COMBINATION OF PROTEINS

COMBINATION OF PROTEINS. A thousand years ago, the Chinese poet Wu Tzu-mu listed the "seven necessities" of life that are still memorized by every Chinese schoolchild: firewood, rice, soy sauce, sesame oil, salt, vinegar, and tea. The list does not include meat. Now as then, we place great value on animal flesh as a food source, but depend on plants. Protein complementarity—or combining certain vegetable foods to achieve complete protein—solves a singular problem with that dependence.

Plant protein is an incomplete source of protein. Protein is built of amino acids, and of the twenty-two found in nature, our bodies can synthesize all but eight. These are termed "essential," and must be found in the diet. Nearly all plants, however, are deficient in the essential amino acid methionine; only soybeans (Glycine max ) and some seeds and nuts, especially the Mongongo (Ricinodendron rautanenii ) and sesame seeds (Sesamum indicum ), can provide over 50 percent of our daily need. Legumes and pulses, such as peanuts (Arachis hypogoea ), lentils (Lens esculenta ), chickpeas (Cicer arietinum ), and lima beans (Phaseolus lunatus ), typically contain about half as much protein by weight as meat, and are good sources of lysine, but lack methionine. Many vegetables such as the squashes (Cucurbita maxima, pepo ), cauliflower (Brassica oleracea ), and runner or green beans (Phaseolus coccineus ) have small amounts of protein, typically 1 to 2 percent by weight, but reasonably balanced amino acids.

Many populations, however, must find their protein in the same starchy carbohydrates that provide their calories. Corn (Zea mays ), about 3 to 4 percent protein by weight, lacks methionine, lysine, and tryptophan, and is overrich in leucine. Rice (Orza sativa, typ), also limited in methionine and lysine, has less total protein than corn by weight. However, its even amino acid profile gives rice greater bioavailability (how many of the amino acids our bodies can utilize). Whole wheat (Triticum aestivum ) has four times more protein than corn or rice, but has low amounts of methionine, lysine, and tyrosine. Some tubers such as potatoes (Solanum tuberosum ) have superior amino acid profiles to cereals, while others such as the yams (genus Dioscorea ) and cassavas (genus Manihot ) offer only about 1½ percent protein by wet weight.

Plants, moreover, contain tannins, phytates, and other indigestible fibers that bind with amino acids, reducing gut absorption and increasing fecal transit rates. Cereal protein digestibility, for instance, depends on whether one eats only the endosperm, source of most amino acids, or includes the fibrous pericarp.

Taking digestibility into account, we can calculate the utility of plant proteins relative to a benchmark—egg white. Soybeans, with a score of 99, are more bioavailable than beef, while peas and beans have scores between 60 and 75, nuts fall to the thirties, and most cereals are in the 20 to 30 range. Culturally important foods such as plantain (Musa spp), yams, and cassava (manioc) begin with modest total protein, contain high fiber, and thus end up with very low protein scores.

Mixing Plant Proteins

With ingenuity, such shortfalls can be met by mixing amino acids from different plant foods. This has become known as protein complementarity. The best-understood example comes from Latin America's menu. Squashes supply lysine, which corn lacks, and beans provide methionine. In sufficient quantity, the triad of squash, corn, and beans provides ample protein.

Moreover, as Solomon Katz has demonstrated, Latin American cultures typically soak their tortilla corn in lime. Besides enhancing availability of the B vitamin niacin, the alkali reaction reduces the amino acid availability of all amino acids except lysine, already deficient in corn. Paradoxically, this is useful. While total protein is reduced, amino acids are "leveled," making a higher percentage of protein (approximately 8 percent by weight) complete. Finally, one of the amino acids reduced is leucine, an overabundance of which relative to lysine may be involved in the etiology of the disease pellagra. Thus lime soaking is a deceptively simple cultural practice that solves complex nutritional dilemmas.

Language itself attests to protein complementarity's importance in East Asia. The Chinese fan literally translates as 'grain' (usually rice), but also signifies 'food'. Ts'ai literally translates as 'edible leaf and stem vegetable', but also signifies 'what goes over rice to complete the meal.' One is reminded of the phrase from the Lord's Prayer, "our daily bread," that signifies all necessary food. Complementarity here rests on the soybean. Rice or wheat is mixed with dozens of soybean products, including curds (tofu), soy milk, boiled or fermented soybeans, and soy sauce. Soy sauce, in turn, typically is made with wheat flour, whose methionine enhances an already strong amino acid profile. Digestion of soy protein, moreover, is enhanced by traditional fermentation, accomplished in water containing dissolved calcium or magnesium. These inactivate the antagonists to trypsin, a digestive enzyme, found in soybeans. Finally, sesame oil, a rich source of methionine, has been a preferred cooking oil in China since the Sung dynasty. Not surprisingly, even in times of famine, straightforward protein malnutrition has been rare in East Asia.

As one proceeds inland west or north, pulses and legumes such as the red bean (Vigna angularis ), broad bean (Vica fava ), the mung bean (Vigna mungo ), the peanut, and the common pea (Pisum sativum ) are mixed with cereals to aid complementarity. Fermented milk products such as yogurt become significant in Central Asia.

In the Middle East, South Asia, and Asia Minor, complementarity typically involves green vegetables, cheese, and lentils or peas with wheat. Wheat and pulses nicely complement each other's lysine and methionine ratios; amino acid scores attain at least 85 percent of egg white. Protein malnutrition is again rare.

