Protein

With the discovery of nitrogen as an element toward the end of the eighteenth century, and the development in France of methods for analyzing the amount of nitrogen in different materials, came the discovery that both animal tissues and the "animal-like" fractions in vegetables contained nitrogen, whereas starch, sugar, fats, and vegetable fibers contained only carbon, hydrogen, and oxygen.

After more work, mostly in Germany, it was concluded that these nitrogenous compounds were really all of one class, the proteins, and that they could be converted one to another by animals, but that as a class they had to come from the vegetable kingdom. Because they were considered the true nutrients, the value of a food or animal feedstuff could be judged by its nitrogen content. It was also thought that the force exerted in the contraction of muscles came from the "explosion" and destruction of some of their protein, with the subsequent breakdown of the residue to urea, a simple nitrogen compound that the kidneys diverted into the urine. It was therefore particularly important for those engaged in hard physical work to have a high-protein diet, and on a dry matter basis, the foods highest in protein were meat, cheese, and eggs.

As a corollary to this idea, carbohydrates (i.e., starch and sugars) and fats were considered "respiratory" foods. These were combusted in the body (i.e., they reacted with oxygen) and this produced heat; this reaction, by "mopping up" oxygen in the tissues, was thought to protect the valuable protein from oxidation.

Because people in affluent countries tended to eat more animal foods, and therefore more protein, it was concluded that the extra protein gave them extra energy, which, in turn, made them prosperous. The diet chosen by healthy, hard workers in the United States was found to contain, typically, some 120 grams of protein per day, and this was adopted generally as a recommended standard at the end of the nineteenth century.

Once set, however, the whole pyramid of ideas collapsed with the discovery that physical labor did not cause a significant breakdown of muscle protein. Fats and carbohydrates were the main energy sources for all forms of activity. Tests showed that 60 grams of protein per day could support health and vigor in an active man. However, in the early years of the twentieth century, it was also discovered that all proteins were not nutritionally equal. Whereas protein molecules themselves were very large, typically containing more than 10,000 atoms, boiling them in a solution of strong acid resulted in the production of 20 or so fairly simple crystalline compounds, all of the same chemical class - the amino acids. Later work showed that proteins had to be broken down to these constituent units in the digestive tract before absorption could take place. The body proteins were then built up anew from these units, or building blocks, with each protein having a fixed composition.

The practical significance of this is that humans, and other members of the animal kingdom, have the ability to convert some amino acids to others, but about half of the amino acids must be provided by the diet. These are called the essential amino acids. In general, the mix of individual proteins contained in foods, even those of plants, includes all the amino acids, but the proportions differ. It was demonstrated in 1915 that growth stopped in young rats when the only protein they received was zein, a particular protein fraction from maize containing none of the amino acids lysine and tryptophan. Ordinary growth was restored by supplementing the diet with both of these chemicals, proving that they were dietary essentials. In later experiments with bean proteins, methionine (present in beans at a low level in relation to the requirements of animals) was determined to be the limiting factor for growth.

For productive farm animals, with their high rates of growth and reproduction, it has proved economic in some circumstances to supplement commercial diets with synthetic lysine or methionine. However, in practical human diets based on mixtures of foods, there is little evidence of a single amino acid being limiting. An exception are the amino acid-deficient diets of infants given some all-vegetable-protein formulas as substitutes for milk.

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