What are nutrients?
Nutrients can be categorized as energy vs. non-energy yielding. An energy nutrient is one which upon burning in the laboratory or in our bodies yields heat (measured as calories).

The energy nutrients are carbohydrates (4 calories per gram), fats or lipids (9 calories per gram) and proteins (4 calories per gram). Other nutrients are involved in helping the body use energy, but none of them actually yields heat or can be converted into a stored form of energy such as fat or glycogen, a carbohydrate. Alcohol, not a nutrient, yields seven calories per gram. These four compounds are our only sources of calories.

Why the energy issue is important to a protein discussion.
The energy issue is important in determining human protein requirements because the first demand of the body is for energy. If calorie/energy requirements are not met, protein will not be spared (or available) to do its jobs of maintenance and growth, thus the term “protein-calorie malnutrition.” However, assuming adequate energy/calories are provided, and the appropriate other nutrients are available to facilitate energy release and other biological mechanisms, then protein can be reserved for its unique functions. Although the daily allowance is expressed as protein, the biological requirement is for amino acids.

What is protein?
Proteins are composed of building blocks called amino acids all of which contain nitrogen. Proteins and other nitrogen containing compounds are degraded and rebuilt continuously in the body. Several times more protein is turned over each day than is typically consumed, indicating that the reutilization of amino acids is a major feature of the economy of protein metabolism. The process of recycling is not completely efficient and some amino acids are lost. Metabolic products of amino acids—urea, creatinine, uric acid and other nitrogen-containing compounds are excreted in urine. Nitrogen also is lost in feces, sweat and other body secretions and in sloughed skin, hair and nails. All of these losses must be replaced by a continuous supply of dietary amino acids which are usually provided through dietary protein.

The importance of the amino acids.
There are about 20 amino acids. However, nine amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—cannot be synthesized by mammals and therefore must be consumed in the diet. They are called essential or indispensable amino acids. The rest of the amino acids can be made by mammals if nitrogen and other needed nutrients are available. Thus the bottom line is that humans must eat protein for two reasons: first, because food protein is the only source of the indispensable amino acids and secondly, because food protein is the only practical source of nitrogen with which to build the dispensable amino acids.

So why are some sources of protein considered better than other sources?
All plant and animal proteins have the same 20 or so amino acids. However, the proportion of the indispensable amino acids in a food compared to the indispensable amino acid requirements of the 3-5 year old child is used by the FAO/WHO and the FDA to determine the quality of a protein source for human nutrition. This method is called the Protein Digestibility Corrected Amino Acid Score (PDCAAS). The best score for protein quality using PDCAAS is 1.0. A score of 1.0 means the protein contains 100% of the amino acids in the correct proportions per gram of protein for the 3-5 year old child. If the requirements for amino acids (measured per gram of protein) for growing children are met, then the requirements for adults will be exceeded. Of course, the absolute amount of protein needed will be higher for an adult. The FDA requires food manufacturers who make protein claims on their labels to take into account protein quality when calculating the % Daily Value. Unfortunately, most food manufacturers do not make protein claims and do not voluntarily provide a % Daily Value for protein (FDA does not require it). Since % Daily Value of protein is not on most labels, it is very difficult for consumers to become familiar with the protein quality of common foods or to learn how a food contributes to their daily protein needs.

What are some high quality protein sources?
We’ve already mentioned that in order for a protein to be high quality, it must contain the indispensable amino acids in the proper proportion to meet the needs of a growing child. Animal sources (meat, poultry, eggs, milk, and fish) and the vegetable protein from soybeans all meet 100% of the amino acid requirements of the young growing child and these are considered the “gold standards” for protein quality.

Other good sources of protein are peanuts, lentils and rice. They score about 0.50 (out of a possible 1.0) when using the Protein Digestibility Corrected Amino Acid Score, the method recommended by FAO/WHO for evaluating protein quality and the method used by FDA. By comparison wheat protein scores 0.40. In typical diets, people consume a combination of protein sources and can actually meet their protein needs entirely from grains, beans, and vegetables.

References

1. Protein Quality Evaluation. Report of a Joint FAO/WHO Expert Consultation. Rome, Italy. Food and Agriculture Organization and World Health Organization. Food and Agriculture Organizations of the United Nations; 1990.

2. Henley EC, Kuster JM. Protein quality evaluation by protein digestibility corrected amino acid scoring. Food Technology. 1994;48:74-77.

3. Title 21, Code of Federal Regulation, Part 100-169 – Food labeling, food standards, good manufacturing practices for foods, low-acid foods, and acidified foods. 101.9(c)(7)p28-9. Edition 4/1/04.