The demand for high-protein supplements and prepared foods has grown in recent years as high-protein diets have become the rage in the weight-loss arena. Current research suggests that different types of protein have varying effects on exercise outcome and physique. Richard B. Kreider, Ph.D., assesses which protein source is best.
Several types of native protein sources, as well as designer blends, are found in dietary products for performance and body composition benefits. Protein sources most commonly used in dietary supplements and functional foods include egg, milk, casein, whey, soy, colostrum and gelatin, with whey being the most prevalent today. Many factors influence the availability of the various amino acids, peptides and fractions within the proteins that have been reported to possess biological activity. These variables include processing methods—for example, hydrolysed, concentrate, isolate, ion-exchange, cross-flow microfiltration; the amino acid profile of the protein; and the mode of delivery, such as foods, bars, powders or ready-to-drink supplements. Although a particular processing method may provide a marketing advantage, the central question remains whether there is really any difference in the protein sources used in products.
Researchers generally determine protein quality by using the protein efficiency ratio (PER) or the protein digestibility corrected amino acid score (PDCAAS). They determine the PER by evaluating the weight gain in growing rats fed a particular protein compared with a standard protein, egg whites being the gold standard. The higher the PER value, the greater the protein quality. The PDCAAS was introduced as a more accurate way to evaluate protein quality for humans, because it uses human, not rat, amino acid requirements to calculate the amino acid score. It compares the amino acid profile of a protein with the essential amino acid requirements for humans according to The Food and Agriculture Organization. When a protein meets this requirement, it gets a score of 1.0. PDCAAS has now been adopted as the official method by the World Health Organisation, the US Food and Drug Administration and the US Department of Agriculture.
Although the PDCAAS method is the internationally recognised standard for comparing proteins for human consumption, it does not allow for differentiation among proteins with a PDCAAS of 1.0, so proteins can have different PERs while still having a PDCAAS of 1.0. This is significant, because researchers are now exploring how variances in specific essential and/or conditionally essential amino acids, as well as the availability of various peptides and micronutrients, affect human physiology. Claims of consumer-relevant superiority or inferiority, when performance of physique modificaitons are concerned, have yet to be supported by population-specific evidence in humans.
(left) lists the PER and/or PDCAAS for the major types of protein used in nutritional supplements. The following discusses the relative strengths and weaknesses of these basic types of proteins that are often used as starting materials for nutritional supplements. Of course, adding deficient amino acids and other nutrients to these proteins may increase the PDCAAS, nutrient value and/or functionality of the protein.
Egg protein is typically obtained from chicken egg whites (ovalbumin) or whole eggs through various extraction and drying techniques. Ovalbumin is recognized as the reference for protein quality comparisons. The PER and PDCAAS of egg protein is similar to milk proteins and only slightly lower than casein, whey and bovine colostrum.
Egg protein has historically been one of the most common protein sources used in nutritional supplements, because consumers perceive it to be a high-quality protein. However, manufacturers are beginning to use other protein sources because egg protein is a relatively expensive form of protein.1
Because egg protein is viewed as the reference protein, researchers have evaluated the effects of egg protein on nitrogen retention and physiological adaptations compared with other proteins. Study results generally indicate that egg protein is as effective as milk protein, casein and whey in promoting nitrogen retention, which is a measure of the adequacy of a dietary protein.2,3,4
Milk Proteins And "Magic Blends"
Milk protein has been one of the most widely used sources of protein for use in nutritional supplements. Milk protein is comprised of about 80 per cent casein and 20 per cent whey protein. Milk protein contains several subtypes of casein (a, b, g, k) and whey protein (a-lactalbumin, b-lactoglobulin, bovine serum albumin, immunoglobulins, lactoferrin, lactoperoxidase) that are each believed to possess unique physiological properties.1
Most researchers have used milk protein to determine the effect of increased dietary protein intake on protein balance, and the results of their research have generally revealed that milk protein is a high-quality protein source. More recently, there has been interest in determining the effects of ingesting casein and whey protein on nitrogen retention, hormonal profiles, protein synthesis and sports training adaptations.
Caseinates are produced from skim milk through a processing technique that separates the casein from the whey, then resolubilising and drying.1 Caseinates used in commercial supplements are available as sodium caseinates, potassium caseinates, calcium caseinates and casein hydrolysates. The processing method affects the amino acid profile slightly, as well as the availability of a, b, g and/or k casein subtypes. There is a lack of research on various caseinate blends as it relates to performance.
