Introduction: A protein is any one of a large number of organic compounds that make up living organisms and are essential to their functioning. First discovered in 1838, proteins are now recognized as predominant ingredients of cells, making up more than 50 percent of the dry weight of animals. The word protein is coined from the Greek proteios, or “primary.”
Protein molecules range from the long, insoluble fibers that make up connective tissue and hair to the compact, soluble globules that can pass through cell membranes and set off metabolic reactions. Humans are made up of an estimated 30,000 different proteins, of which only about 2 percent have been adequately described. Proteins in the diet serve primarily to build and maintain cells, but their chemical breakdown also provides energy, yielding close to the same 4 calories per gram as do carbohydrates.
Besides their function in growth and cell maintenance, proteins are also responsible for muscle contraction. Digestive enzymes are proteins, as are insulin and most other hormones. The antibodies of the immune system are proteins, and proteins such as hemoglobin carry vital substances throughout the body. Not all proteins are equal, just as all carbohydrates are not the same. While glycogen is formed by chains of glucose subunits, proteins are composed of chains of amino acids. The chains of amino acids in proteins are linked by peptide bonds and each protein is a unique combination of amino acids.
Protein is composed of about 20 different amino acids which in turn, are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. In a protein molecule these acids form peptide bonds which are bonds between amino and carboxyl (COOH) groups-in long strands (polypeptide chains). The almost numerous combinations, in which the acids line up, and the helical and globular shapes into which the strands coil, help to explain the great diversity of tasks that proteins perform in living matter.
Of the 20 amino acids in the body, there are two types: essential and non-essential. There are nine essential amino acids necessary to maintain health – Branched Chain amino acids (leucine, isoleucine & valine), histidine, lysine, methionine, phenylalanine, threonine, and tryptophan — that cannot be synthesized by the body. Because of this, your diet must provide these essential amino acids via protein intake. Complete proteins include all of the essential amino acids, and are therefore better quality proteins. Incomplete proteins are missing one or more of the essential amino acids, and are therefore of lower quality. Because the rate of protein synthesis in the body is limited by the quantity of the lowest available essential amino acid, it is important to consume equal quantities of all of them. Proteins produced in the seeds of plants (soy, for example) do contain essential amino acids, but because plant sources are often weak in lysine, methionine and tryptophan, nutrition experts advise supplementing the diet with animal protein from meat, eggs, and milk, which contain all the essential amino acids. The other non-essential amino acids are equally important, but can be synthesized in the body at a rate that equals demand so dietary supplementation is not crucial.
Protein measurement tools: More important than looking at the amino acid profile of certain proteins are the analytical tools developed to measure protein’s effectiveness. Various tools are used to measure digestibility, amino acid profile, muscular growth and absorption. Though no single tool gives a complete picture of a protein’s effectiveness, a chart outlining all the measures allows for a more complete assessment.
*The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is the USDA’s officially approved method of scoring protein quality.
*Amino Acid Score is a count of total amino acids in a protein, adjusted for the protein’s composition.
*Another method used to measure protein quality is the Protein Efficiency Ratio (PER). The PER rating is based upon the evaluation of the growth of animals consuming a fixed amount of dietary protein from a single source. As the PER increases, so does the quality of the protein.
*Biological Value (BV), another measure of protein quality, measures the amount of protein that is retained from the absorbed protein for maintenance and growth. It measures the fraction of the nitrogen in the diet that remains after the nitrogen losses in the waste products have been subtracted.
Casein: Casein is the major protein found in cow’s milk (80-82%). In fact, Casein is what is used to make cheese. Most meal replacement products, and some protein powders, today use Casein as one of their main ingredients. Casein contains a high quantity of alpha Casein-a protein found only in animal milk, which is not easily digestible by humans. In fact, science shows that Casein in high amounts may actually delay gastric emptying. This is why Casein is considered a “slow” protein, meaning nitrogen and amino acids are released more slowly.
Whey Protein: Whey protein is a high quality, complete protein and a rich source of branched chain amino acids (BCAAs) and essential amino acids which are critically important for individuals who are involved in sports, exercise, or do resistance training. Whey protein is a dairy protein and comes from cow’s milk, not animal flesh. In the United States whey protein is a by-product of the cheese making process and in most cases a microbial type of rennet is used in the manufacturing process. Whey protein is acceptable for any vegetarian diet that allows dairy products including lacto-ovo, lacto, and ovo types of vegetarian diets.
