Evolution of the Concept

Differences in the physical properties of foods and in their content of medicinal and toxic substances were considered to be important in the prevention and treatment of diseases in ancient times, but knowledge that foods contain many substances essential for life has been acquired only during the past two centuries ( 7, 8, 9, 10 and 11). Although the Hippocratic physicians in Greece practiced a form of dietetic medicine some 2400 years ago, they had no understanding of the chemical nature of foods and believed that foods contained only a single nutritional principle—aliment ( 9). This belief persisted until the 19th century, but a few earlier observations presaged the concept of nutritional essentiality (11). During the 1670s, Sydenham, a British physician, observed that a tonic of iron filings in wine improved the condition of chlorotic (anemic) patients, and in the 1740s, Lind, a British naval surgeon, found that consumption of citrus fruits, but not typical shipboard foods and medicines, cured scurvy in sailors. McCollum (9) cites Syednham's report as the first evidence of essentiality of a specific nutrient, but it was not recognized as such at the time.

Between 1770 and 1794, through experiments on the nature of respiration in guinea pigs and human subjects, Lavoisier and Laplace discovered that oxidation of carbon compounds in tissues was the source of energy for bodily functions (7). For the first time, a specific function of foods had been identified in chemical terms. Lavoisier and his colleagues also established the basic concepts of organic chemistry, thus opening the way for understanding the chemical nature of foods.

Scientists interested in animal production then began to examine food components as nutrients. The first evidence of nutritional essentiality of an organic food component—protein—was the observation of Magendie in 1816 that dogs fed only carbohydrate or fat lost considerable body protein and died within a few weeks, whereas dogs fed on foods containing protein remained healthy. A few years later, in 1827, Prout, a physician and scientist in London, proposed that the nutrition of higher animals could be explained by their need for the three major constituents of foods—proteins, carbohydrates, and fats—and the changes these undergo in the body. This explanation, which was widely accepted, sounded the death knell of the single aliment hypothesis of the Hippocratic physicians.

During the next two decades, knowledge of the needs of animals for several mineral elements advanced. Chossat found that a calcium supplement prevented the mineral loss observed in birds fed a diet of wheat; Boussingalt, using the balance technique, showed that pigs required calcium and phosphorus for skeletal development and also noted that cattle deteriorated when deprived of salt for a prolonged period. Liebig, a leading German chemist with a major interest in agricultural problems, found that sodium was the major cation of blood and potassium of tissues. Thus, by 1850, at least six mineral elements (Ca, P, Na, K, Cl, and Fe) had been established as essential for higher animals (11).

During this time also, Liebig postulated that energy-yielding substances (carbohydrates, fats) and proteins together with a few minerals were the principles of a nutritionally adequate diet. Liebig's hypothesis, however, was questioned by Pereira (1847), who noted that diets restricted to a small number of foods were associated with development of diseases such as scurvy, and by Dumas, who observed that feeding children artificial milk containing the known dietary constituents had failed to prevent deterioration of their health during the siege of Paris (1870-71). Still, owing to his great prestige, Leibig's concept continued to dominate thinking throughout the 19th century (9).

In 1881, Lunin in Dorpat, and 10 years later, Socin in Basel, found that mice fed on diets composed of purified proteins, fats, carbohydrates, and a mineral mixture survived less than 32 days. Mice that received milk or egg yolk in addition remained healthy throughout the 60-day experiments. Lunin and Socin concluded that these foods must contain small amounts of unknown substances essential for life. Their observations, nonetheless, did not stimulate a vigorous search for essential nutrients in foods, probably because of the skepticism of prominent scientists. Von Bunge, in whose laboratories Lunin and Socin worked, attributed inadequacies of purified diets to mineral imbalances or failure to supply minerals as organic complexes. Voit, a colleague of Liebig, assumed that purified diets would be adequate if they could be made palatable.

During the early 1880s, Takaki, director general of the Japanese Navy, noted that about 30% of Japanese sailors developed beriberi, although this disease was not prevalent among British sailors, whose rations were higher in protein. When evaporated milk and meat were included in the rations of the Japanese Navy, the incidence of beriberi declined remarkably. He concluded correctly that beriberi was a dietary deficiency disease, but incorrectly that it was caused by an inadequate intake of protein. In the 1890s, Eijkman, an army physician in the Dutch East Indies who was concerned with the high incidence of beriberi in the prisons in Java (Indonesia), where polished rice was a staple, discovered that chickens fed on a military hospital diet consisting mainly of polished rice developed a neurologic disease resembling beriberi, whereas those fed rice with the pericarp intact remained healthy. He proposed that accumulation of starch in the intestine favored formation of a substance that acted as a nerve poison and that rice hulls contained an antidote.