The advantage of casein is that it is relatively inexpensive and is available in a range of grades that vary in quality, taste and mixing characteristics.1
The major disadvantage of casein for manufacturers and consumers is that it is not readily soluble in liquid and tends to clump, particularly in an acidic fluid. However, research indicates that several factors influence protein synthesis, including the amount of calories consumed, the quantity and quality of protein ingested, the insulin response to the meal, and the digestibility of the food.5
The digestion rate of a particular food influences the time release of amino acid in the blood. Foods containing proteins that are digested quickly, such as whey, often expedite the amino acid release in the blood. Conversely, foods containing proteins that are digested slowly, such as casein, often result in a reduced but more prolonged increase in amino acids.6
Research from Boirie and colleagues and Dangin and associates suggests that casein may be superior to whey because of the prolonged amino acid release and influence on anticatabolic hormone profiles.7,8 Some supplements manufacturers now market casein as a superior form of protein.
A study by Demling and co-workers is often cited to support contentions that casein supplementation during athletic training is more effective than whey protein. However, this study actually compared ingesting a vitamin/mineral-fortified carbohydrate/protein meal replacement powder containing a "unique blend of milk protein isolates (caseinate, glutamine, whey protein concentrate, egg white)" with a whey protein supplement.9 Consequently, the greater gains in muscle mass and strength observed in the study could not be attributed to ingesting casein alone.
Although casein is a quality protein source, there is little evidence that it is more effective than soy, egg, whey or bovine colostrum. In fact, it has often been used as the control protein in supplements studies.
The slow and fast concept of protein digestion is certainly interesting. However, more research is needed to explore the role of ingesting various mixtures of slow and fast proteins with and without other macro- and micronutrients.
Is Whey The Way?
Whey protein is currently the most popular source of protein used in nutritional supplements, particularly in the sports nutrition market. Whey proteins are available as whey protein concentrates, isolates and hydrolysates. Whey protein concentrates (about 80 per cent protein) are produced from liquid whey by clarification, ultrafiltration, diafiltration and drying techniques.1 Whey protein isolates (about 90 per cent protein) are typically produced through ion-exchange (as with Davisco Ingredients' BiPro) or cross-flow microfiltration (as with Glanbia Ingredients' CFM/Provon) techniques. Whey protein hydrolysates (about 90 per cent protein) are typically produced by heating with acid, or preferably treatment with proteolytic enzymes, followed by purification and filtration.
Different processing methods affect amino acid availability as well as concentration of whey protein subtypes, peptides and amino acids (i.e., a-lactalbumin, b-lactoglobulin, bovine serum albumin, immunoglobulins, lactoferrin, lactoperoxidase). Theoretically, differences in protein subtypes and/or peptides may influence the physiological function of a protein.
In recent years, marketing experts have focused on distinguishing differences in the quality of whey proteins based on how they were processed. Some contend that ion-exchange (IE) whey protein is superior to other forms. However, cross-flow microfiltration (CFM) and proteolytic enzyme-treated whey hydrolysates may be the best choice because they better preserve the availability of whey protein subtypes and peptides.
Research suggests that the availability of whey protein subtypes and peptides may be more health-promoting constituents than whole whey.10,11 For this reason, most researchers who have evaluated the health and/or performance benefits of whey protein have used whey protein produced using CFM or proteolytic enzyme hydrolysis.1
Compared to casein, whey protein is digested more quickly, with better mixing characteristics and a general reputation for higher quality. Research has indicated that the rapid increase in blood amino acid levels following whey protein ingestion stimulates protein synthesis to a greater degree than casein.7,12 However, the researchers found that a single dose of whey protein had less impact over time on protein catabolism in comparison to casein. Theoretically, individuals who consume whey protein frequently throughout the day may optimize protein synthesis.
In support of this contention, Dangin and associates concluded that frequent ingestion of a small amount of whey protein increased protein synthesis to a greater degree than less-frequent ingestion of various proteins.8 Whey protein may also offer a number of health benefits compared with casein, including greater immunoenhancing and anticarcinogenic properties.10,11,13,14,15 For example, Lands and colleagues reported that 20 healthy young adults who took 20g/day whey protein during 12 weeks of athletic training experienced enhanced immune function, performance and body composition alterations over casein.16 These findings and others have helped position whey protein as a superior protein source for nutritional supplements.
Soy protein is obtained from soybeans through a process of water extraction, precipitation, washing and drying that yields soy protein concentrate (about 70 per cent protein) or soy protein isolates (about 90 per cent protein).1 Although soy is lower in the essential amino acid methionine, it has a relatively high concentration of the remaining essential amino acids—particularly arginine and glutamine—and is therefore considered a complete protein. The PER and PDCAAS of soy protein is similar to dietary meat or fish and slightly lower than egg, milk, casein, whey and bovine colostrum.
