The metabolic process by which an apple, a tomato or a portion of cabbage is made into hair, or muscle or nerve, or some of the cells of the eyes, or into a hormone of some of the ductless glands is beyond our present comprehension, although a few steps in this process are supposed to be known. What we do know is that if the foods we eat are to be made into tissues and secretions, they must contain adequate amounts and due proportions of all the elements needed in the production of these things. Just as we cannot make concrete with cement alone or with sand alone, but must, if our concrete is to be good, have due proportions of both, so, if we are to have good structures in our bodies, we must have adequate amounts and proper proportions of all the food elements that go into the construction of these structures.

To make the best use of any food element, such, for example, as the proteins, certain other food substances and possibly a variety of them must be available in the diet in definite proportions. No food element is of itself of more importance in the body than another. It is only by the combination of all the necessary ones that the whole may be obtained.

Liebig laid down a "law of the minimum" in these words: "The development of living beings is regulated by the supply of whichever element is least bountifully provided."

The law of the minimum implies that the nutritive value of any food-mixture, however abundant most of the food elements therein may be, is limited by the minimum quantity of any essential element it may contain; unless, as it happens in certain food constituents, the lacking factor may be synthesized from some of the more abundant food elements, fat from sugar, for example.

Calcium cannot be utilized in producing enamel in the absence of phosphorus. If phosphorus is present in inadequate amounts, the enamel will be of poor quality. If iron is lacking, there will also be poor enamel. If any one of the vitamins is lacking poor enamel is the result.

The united "action" of minerals, proteins, vitamins, etc., in the production of tissues and fluids in the body is called "synergistic action." Since they do not act, but are used, it may be more correct to say that the body makes correlated use of them. At any rate, a knowledge of the "synergistic action" of the food elements and vitamins is of vast importance and helps to make clear to us the Law of the Minimum.

Tooth enamel is made up largely of calcium and phosphorus with small quantities of iron. But the fusion of these substances into enamel requires the presence of vitamines A, C and D. At least half a dozen elements and factors are essential to the production of this tissue. In the absence of either of these factors the enamel of the teeth cannot be made. It is folly to feed much calcium in an effort to produce good teeth and to ignore the other essential elements of the teeth.

Many different tissues are being built and repaired in the body at all times. These tissues are constantly engaged in a wide variety of activities. This means that there is a continuous use of vitamins, minerals, amino acids, sugars and fatty acids in the body. Consuming an abundance of a single vitamin or of a single mineral is not sufficient to meet the needs of the many "synergistic actions" that are in constant process in the body. All of the vitamins and all of the minerals are required. If all the nutritive elements are present in adequate quantities and in due proportions, all the synergisms of all the minerals, vitamins, amino acids, sugars, etc., are at work piling up their benefits.

It is well to keep in mind that these "synergistic actions" are far more complex and broader than our description of them would indicate. The many functions of the body are also synergistic. The function of one part of the body is dependent upon the functions of all other parts. Feeding vitamin B in certain forms of nervous trouble with the aim of remedying the nervous impairment, and ignoring the correlated functions of the body that support nerve function, is doomed to failure. All of the "synergistic chemical actions" in all parts of the body must be adequately provided for before the actions of any part can be ideal. This is the thing that makes specific diets so unsatisfactory.

The work of potassium in promoting the formation of proteins, fats and glycogen, is not essentially unlike the work ascribed to vitamins. Zinc is thought to be connected with the action of the vitamins, which it seems to be able to replace to some extent, at least in the animal organism. Lithium is also thought to influence metabolism in a way not unlike that of the vitamins. The same is true of copper, nickel and of arsenic. It should be borne in mind that little is known about the offices of zinc, lithium, copper, nickel and arsenic in the body and that experimenters, carrying out their experiments, have wholly neglected these elements.

In plant life and growth the Law of the Minimum has long been known. If perfect growth is to be procured in plants, these must find a certain minimum of each of several elements in the soil in which they are grown. If only half the needed amount of potassium, for example, is present, then, regardless of how abundant all the other soil elements may be, their normal utilization is reduced one-half. The rate of growth of the plant and its ultimate development are correspondingly depressed.

Prof. Osterhout showed that seeds placed in distilled water grow better than when placed in water containing but one salt, and that each salt exerts a specific toxic effect according to its chemical nature. He found that one salt "counteracts" the effects of another.

An excess of nitrogen in the soil retards the formation of grain, roots and tubers and gives rise to sickly plants. Nitrogen excess in the soil of rose beds leads to the production of soft, sappy tissues, a luxuriant growth of leaves, and roses with little or no perfume.

Plant processes, from germination to ripening of their seed, are a drama of physiological minerals and gases. Without a sufficiency of lime or potash in the soil, acids, sugars and starches cannot be formed; without iron no chlorophyll and no albumen can be formed. Without silica no fibre or plant skeleton, etc. Each mineral plays a definite role in the growth of healthy plants.

The actual amount of potassium requisite for plant growth is very small compared with the needed amount of carbon or nitrogen. But if this relatively small quantity is not available the utilization of other constituents in tissue growth or repair is always deficient.

If one essential food element, which normally should compose one per cent of the food eaten, is present in only half this amount, then, the body will only be able to utilize the other elements, in tissue building, in the same reduced quantity.

The organic whole--the unit--is official to animal nutrition. Not the sum total of the minerals consumed, but their relative proportions, determines the nutritive value of any given dietary.