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Foods and Diets


Humans have evolved to depend on a continuous supply of energy (calories) and nutrients from edible plants and animals. From the dawn of civilization, humans have been omnivores. Early humans hunted or gathered whatever foods were available in the immediate environment, as determined by geography, climate, and season. The precise number of foods eaten by humans is uncertain; scientists estimate that at least twenty thousand plants have edible parts. To these must be added fungi and algae, as well as foods derived from mammals, fish, seafood, birds, eggs, milk, and exotic animals. Among food plants, about 150 have been domesticated and are cultivated on a large scale, but only about twenty constitute principal energy sources, and just threewheat, corn, and ricedominate world markets. What people choose to eat depends largely on culture and economics. Cultural factors explain why Asians eat rice or noodles, whereas Mediterraneans consider no meal complete without bread. There are also religious prohibitions, such as those against consuming beef, pork, or alcoholic beverages. People can choose freely from available foods only if they can afford them; without an adequate income, food choices are limited.

Foods vary in quality as well as variety. Their composition is exceedingly complex; foods contain hundreds of chemical componentsincluding the familiar nutrients and energy sources, but also fiber, phytochemicals, and other substances that may affect health. As recently as the mid twentieth century, most foods were produced and eaten locally. As transportation improved, production became increasingly centralized and foods were processed to permit transport and storage. Processing removed essential vitamins, minerals, and other nutritious components and added preservatives, texturizers, flavors, and colors. While highly efficient, centralized production permitted biological and other contaminants to affect larger numbers of people. Increased efficiency led to cheaper food, but it also led to other, less desirable effects of overproduction.

The food systems of industrialized countries produce more food than their population can consume. As a result, they compete for consumer food dollars. About 80 percent of every food dollar is spent for processing beyond the food itself, including transportation, packaging, and advertising. Potatoes are cheap; it is much more profitable to sell potato chips. Food companies introduce more than twelve thousand new food productsmany of them candy, snacks, soft drinks, and dessertsinto the American food supply each year. A typical supermarket stocks more than thirty thousand different food items, and manufacturers market them with about $30 billion worth of annual advertising. Marketing affects dietary choices. Thus, the changing food supply favors consumption of processed foods higher in energy and relatively lower in nutritional value than the basic foods from which they were derived.


An optimal diet, by definition, is one that maximizes health and longevity, prevents nutrient deficiencies, reduces risks for diet-related chronic diseases, and is obtained from foods that are available, affordable, safe, and palatable. Throughout human history, societies have developed a variety of dietary food patterns that take advantage of the foods available. Ancestral diets of societies surviving to the present must have been sufficient enough to support growth and reproduction, even if they did not promote optimal adult health.

At issue is how to select a health-promoting diet from the array of possible choices. When the leading causes of illness and death were infectious diseases made worse by inadequate diets, health officials advised people to eat more foods from specific groups such as dairy, meat, fruits and vegetables, and grains. As diseases such as coronary heart disease, certain cancers, diabetes, and stroke overtook infectious diseases as the leading causes of death, new recommendations were needed to address these chronic conditions. A large body of biochemical, animal, epidemioligic, and clinical research indicated that diets high in energy, saturated fat, cholesterol, sugar, salt, and alcohol raise risks for multiple chronic diseases, whereas diets high in fruits, vegetables, and whole grains reduce chronic-disease risk factors such as obesity, high blood cholesterol, and high blood pressure. This evidence established the basis for new dietary recommendations.


Throughout the world, governments issue dietary guidelines to help their populations make healthful food choices. Guidelines may be quantitative ("Reduce fat intake to 30 percent of energy or less") or qualitative ("If you drink alcoholic beverages, do so in moderation"). They may focus on food, nutrients, dietary behavior, or other issues. A typical food-based guideline is, "Eat more fruits and vegetables every day." Nutrient-based guidelines suggest choosing foods low in saturated fat, cholesterol, sugar, or salt. Because obesity has emerged as a major health problem among people in industrialized countries and is increasingly a problem in developing countries, guidelines focus on behavioral changes to reduce energy intake ("eat less") and increase energy expenditure ("move more"). Finally, guidelines address the need to "eat a variety of foods," or to "keep foods safe to eat."

