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HEALTH AND DISEASE. The relationships among food, health, and disease are myriad and complex. We consume food every day, and it provides the resources we need to carry out life-sustaining functions. Hence it comes as no surprise that one's diet can affect profoundly one's daily and long-term physiological health and wellbeing. Qualities of a diet and the foods that comprise it have the potential to make one sick, but they also can act to reduce one's risk of acute or chronic diseases.

All of the formal medical traditions of the world recognize a close connection between diet and an individual's health. One theme common to Mediterranean, Middle Eastern, and South and East Asian traditional medical systems is the ascription of humoral qualities to foods (for example, foods that are "heating" or "cooling" to the body). In these traditional systems, an individual's diet is manipulated to include or exclude foods with specific properties in order to correct putative humoral imbalances or disease states. In contemporary biomedicine, the link between food and disease most often is articulated with regard to the compositional qualities of foods and the ways that diets high or low in specific foods (and hence nutrients and other plant constituents) have harmful or beneficial effects on the body.

Benefits of Nutrient Diversity

There are numerous ways in which diets comprised of specific foods containing or lacking a given nutrient contribute to health or disease. For example, a diet that includes few or no animal products may result in anemia due to a deficiency in iron and/or vitamin B₁₂. On the other hand, a diet high in animal products but low in fruits and vegetables may contribute to specific vitamin deficiencies. Scurvy (a disease caused by vitamin C deficiency) was recognized first among sailors on long-distance sea voyages, as they had no source of fresh fruits or vegetables. An unprocessed corn-based diet is known to result in the disease pellagra, caused by a deficiency of niacin, one of the B vitamins. Up through the early twentieth century, there was a well-defined "pellagra belt" through the southern United States, where corn was consumed widely.

A diet comprised of diverse foods generally is considered to be the best way to prevent nutrient-deficiency diseases. Early humans lived by hunting and gathering, and they ate a broad array of plant and animal foods, although this varied by season and geography. Modern hunter-gatherers of the Kalahari Desert in southern Africa are known to exploit more than eighty species of plant foods, and no specific nutrient deficiencies have been reported among these groups. However, with the transition to agriculture, which happened in many parts of the world around 10,000 years ago, dietary diversity declined notably as populations began to cultivate a narrow array of staple crops (such as wheat, rice, potatoes, and millet). Iron deficiencies and severe growth deficits due to undernutrition become apparent in the skeletons of early farmers.

Effects of Food Processing

Some of these nutritional problems were resolved as populations evolved different means of processing staple foods that enhanced dietary nutrient profiles. Indeed, in the postagricultural period, food-processing techniques became crucial for reducing the negative health impacts of reliance on a few foods. Native populations of the Americas that had a long tradition of reliance on maize (corn) prepared it in such a way as to avoid the problem of niacin deficiency. Corn was boiled in a solution containing lime (calcium carbonate, ash, etc.); this process resulted in the liberation of niacin from an undigestible complex, and also improved the food's amino-acid balances. When corn was introduced to Europe during the Columbian period, the lack of a tradition for its processing led to outbreaks of pellagra. A similar example is the leavening of wheat to make bread, or fermentation to make beer. Both of these processes increase the bioavailability of the minerals calcium, iron, and zinc. When soybeans are processed into bean curd, as is common in East Asia, they lose their protease inhibitors, which interfere with protein digestion.

On the other hand, it is also the case that some food-processing techniques—such as heating, boiling, or drying—can destroy vitamins in foods. Vitamin C degrades in the presence of heat and aridity; folic acid and thiamine likewise are sensitive to heat. Some of the other B vitamins break down in the presence of alkaline or acidic conditions. Others, such as vitamins B₆ and B₁₂, are quite stable under most cooking conditions. Milling and polishing rice into smooth white grains, which are valued highly in East Asian cuisine, reduce the protein and thiamine content of rice, and contribute to the risk of the disease beriberi (thiamine deficiency). Industrial processing of foods often reduces their nutrient profile, but many foods, especially those that are consumed widely such as cereals, are enriched to replace lost nutrients. In addition, grilling or broiling meats until they are well-charred has been associated with the production of the chemical compound Benzo(a)pyrene, which has been linked to gastrointestinal cancers.

Nonnutritive Food Components

When diets are derived largely from plant foods, particular combinations of food are known to improve the overall dietary quality, particularly with respect to the balance of essential amino acids. Corn, for example, is low in the amino acids lysine and tryptophan, but in native American cuisine, corn is often combined with legumes that are rich in those amino acids. Likewise, the combination of rice and legumes can provide the full array of essential amino acids. A peanut butter sandwich, a staple in the diet of many American children, contains complementary amino acids from the wheat and peanuts.