Comparatively protein-poor Oceanic foods such as breadfruit (Artocarpus communis ), bananas (Musa ), taro (Colocasia esculenta ), and yams tend to be served together, often with green vegetables. Fish or pork is typically added in small amounts. Except in mountainous areas of New Guinea, protein malnutrition is rare.

Both Southeast Asia and coastal West Africa illustrate a dilemma stemming from substitution of meat for plants. The dominant carbohydrate, rice, typically is topped by small amounts of fish, as a fermented paste (Southeast Asia), or dried fragments (West Africa); peanuts and green vegetables may also contribute to the sauce. With much rice and little fish, protein can become quite diluted. In northern Thailand, for instance, small children may not eat enough rice with fish paste to meet caloric needs. However balanced, the protein is broken down for energy. In Senegal, among the Wolof, dried fish tends to fragment during cooking and be diluted throughout the rice gruel. Adults consider this less tasty, and following their example, children may resist eating sufficient rice.

Origins of Complementarity

The ingenuity and specificity of protein complementarity demands explanation. How did so many cultures independently discover protein mixing? Simple trial and error is unlikely, since protein undernutrition has ambiguous symptoms; even obvious malnutrition (kwashiorkor) has been variously attributed to supernatural intervention, inappropriate parental morals, or a failure of the child's will. On the other hand, dietary practices show good congruence with underlying biochemical advantages in growth and resistance to infection. Folk knowledge of these connections, however, remains poorly documented.

The origins of protein complementarity may be ecological. Optimization strategies must lead to a cost-benefit honing of the total menu available within any ecosystem. Nutritional benefits will be balanced against procurement costs. The !Kung San, for instance, select plant foods on the basis of abundance, ease of acquisition, and nutrient value. Elaborate taboos are reserved for meat, which costs more calories to obtain than it yields. Ultimately, any population whose long-term optimizations transgress nutritional requirements will suffer demographic collapse. We may, in other words, see close linkages between food choice and protein scores because cultures that ignored such an association are no longer extant.

Practices that enhance complementarity may operate through individual health-seeking behavior. Pairing introduced foods with familiar, liked components, such as mixing cereals with pulses, has been shown to facilitate acceptance in humans, but not in animals. The !Kung San typically equate nutrient value with tastiness, including texture, flavor, and smell. Such findings locate protein complementarity within what Pierre Bordieu terms "cultural habitas": a habitual, unremarked, individual practice that reflects group consensus about "how things are done." This habitas serves to internalize biologically adaptive food choices.

Over evolutionary time spans, finally, some behaviors may have become encephalized, or incorporated into neural functioning. Monoamine neurotransmitters such as serotonin, dopamine, and norepinephrine are synthesized from two amino acids, tryptophan and tyrosine. Neuronal levels of these amino acids in plant-eating monkeys, for instance, vary systematically with intake when dietary protein is scant, but are insensitive to intakes above 10 percent. Reductions in neuronal level limit neurotransmitter synthesis in the hypothalamus, which in turn regulates appetite. The brain, then, may receive constant information about the amino acid balance of our diet, and mediate appetite to achieve optimal rates of utilization when protein is scarce. Individuals would not be conscious of such nutrient-seeking appetites. Rather, diets that yielded appropriate mixes would become associated with elevated affect; they would taste richer, or better satisfy cravings, thus conditioning individuals to seek those foods. Protein complementarity consequently appears to follow multiple adaptive paths, individual and social, using both biology and culture.

See also Legumes; Maize; Nuts; Proteins and Amino Acids; Rice; Soy; Squash and Gourds; Wheat .

BIBLIOGRAPHY

Chang, K. C. Food in Chinese Culture: Anthropological and Historical Perspectives. New Haven: Yale University Press, 1971. Definitive source for historical development of Chinese diet.

Davidson, Stanley, R. Brock Passmore, and J. F. Truswell. Human Nutrition and Dietetics. 8th ed. Edinburgh, London, and New York: Churchill Livingston, 1986. Unusual for its detailed coverage of specific food groups.

Fernstrom, John, and Madelyn Fernstrom. "Monoamines and Protein Intake: Are Control Mechanisms Designed to Monitor a Threshold or a Set Point?" Nutrition Reviews 59, no. 8, part 2 (2001): 60–65. Review of recent psychobiology of food choice.

Guthrie, Helen. Human Nutrition. St. Louis: Mosby, 1995. Solid overview of human nutritional needs.

Harris, Marvin. Good to Eat: Riddles of Food and Culture. New York: Simon and Schuster, 1985. Chief advocate of ecologically adaptive food choice.

Katz, Solomon, Mary Heidiger, and L. Valleroy. "The Anthropological and Nutritional Significance of Traditional Maize Processing in the New World." In Biosocial Interrelations in Population Adaptation, edited by Elizabeth Watts, Francis Johnson, and Gabriel Lasker, pp. 195–234. The Hague: Mouton, 1975. Seminal exploration of a specific adaptive practice.

Katz, Solomon, and Sara Schall. "Fava Bean Consumption and Biocultural Evolution." Medical Anthropology 3 (1979): 459–476. First link of specific food choice to human genetic variation.

Lee, Richard Borshay. The !Kung San: Men, Women, and Work in a Foraging Culture. Cambridge: Cambridge UniversityPress, 1979. Includes detailed investigation of diet among hunter-gatherers.

Rozin, Paul. "Psychobiological Perspectives on Food Preferences and Avoidances." In Food and Evolution: Toward a Theory of Human Food Habits, edited by Marvin Harris and Eric Ross. Philadelphia: Temple University Press, 1987. Useful introduction to the psychology of food choice.

Stephen M. Bailey