Protein
Reviewed by Sally Warner, PhD

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 approximately 4 calories per gram, similar to carbohydrates.

Proteins are 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 between amino and carboxyl (COOH) groups in long strands (polypeptide chains). The 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.

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.

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 that cannot be synthesized by the body (branched chain amino acids: leucine, isoleucine & valine; histidine, lysine, methionine, phenylalanine, threonine, and tryptophan). Therefore, your diet must provide these essential amino acids through dietary protein intake. Complete proteins include all of the essential amino acids, and are therefore higher 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 adequate quantities of all amino acids. 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 (as long as nitrogen is available).

Protein measurement tools: More important than looking at the amino acid profile of certain proteins are the analytical tools developed to measure protein’s bioavailability. Various tools are used to measure digestibility, amino acid profile, impact on muscular growth and absorption. Although no single tool gives a complete picture of a protein’s effectiveness, the following methods can be used to assess protein quality.

• 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.
• Protein Efficiency Ratio (PER) 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) 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.

Protein Quality Comparison Chart

*Whey protein has a PDCAAS of 1.14. The reported score is 1.0, which is the maximum value allowed by the USDA for reporting purposes.

Proteins Defined:

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. There is recent evidence showing casein plays a role in the later phase of protein synthesis following exercise.

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 found in cow’s milk, but 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 positive 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. However, the yolk is also the portion of the egg that contains many important vitamins. Egg protein as a supplement can be difficult to work with. The mix ability 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 while remaining very high quality. High quality sport nutrition products and infant formulas often use hydrolyzed proteins for these reasons. Concentrates, considered the lowest quality of Whey proteins, can vary in protein (25% - 89%), fat, lactose and mineral content. 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.

Recommendations: 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 translates into 84g - 126g of protein daily, preferably consumed in five or more meals per day. Although many advocate whey Isolate or hydrolysate as the protein of choice for athletes, there is a strong argument for soy isolate due to its bone-building and heart-health benefits.

Daily Protein needs:

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:

The International Society of Sports Nutrition published a position stand on protein and exercise in 2007 (Campbell et al.). This document is a useful guide that reviews all of the most current and important research available on protein and exercise. The take home points for endurance athletes include the following. They recommend protein intakes ranging from 1.4-2.0 g/kg per day and report that endurance athletes fit into the lower end of this range. They report protein intakes above the RDA as being safe for healthy and active adults. They support the use of PDCAAS score as the most accurate way to assess protein quality. They endorse whole foods as the primary source of protein yet suggest the next best options are supplements containing whey or casein. They also address the important roles of BCAA’s in maintaining mental function by reducing perception of fatigue, delaying muscle glycogen depletion, aiding in glycogen resynthesis and stimulating protein synthesis. They encourage consumption of high quality protein before, during and after exercise.

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 muscle glycogen and allow you to exercise longer. While the latest research indicates that the addition of protein during exercise does extend time to exhaustion, the reason is not exactly clear. The exact mechanism has yet to be clarified, though researchers speculate that the amino acids found in dietary protein may help 1) delay central fatigue and 2) maintain precursors to Krebs Cycle intermediaries (see side-bar by Bob Seebohar MS CSCS).

Recently, Greer et al 2007 studied BCAA supplementation and muscle damage during 90 minute bouts of endurance cycling in untrained college-aged men. They found decreased 24hrs post exercise muscle soreness, lower creatine kinase activity from 4-48hrs post exercise. They concluded that BCAA supplementation before and during exercise can attenuate muscle damage during endurance exercise.

Another positive role of protein during exercise was recently researched by Carvalho-Peixoto et al. (2007). In this study, blood ammonia accumulation (ammonemia) was suppressed by glutamine in the second half of a 2 hour run in high level male runners. Ammonemia is thought to play a role in central and peripheral fatigue during prolonged endurance exercise.

In another recent study of the glycogen sparing effect of protein, aspartate and asparagine were administered to trained male triathletes during high intensity endurance exercise (Parisi et al 2007). Unfortunately, no significant performance benefits were observed in this trial to support this hypothesis.

A study of cycling endurance times showed an increase of 29-40% at 85% VO2max with a drink including protein (1.8%) and carbohydrates (7.3%) versus carbohydrate only (Saunders et al 2004). Additionally the post-exercise levels of muscle damage were 83% lower during exercise with the protein and carbohydrate drink. On the other hand, Van Essen and Gibala (2006) found the addition of protein to a carbohydrate drink provided no additional performance benefit during a cycling time trial. Romano-Ely et al. (2006) also observed no differences between consuming a carbohydrate-protein-antioxidant (CHOPA) beverage and carbohydrate-only (CHO) beverage in time to fatigue in cycling. However, the CHOPA beverage attenuated postexercise muscle damage compared to the CHO beverage (beverages were isolcaloric).The existing research provides mixed results with regard to adding protein to carbohydrate drinks during exercise. What must be considered is that sport nutrition is as much an art as it is a science and therefore must be individualized based on the individual’s nutritional status as wells as duration and intensity of exercise.

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.

Post-Exercise: There are three primary components to complete recovery: 1) glycogen replenishment, 2) water and electrolyte replenishment, and 3) muscle repair. As a recovery tool, protein’s main role is to repair damaged muscle tissue. Without a positive nitrogen balance through the consumption of protein, your body will not have the tools necessary to repair exhausted muscles and in time you will slowly but surely lose muscle. And consuming a high protein shake or food following exhaustive exercise without paying attention to the other two components is a common mistake that should be avoided.

Post-exercise, it is critical that you consume a high quality protein with a high glycemic carbohydrate. High glycemic carbohydrates help shuttle glycogen and protein to the cell. This is the foundation of proper recovery and should be done religiously. The best ratio of carbohydrates to protein for recovery is somewhere between 3:1 and 5:1. Also keep in mind that there is a short window of opportunity immediately following exercise when your insulin sensitivity is at its highest. Recovery is maximized if you consume your carbohydrate/protein drink immediately following exhaustive exercise. Timing is particularly important if your next workout is ≤ 8 hours (i.e. 2 a day practice/training session). (See side-bar by Neal Henderson MS CSCS)

References:
Soy Protein Council \0022″”>(http://www.spcouncil.org/SoyProtein.pdf) 

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