However, it is not only the nutrient composition of foods that is relevant to disease. Other qualities of foods —especially plant foods—recently have been found to contain other chemicals that reduce the risk of certain diseases. Phytochemicals derived from plant foods may reduce the risk of some cancers, while others may protect against heart disease and/or diabetes. Some potentially important phytochemicals include polyphenols (in red wine and green tea) and carotenoids (in orange, yellow, and green vegetables). Many of these have been found to have antioxidant effects and may prevent cell damage from oxygen-free radicals. Widespread consumption of red wine has been credited by some with the "French Paradox," the observation that, although the French tend to eat foods high in fat, their consumption of red wine may offset some of the risk of cardiovascular disease usually associated with such diets. Phytoestrogens, a form of isoflavones found in legumes such as soybeans, may reduce the risk of many cancers, especially breast cancer, by binding to estrogen receptors, and these also may reduce bone loss associated with osteoporosis. Proteins in soybeans also may reduce cholesterol levels and thus reduce the risk of heart disease. The organosulfur constituents of garlic may inhibit platelet aggregation and reduce blood lipids, thereby reducing the risk of coronary heart disease. Tannins (found in tea, coffee, cocoa, red wine, and some legumes and grains) and phytates are hypoglycemic, and may contribute to reduced risk of diabetes.

Other plant compounds have links to infectious disease, such as the protozoan disease malaria, which is a common disease (and often life-threatening) in tropical and semitropical areas. Manioc (Manihot esculenta; also called cassava or yuca), a widely cultivated root crop in the tropics, contains cyanogens, which appear to inhibit the growth of the malaria parasite in red blood cells. Likewise, fava beans contain vicine, a potent oxidant that disrupts malarial reproduction in red blood cells. However, individuals who are deficient in the enzyme G-6PD (a deficiency most common in Mediterranean populations) are susceptible to the potentially fatal anemia, favism, because their red blood cells are extremely vulnerable to destruction by potent oxidants such as vicine.

Many secondary compounds in plants do not have such salutary effects, or their benefits are tempered by potential negative effects on health. The cyanogens in manioc, lima beans, and other foods can interfere with thyroid function, glucose metabolism, growth and development, and other important physiological functions. Cruciferous vegetables such as cabbage contain thiocyanate compounds that act as goitrogens, and thereby contribute to thyroid disease. Tannins, which are distributed widely among plant foods, inhibit protein digestion and interfere with iron absorption. The ingestion of solanine, a glycoalkaloid found in commercial strains of potatoes that have been exposed to light, or in many wild varieties, can lead to serious gastrointestinal and neurological symptoms. Interestingly, traditional modes of consuming potatoes among Andean populations appear to reduce the risk of solanine exposure; their potatoes are consumed often with a clay-based slurry, which effectively detoxifies them.

Food-Consumption Concerns Linked to Population Profiles

There are cases in which the health effects associated with the consumption of particular foods vary in significant ways among diverse populations. For example, the ability to produce the enzyme lactase (which breaks down the milk sugar lactose) in adulthood is rare among human populations. This ability persists in highest frequencies through adulthood among northern Europeans and pastoral populations in Africa and other areas. Fresh milk consumption played an important role in maintaining health in the history of these populations, and they evolved lactase persistence as a dietary adaptation. When adults with low levels of small-intestinal lactase activity consume fresh milk (the food highest in lactose), they often experience cramps, bloating, diarrhea, and other forms of gastrointestinal distress. This is less of a problem when milk is consumed after processing into yogurt or cheese, as lactose is either fermented or removed during their production.

In populations that only recently have begun relying on wheat production there is a high frequency of celiac disease, an allergic response to wheat protein (gluten). There is some suggestion that African Americans may be more sensitive to salt than are other sectors of the population, and that, consequently, salt consumption by African Americans increases blood pressure and contributes to an incidence of hypertension greater than in other groups.

In the most general sense, both underconsumption and overconsumption of foods can lead to chronic disease. Not surprisingly, these two ends of the consumption spectrum tend to occur in poor and wealthy populations, respectively. It is estimated that more than 1.2 billion people suffer from deficiencies of calories and protein. A similar number suffer from problems related to the overconsumption of calories. Both are associated with deficiencies of micronutrients. It has been suggested that more than half of the world's disease burden derives from nutrition-related sources.

Overconsumption of calorie-rich foods became the norm in wealthy countries during the late twentieth century. Such foods became mass produced, more readily available, and relatively inexpensive. Today supermarket shelves are lined with potato chips, candy, cookies, crackers, soda, and all kinds of other calorie-dense foods. Fast-food restaurants specialize in ever-larger servings of high-calorie foods that are quickly prepared and consumed. Most of these are highly processed, and although they are rich in calories, they are often low in vitamins, minerals, and phytochemicals. It is widely accepted that, when combined with a sedentary lifestyle, diets high in such foods contribute to a broad array of chronic health conditions, most significantly cardiovascular disease (CVD), diabetes, cancer, and hypertension. It is now estimated that more than half of Americans are overweight, and almost one-quarter are obese, which is itself a risk factor for these diseases. In addition, an increasing number of children are now obese, and "adult-onset" (Type 2) diabetes is appearing with alarming frequency in adolescents. More than 75 percent of all mortality in the United States is due to CVD and cancer, but death rates from stroke and heart attacks have declined since the 1970s. This has been attributed, in part, to reduced consumption of saturated fat from red meat, whole milk, butter, and lard. There are several studies indicating that a low-fat diet based largely on vegetables, fruits, whole grains, legumes, with relatively small amounts of animal protein (especially from fish) is associated with increased longevity and reduced risk of chronic disease.