In the United States, collections of such precepts are published as the Dietary Guidelines for Americans. Issued every five years since 1980, and required by Congress since 1990, the Guidelines document is a policy statement that governs federal nutrition programs and educational activities. The separate guidelines are meant to be followed as a whole, and they define a distinct dietary pattern. When translated into food choices, this pattern derives most daily energy from grains, vegetables, and fruits, with less from meat and dairy foods, and even less from fats and sweets. To help the public translate this advice into healthful food choices, the U.S. Department of Agriculture (USDA), with the assistance of the U.S. Department of Health and Human Services, developed the food guide pyramid. The pyramid suggests that people consume specified numbers of daily servings of certain food groups. Its design indicates that the foods are hierarchical. People are supposed to eat more foods from the base of the pyramid (the plant food groups) but to eat fewer servings from the upper sections (meat, dairy, and processed foods). In contrast to earlier advice to "eat more," this pattern demands "eat less" in order to prevent chronic diseases.


Recommendations to limit intake of fat, saturated fat, and cholesterol mean eating less of the food

sources of these nutrients: meat, dairy, eggs, and processed foods high in fat. Eating less salt and sugar means less snack foods, soft drinks, and desserts. Although the first such recommendations appeared in the mid-1950s and have hardly changed since, they consistently have elicited controversy. Groups have argued that "eat less" advice is unjustified by the evidence, inappropriate for the general public, and economically unsound. Scientific concerns derive from the difficulties inherent in conducting studies that meet rigorous standards of proof among diverse populations who consume varied diets and develop diseases with multiple causes. Dietary recommendations apply to all individuals over the age of two years, but some people will benefit more from the advice than others. Some authorities argue that public health guidelines are unnecessary because individuals with chronic-disease risk factors can be identified and treated, and because young and old individuals may not be helped by such advice. But high-risk individuals cannot always be identified before they develop symptoms, and health care systems do not reach everyone who is at risk. Because similar recommendations apply to so many chronic diseases, most health officials believe that the advice in the dietary guidelines and food pyramid is prudent.

Economic concerns about guidelines derive from their impact on food producers. Meat and dairy foods together account for about 40 percent of the total fat, 60 percent of the saturated fat, and all of the cholesterol in the food supply, and processed foods are often high in energy, salt, and sugar. Soft drinks, for example, are a leading source of added sugars in American diets. Dietary recommendations to reduce such nutrients necessarily translate into decreased intake of meat, dairy, and processed foods. Since 1977, any federal recommendation to "eat less" has been stronglyand often effectivelyprotested by interested commodity groups. In 1992, objections of meat producers to the location of their products on the pyramid led the USDA to suspend publication until additional research confirmed its effectiveness. Given such considerations, government advice about healthful diets elicits more attention than might be expected for messages that have not changed in years.

Virtually every government or health organizationnational and internationalthat has examined research linking diet to health has issued similar dietary guidelines. This international policy consensus, based on research that is uncertain, incomplete, sometimes contradictory, and endlessly debated, can be explained by the fact that scientific arguments usually focus on the role of single nutrients such as fat, specific fatty acids, cholesterol, fiber, sugar, or sodium. But individuals do not eat single nutrients; they eat food. Evidence for the substantial health benefits of dietary patterns that follow recommendations has remained constant, despite debates over nutritional details. If this point is not widely recognized, it may be because the underlying message to "eat your vegetables" is not headline news, is difficult to follow in societies where meals are increasingly consumed outside the home, and is likely to lead to politically unpopular changes in food patterns. From the standpoint of health, pyramid-like diets make sense. In the light of current societal pressures that encourage people to eat more, not less, the challenge is to find ways to make it easier to follow the pyramid's recommendations.

Marion Nestle

(see also: Nutrition; Nutrition in Health Departments; United States Department of Agriculture [USDA] )


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