The intake of food must always be sufficient to supply the metabolic needs of the body. Since different foods contain different proportions of proteins, carbohydrates, fats, an appropriate balance must be maintained among these different types of food so that all segments can be supplied with the requisite materials.

The energy liberated from each gram of carbohydrate as it is oxidized to carbon dioxide and water is 4:1 calories (cal), and that liberated from fat is 9.3 calories. The energy liberated from metabolism of the average protein of the diet as each gram is oxidized to carbon dioxide, water and urea is 4.35 calories. Also these different substances vary in the average percentages that are absorbed from the gastrointestinal tract approximately 98% of carbohydrate, 95% of the fat, 92% of the protein.

Average American receives approximately 15% of their energy from protein, 40% from fat and 45% from carbohydrates. In most of the other parts of the world the quantity of energy derived from carbohydrates far exceeds that derived from both proteins and fats.

High proportions of fats and proteins are present in meat products and high proportions of carbohydrates are seen in vegetables and grain products. Fat is deceptive in the diet for it usually exists as 100% fat whereas both proteins and carbohydrates are mixed in watery media so that each of these normally represents less than 25% of the weight as food. Therefore, the fat of one pat of butter mixed with an entire helping of potatoes often contains as much energy as all the potato itself.

20 to 30 gms of the body proteins are degraded and used for producing other body chemicals daily. Therefore, all cells must continue to form new proteins to take the place of those that are being destroyed, and a supply of protein is needed in the diet for this purpose. An average person can maintain normal stores of protein provided the daily intake is above 30 to 55 gms. In general, proteins derived from animal foodstuffs are more nearly complete than are proteins derived from the vegetable and grain sources. Animal proteins have all the amino acids whereas many of the vegetable or grain proteins are only partial proteins.

When the diet contains an abundance of carbohydrates and fats almost all the body's energy is derived from these two substances and very little is derived from the proteins. Therefore, both carbohydrates and fats are said to be protein sparers. On the other hand, in starvation after the carbohydrate and fats have been depleted the body's protein stores are then consumed rapidly for energy, sometimes at rates approaching several hundred grams per day rather than at the normal daily rate of 30 to 55 gms.

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