) and should consider known toxicities.
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Excerpted from BIOACTIVE FOODS IN PROMOTING HEALTH: FRUITS AND VEGETABLES Copyright © 2010 by Elsevier Inc. . Excerpted by permission of Academic Press. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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