Grijns extended Eijkman's investigations and showed through feeding trials with chickens that the protective substance in rice hulls could be extracted with water. In 1901, he concluded that beriberi was caused by the absence from polished rice of an essential nutrient present mainly in the hulls. He provided, for the first time, a clear concept of a dietary deficiency disease, but the broad implications of his discovery were not appreciated. The authors of a British report ( 8) noted that facts brought to light by research done between 1880 and 1901 had "little or no effect on orthodox views and teaching concerning human nutrition." Another 15 years of research was required before the concept that foods contained a variety of unidentified essential nutrients gained widespread acceptance. Establishing the Concept

The first evidence of essentiality of a specific organic molecule was the discovery by Willcock and Hopkins ( 12) in 1906 that a supplement of the amino acid tryptophan prolonged the survival of mice fed on a diet in which the protein source was the tryptophan-deficient protein zein. The following year, Holst and Frölich in Norway reported that guinea pigs fed on dry diets with no fresh vegetables developed a disease resembling scurvy, which was prevented by feeding them fresh vegetables or citrus juices. This was further evidence that foods contained unidentified substances that protected against specific diseases ( 9, 10).

Also, in 1907, Hart and associates at Wisconsin initiated a direct test of the validity of Liebig's hypothesis that the nutritive value of foods and feeds could be predicted from measurements of their gross composition by chemical analysis. They fed heifers on different rations designed to contain essentially the same amounts of major nutrients and minerals, each composed of a single plant source—wheat, oats, or corn—using all parts of the plant. The study lasted 3 years and included two reproductive periods. Animals that ate the wheat plant ration failed to thrive and did not produce viable calves; those fed the corn plant ration grew well and reproduced successfully. The results of this study, published in 1911, demonstrated that Liebig's hypothesis was untenable and stimulated intensive investigation in the United States of nutritional defects in diets (13).

In experiments undertaken between 1909 and 1913 to compare the nutritional value of proteins, Osborne and Mendel at Yale had initially been unable to obtain satisfactory rates of growth of rats fed on purified diets. They solved this problem by including a protein-free milk preparation in the diets. They then demonstrated that proteins from different sources differed in nutritive value and discovered that lysine, sulfur-containing amino acids, and histidine were essential for the rat ( 14).

During this time, Hopkins also observed that including small amounts of protein-free extracts of milk in nutritionally inadequate, purified diets converted them into diets that supported growth (10). In 1912 he commented: "It is possible that what is absent from artificial diets...is of the nature of an organic complex...which the animal body cannot synthesize." In 1912 also, in a review of the literature on beriberi, scurvy, and pellagra, Funk in London, who had been trying to purify the antiberiberi principle from rice polishings, proposed that these diseases were caused by a lack in the diet of "special substances which are in the nature of organic bases, which we will call vitamines" (9).

In studies of the nutritional inadequacies of purified diets McCollum and Davis, at Wisconsin, noted that when part of the carbohydrate was supplied as unpurified lactose, growth of rats was satisfactory if the fat was supplied as butterfat. When butterfat was replaced by lard or olive oil, growth failure occurred. In 1913 they concluded that butterfat contained an unidentified substance essential for growth. Meanwhile, Osborne and Mendel observed that if they purified the protein-free milk included in their diets, growth failure of rats again occurred, but if they substituted milk fat for the lard in their diets, growth was restored. They also concluded in 1913 that milk fat contained an unidentified substance essential for life.

McCollum and Davis extracted the active substance from butterfat and transferred it to olive oil, which then promoted growth. They called this substance "fat-soluble A." They then tested their active extracts in a polished rice diet of the type used by Eijkman and Grijns and found that even though the diet contained fat-soluble A, it failed to support growth. The problem was remedied when they added water extracts of wheat germ or boiled eggs. They concluded that animals consuming purified diets required two unidentified factors—fat-soluble A and water-soluble B (presumably Grijns' antiberiberi factor) ( 9, 10). Thus, by 1915, six minerals, four amino acids, and three vitamins—A, B, and the antiscorbutic factor—had been identified as essential nutrients.

The concept that foods contained several organic substances that were essential for growth, health, and survival was by then generally accepted. By 1918, the importance of consuming a wide variety of foods to ensure that diets provided adequate quantities of these substances was being emphasized in health programs for the public in Great Britain and the United States, and by the League of Nations.

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