Problems related to the overconsumption of high-calorie foods are not unique to the industrialized world. As countries are integrated into the global economy and populations increasingly become urbanized, there has been a global shift in dietary patterns and health conditions that appear to accompany those new consumption habits. Interestingly, such changes are remarkably consistent across countries, and may reflect a panhuman preference for foods rich in calories, which historically have been quite limited in the diet. Generally the consumption of fats and sweets has increased, and the use of traditional whole-grain foods and traditional modes of processing has declined. Fast-food outlets such as Kentucky Fried Chicken and McDonalds have become ubiquitous in urban centers throughout the world. Active lifestyles are being replaced with sedentism, as people move away from subsistence agriculture into clerical and factory jobs. As a result, the chronic diseases that heretofore had predominated in wealthy industrialized countries are becoming globalized. For example, the global diabetes rates seen in 2000 are expected to double by 2025, with the majority of that growth occurring in developing countries.

On the other hand, undernutrition, also referred to as protein-energy malnutrition (PEM), often occurs under conditions of food scarcity and is associated with a wide range of negative health effects. More than 10 percent of the world's population suffers from chronic hunger, and undernutrition may be responsible for as many as twenty million deaths per year. It is important to realize that hunger is not the result of too little food being produced for too many people in the world; it is essentially a problem with the way that food is distributed unevenly among the world's populations. Children are especially vulnerable to PEM, as they have higher protein and energy needs per unit of body weight than do adults. When calories and protein are chronically scarce in childhood, permanent stunting and retarded development occur. In its acute form, PEM results in wasting (dramatically reduced weight relative to height) and it is potentially fatal. More routinely, PEM increases vulnerability to infectious disease, since energy, protein, and certain vitamins and minerals play crucial roles in immune function. In environmental contexts in which infectious disease (especially diarrheal disease) is common, the combination of PEM and infection can provoke a rapid deterioration of health that can lead to death. A common stage for this progression to manifest itself is weaning, the period when children make the transition away from breast milk (which contains nutrients and disease-suppressing maternal immunoglobulins) to an adult-type diet. It is not uncommon for children to become more vulnerable to infection when they are weaned prematurely and are unable to consume sufficient nutrientdense foods to maintain growth.

In the 1800s, baby bottles were developed and cow's milk was developed into infant formula as an alternative to breast milk. The practice of formula-feeding peaked in the United States in the years following World War II; breast-feeding is now on the rise again in most parts of the world, although it remains uncommon past the early months in most industrialized countries. Most research amply demonstrates the health benefits of breastfeeding: substitution of formula for breast milk is associated with increased risks of numerous health problems including SIDS (sudden infant death syndrome), ear infections, diabetes, breast cancer, and allergies.

Controversy erupted in the early 1970s over the promotion of formula by multinational corporations in the developing world. Formula was marketed heavily and inappropriately, and health personnel began to encourage mothers to feed their children formula rather than nurse them. Formula, which was costly, often was prepared in dilute form with contaminated water. Its use in this way increased infant morbidity and mortality and generated much attention among the media and international health organizations, ultimately resulting in a ban on formula promotion by multinational corporations.

As the links between diet and disease have become more widely known, there has been a trend toward more healthful eating habits in industrialized societies. However, this trend is not uniform within such populations. Numerous studies have shown that obesity, the eating habits that contribute to it, and the diseases associated with it, especially diabetes, have increased among lower socioeconomic groups. The reasons behind this trend are complex, but as noted above, foods high in starches, fats, and sugars are now cheap and readily available. Those high in protein (meat, dairy products) and fresh fruits and vegetables are relatively less accessible and more expensive, and are consumed less commonly by the poor. Moreover, in the United States, fast-food outlets are locating preferentially in areas serving poorer communities. This has led to the curious, yet commonplace, phenomenon in wealthy countries whereby weight is correlated inversely with wealth. Historically, of course, the reverse would have been the case, as is still evident in many developing countries.

See also Anthropology and Food; Baby Food; Disease: Metabolic Diseases; Fast Food; Food Politics: United States; Lactation; Malnutrition; Medicine; Milk, Human; Niacin Deficiency (Pellagra); Nutrients; Nutrition; Obesity; Paleonutrition, Methods of; Political Economy; Population and Demographics; Salt; Sodium; Vitamins.

Andrea S. Wiley