The Fall of Vitalism. Nitrogen and the Diet

However unsettling Darwin's theory of evolution by natural selection might have been to many of mankind's settled beliefs, it did, viewed properly, enhance the marvel of life. From very simple beginnings, life had striven endlessly, under the stress of environment, to achieve ever greater complexity and efficiency. There was nothing to compare with that in the changeless world of the in- animate. Mountains might rise but there had been other mountains eons before; life forms, on the other hand, were ever new, ever different.

Darwinian theory might therefore be interpreted at first blush as favorable to vitalism, to the great barrier thrown up in men's minds between life and nonlife. And indeed vitalism reached a new height of popularity in the latter half of the nineteenth century.

The great danger to nineteenth-century vitalism lay in the advances made by organic chemists (see page 53). Against this, however, the vitalists raised the protein molecule as a shield and down almost to the very end of the century, that shield held firm. The biochemistry of the nineteenth century was very largely concerned with that protein molecule.

The importance of protein to life was first made completely clear by the French physiologist, Francois Magendie (1783-1855). The economic dislocations brought on by the Napoleonic wars had brought a period of food scarcity, and the condition of the poor was worse than usual. Governments were beginning to feel a responsibility for the condition of the people, and a commission was appointed, with Magendie at its head, to investigate whether a nourishing food could be made out of some-thing as cheap and available as gelatin.

Magendie began, in 1816, by feeding dogs on a protein-free diet, one that contained only sugar, olive oil, and water. The animals starved to death. Calories alone were not sufficient; protein was a necessary component of the diet. Furthermore, not all proteins were equally useful. Unfortunately, where gelatin was the only protein in the diet, the dogs still died. Thus was founded the modem science of nutrition, the study of diet and its connection with life and health.

Proteins differed from the carbohydrates and lipids in that the former contained nitrogen and the latter did not. For that reason, interest focused on nitrogen as a necessary component of living organisms. The French chemist, Jean Baptiste Boussingault (1802-87), set out in the 1840s to study the nitrogen requirements of plants. He found that some plants, such as the legumes (peas, beans, etc.), could grow readily in nitrogen-free soil while being watered with nitrogen-free water. Not only did they grow, but their nitrogen content increased steadily. The only conclusion he could come to was that these plants gained their nitrogen from the air. (We now know that it is not the plants themselves that do this, but certain strains of "nitrogen-fixing bacteria" growing in root nodules that do so.)

Boussingault, however, went on to show that animals could obtain no nitrogen from the air, but only from food. He sharpened Magendie's rather qualitative studies by actually analyzing the nitrogen content of some foods and comparing the rate of growth with the nitrogen content. There was a direct relationship, provided a single food was used as nitrogen source. However, some foods were more efficient than others at bringing about growth with a given nitrogen content. The conclusion could only be that some proteins were more useful, nutritionally, to the body than others were. The reason for this remained obscure till the end of the century, but, by 1844, Boussingault could, on purely empirical grounds, list the relative usefulness of various foods as sources of protein.

This was carried further by the German chemist, Justus von Liebig (1805-73), who over the following decade prepared detailed lists of this sort. Liebig leaned strongly toward mechanism, and he applied this viewpoint to the problems of agriculture. He believed that the reason for loss of soil fertility after years of farming lay in the gradual consumption of certain minerals in the soil which were necessary for plant growth. Plant tissues contained small quantities of sodium, potassium, calcium, and phosphorus, and these had to come from soluble compounds in the soil, which the plant could absorb. It had been customary from time immemorial to bolster soil fertility by the addition of animal refuse, but to Liebig this did not signify the addition of something "vital" to the soil, but merely that of the mineral content of the wastes to replenish that which had been taken out of the soil. Why not add the minerals themselves, pure, clean, and odor-less, and do away with the necessity of dealing with wastes?

He was the first to experiment with chemical fertilizers. At first, his products were failures because he relied too heavily on Boussingault's finding that some plants obtain nitrogen from the air. When Liebig realized that most plants, after all, obtain nitrogen from soluble nitrogen compounds ("nitrates") in the soil, he added these to his mixture and produced useful fertilizers. Between them, Boussingault and Liebig founded agricultural chemistry.