Soy Protein: Soy protein is an abundant and inexpensive protein source. Even though soy protein is vegetable based, it contains a wide range of essential amino acids. The amino acids in soy protein parallel those of animal based protein, however soy protein is lower in methionine. Although methionine is considered a rate limiting amino acid, the FDA suggests the typical American diet generally makes up for its lower availability in soy products. Soy protein has very high concentrations of the amino acids lysine and leucine, which are necessary for maintenance of nitrogen levels. Long and short-term studies have supported the use of soy protein as the sole source of protein to maintain healthy nitrogen balance in healthy adults. Research supports the digestibility of soy being comparable to that of other high quality protein sources such as dairy and animal based products. While soy flour can cause flatulence (due to our lack of the enzyme alpha galactosidase), the process of conversion to soy isolates, those generally found in supplements, reduces this side effect. Beyond these effects, the isoflavones from soy protein have been associated with prevention of bone loss and the symptoms of menopause. While the lower cost and health benefits of soy make it an attractive option when considering protein supplementation the gastric side effects and mild estrogenic activity may turn off some athletes.
Egg Protein: Egg protein is considered by many to be the gold standard in food proteins. Egg is a complete protein, easily digestible and assimilated. The only downside to egg protein is found in the high cholesterol content of the yolk. As a food there is no better source of protein available, however when you turn to supplements egg proteins are more difficult to work with. The mixability and palatability of concentrated egg proteins is not as good as soy, whey or milk which is why you rarely see egg as the primary protein in supplements.
Isolate, Hydrolysate, Concentrate:
Isolates are the purest form of protein and contain 90 – 95% protein. Isolates are formed using either ion exchange or cross-flow filtration, with either method resulting in a protein with little (if any) fat or lactose (in milk proteins). Hydrolyzed, or Whey Hydrolysis is a chemical process which breaks the protein chains down into smaller segments called peptides. Due to its low molecular weight, Hydrolyzed protein is more easily digested (BV=100) and has a reduced potential for allergic reactions versus non-hydrolyzed proteins while remaining very high quality. High quality Sports nutrition products and infant formulas often use hydrolyzed proteins for these reasons. Concentrates, considered the lowest quality of the Whey proteins are available in a number of different types from 25% protein content to 89% protein content, containing some fat, lactose and minerals. As a rule, when the protein level decreases the amount of lactose increases. Whey protein concentrate at 80% protein content is the form most readily available as a protein powder supplement. Due to the inclusion of fat and lactose, Whey concentrate is not digested as easily as other sources.
It is a commonly held belief that athletes, particularly bodybuilders, have greater requirements for dietary protein than sedentary individuals. The evidence in support of this contention is controversial, although a growing number of researchers advocate protein supplementation for endurance athletes between 1.2 – 1.8g/kg. For a 70kg (155lb person) this means 84g – 126g of protein daily, preferably consumed in five or more meals per day. Though the case for athletes favors whey Isolate or hydrolysate as the protein of choice, there is a strong argument for soy isolate due to its bone-building and heart-health benefits.
While most athletes with a healthy diet do achieve the recommended daily intakes of dietary protein, it is easy to be distracted by carbohydrate needs as an endurance athlete. Those on low fat diets typically avoid meat and dairy sources of protein because they can contain saturated fats. You can purchase low fat versions of most of these and trim or drain excess fats during preparation. A fine balance in food selection that includes variety and moderation is optimal. Here is a list of some common foods that contain protein:
During Exercise: There is some evidence, although limited, that protein consumption during exercise can help shuttle glycogen to your working muscles. This, in effect, will spare glycogen and allow you to workout or race longer. While the latest research indicates that the addition of protein during exercise does extend time to exhaustion, the reason why isn’t exactly clear. The exact mechanism has yet to be clarified, though researchers speculate that the amino acids found in protein may help 1) delay central fatigue and 2) maintain precursors to Krebs Cycle intermediaries (see side-bar by Bob Seebohar MS CSCS). The question of whether or not to add protein to carbohydrates during exercise has certainly made a significant step in determining possible answers, however, there is by no means conclusive evidence that states it is absolutely necessary and for what type of athlete and duration/intensity of exercise. Sports nutrition is as much of a science as it is an art, which must be individualized to each athlete under different conditions. A recent study (Saunders et al 2004) found 29-40% longer endurance times at 85% VO2max cycling with a drink that included protein (7.3% carbs and 1.8% protein) versus carbohydrate only drink. Additionally the post-exercise levels of muscle damage were 83% lower with during exercise protein consumption. The current recommendation is to consume either a small portion of protein or individual amino acids along with your carbohydrate drink during exercise, though this should not be done at the expense of an easily digested drink. More research is needed in this area to further solidify this theory.