The Importance of Proteins. Because they are found universally in the cells of plant and animal life it seems appropriate that we should first consider proteins as we begin the study of food substances. The name is formed by modifying the Greek word, PROTOS, meaning first. Proteins then may be thought of as first or fundamental substances in the scheme of life as we know it. Their chemical composition includes chiefly the elements of carbon, hydrogen, oxygen, nitrogen, and sulphur. Other elements, such as phosphorus, may be present in small amounts. The proportion of proteins present in living materials varies greatly in total amount, but in some measure they are found in every cell.
With activity and function of the cells there are changes which require that the tissues must be supplied with sub-stances that will replace this wear and tear, or build new cells when needed. Proteins are made up of amino acids, which become available in the digestive tract of man as protein-containing foods are digested. These are absorbed into the blood stream, and built into the particular proteins needed by our bodies.
Sources and Quality of Proteins. Proteins are obtained from various sources, the most important of which are milk, cheese, eggs, meat, fish, fowl, and nuts. Cereals, vegetables, and fruits contain variable amounts.
The quality of protein is determined by the kinds and proportions of amino acids present. They are classified as complete or incomplete, according to whether or not all of the essential amino acids are present. For example, the proteins of milk and eggs are complete, while gelatin and one of the proteins of corn are incomplete. Experimental animals will not grow on diets containing those of the in-complete type only. All foods contain more than one protein, so even those containing an incomplete protein may also contain others that are complete.
The proportion in which these amino acids exist is important. One food may be of higher nutritive value than another, because it contains these elements in more nearly the proportion that the body needs. Milk and eggs contain proteins of the high nutritive type. According to McCollum, as high as 65 per cent of the proteins found in these foods is convertible into body tissue during growth. Meat protein is lower. He further states that the proteins of grains, peas, beans, and nuts are without exception of rather low quality. Those of wheat, corn, and rice can be utilized only to the extent of 23 to 25 per cent, while those of peas and beans are available in still lower degree. Proteins of tubers and roots are about the same in quality as those of grains. The proteins of leaves of plants are higher, being comparable to those of animal foods. He states that the supplementing of the proteins of milk, eggs, meat, and leafy vegetables improves the utilization of proteins of lower nutritive value, and concludes that a mixed diet is preferable to a strictly vegetarian one.
When Protein is Needed. Protein is the chief constituent of muscle tissue. The body uses the amino acids found in animal and vegetable foods in any way in which it needs them. The protein content of the diet is most important when there is .new muscle tissue to be built. This occurs during pregnancy, childhood, athletic training, and convalescence from a wasting disease. The protein requirement of the growing child is two or three times greater than that of the adult, per unit of weight. Often parents fail to realize this, and serve protein food in proportion to size rather than according to needs. Active children often crave proteins, and in a general way this craving is an excellent guide to their needs for this particular substance.
In the normal adult during work or rest, a certain amount of protein is broken down each day and must be replaced. Contrary to popular belief, however, normal activity or heavy physical work does not destroy muscle tissue, so that the laborer does not require a larger amount of protein than the man of sedentary habits. It is used only during starvation, or when extra work is being done with an inadequate supply of food. After the protein that is needed for tissue building has been used, any extra amount is broken down and used as fuel, the nitrogenous part being excreted by the kidneys. It is common to find men using as much as two hundred to three hundred grams of protein per day. This intake is in excess of the body needs and as the body does not store protein, the surplus is used as fuel.
Amount Needed. There is no actual agreement on the amount of protein needed in the diet, but it is agreed that the biological or nutritive value must be taken into consideration. Sherman states that there is no danger of protein deficiency in the American diet. As a result of one hundred experiments of several days duration on healthy men and women, the amount needed was found to be 44.4 grams per day for a person weighing 154 pounds. The amount actually consumed in 200 studies was 106 grams per day. The consensus of opinion seems to be that while excessive protein consumption is not advisable, a fairly liberal mixture of proteins is conducive to optimum health. McCollum states that 3 to 4 ounces of protein obtained from a mixture of vegetable and animal foods is a safe amount. It has been estimated that the average adult requires from two-thirds to one and one-half grams per kilogram of average body weight. Children require from 2 to 4 grams, according to age.
Effects of Excess or Insufficient Amounts. There is considerable difference of opinion about the harmful effects of an excessive amount of protein. We believe that kidney and liver damage may result after continued use of abnormal quantities. The liver is overworked in breaking down the excess amino acids, whereby nitrogen is re-leased into the blood stream in the form of urea or uric acid. In turn the kidneys may be damaged and likely injury come to the small blood vessels. At the same time too little protein in the diet may result in stunted growth, lack of endurance, decreased efficiency, and possibly an anemic condition. In extreme cases of protein deficiency, a type of edema may be produced, such as was seen in several European countries during the World War, where the meat supply, including milch cows, was taken for the army. Peasants were forced to subsist on turnips and sawdust breadpoor sources of protein, and of the necessary minerals. Experimentally, edema can be produced in animals by similar rations. Studies by physicians at that time of patients with edema lead us to believe that the shortage of protein made the condition possible, but that an abnormal mineral balance actually produced the water retention in the tissues.