Studies have shown that soy protein helps build and maintain muscle mass and lean body tissue. Dragan and colleagues conducted trials with the Romanian Olympic rowing team and found that 1.5g/kg body weight Supro soy protein, from Dupont Protein Technologies, in addition to regular intake of 2.0g/kg protein in the diet may be responsible for reduced post-exercise fatigue.17
Other studies provide evidence that soy can exert antioxidant effects in people, particularly for exercise-induced oxidant stress. Robert DiSilvestro, PhD, and co-workers at Ohio State University fed 10 young adult males a soy beverage containing 40g/day protein and 44mg genistein/day. Compared to a group of 10 males who consumed a whey beverage containing 40g protein/day, the Supro soy group experienced superior plasma total antioxidant status and a lower rise in plasma creatine kinase activity (an indicator of muscle tissue breakdown).18
Some researchers also have expressed concern over the long-term health impact of diets high in phytoestrogens. There has also been some concern that male consumption of phytoestrogens may decrease testosterone levels and/or increase the infertility rate.19,20 Finally, there is some indication that soy-enhanced diets may interfere with thyroid function and absorption of the thyroid medication levothyroxine in infants who have hypothyroidism.21,22 Although long-term studies evaluating soy protein consumption generally indicate no significant adverse effects on health, extremely high consumption of soy has not been adequately evaluated.
A fairly new entry into the sports nutrition market has been bovine colostrum (BC), the milk produced by cows during the first few days after calving.23,24 It has greater nutrient density and higher protein quality than ordinary dairy milk. Additionally, it has a PER comparable to egg, casein and whey protein (See Table 1). Consequently, BC is an excellent source of quality protein. In addition, BC also contains fairly high concentrations of growth factors (IGF-I, IGF-II, TGF b), immunoglobulins (IgG, IgA, IgM) and antibacterials (lactoperoxidase, lysozyme, lactoferrin) in comparison to other forms of proteins like milk and whey.24,25,26,27,28,29
Although adult digestive enzymes are believed to degrade the majority of these compounds, several studies suggest that milk30 and BC supplementation24,31 may slightly elevate levels of growth factors and immunoglobulins in the blood. Theoretically, BC supplementation during athletic training may therefore increase the availability of essential amino acids, growth factors, and/or immunoglobulins, leading to greater gains in strength and/or muscle mass during intense training.
In support of this theory, Mero and colleagues reported that nine male sprinters significantly increased IGF-I levels by 10 to 20 per cent in a linear manner during training upon supplementing with 67.5mL BC (providing about 20g BC) twice daily for eight days.24 The change in IGF-I was significantly correlated to changes in insulin levels and was not seen as attributable to absorption of the BC-derived IGF-1.
In a series of preliminary studies, Buckley and colleagues reported that 60g/day BC supplementation during eight to nine weeks of training improved run time-to-exhaustion, vertical jump performance and rowing performance.32,33,34
Results of these studies could not be attributed to changes in IGF levels, suggesting that other factors such as amino acid profile and/or other protein subtypes may play a role. Interestingly, BC was compared to whey protein placebo in these studies.
More recently, Antonio and associates reported that 20g/day BC supplementation for eight weeks during training promoted significantly greater gains in fat-free mass (1.5 kg) compared with subjects taking whey protein (-0.1 kg).35 However, they observed no significant differences between groups in muscle strength and/or endurance.
Researchers from my lab evaluated the effects of using BC or whey protein as the source of protein in fortified and non-fortified supplements during 12 weeks of resistance training.36,37 Results revealed BC served as an effective source of protein in comparison to whey, particularly when coingested with a vitamin/mineral-fortified supplement containing creatine. Although these preliminary findings are promising, BC is currently an expensive source of protein, which may limit marketability.
Gelatin (collagen) was once a mainstay in the nutrition industry. However, it is a poor-quality protein that has fallen out of favor in the eyes of consumers.1 More recently, use of gelatin supplements during training have been suggested as a means to maintain joint health in athletes.38 Interestingly, gelatin has experienced a mild re-emergence in various weight-loss products such as bars. Whether this renewed interest takes off will be judged by research and consumer experience with these products.
Which Protein Is Best?
Different types of protein may offer specific advantages over others, depending on the population targeted or degree of performance of body composition goals. For example, egg protein is generally well accepted among consumers and may be an attractive alternative for lacto-ovo vegetarians. Milk proteins are inexpensive and serve as a quality source of casein and whey protein for individuals who are not lactose intolerant. Casein may serve as a good source of protein to minimize protein catabolism during prolonged periods between eating, such as during sleep, or in people maintaining a low-calorie diet.
Frequent ingestion of whey protein may optimise protein synthesis and immune function. Soy protein may be the best choice for vegetarians and individuals interested in increasing dietary availability of isoflavones. Plus, soy is typically the least expensive protein source, and it has relatively good organoleptic (taste and texture) qualities. Bovine colostrum appears to be a high-quality, but high-priced, protein that may enhance training adaptations.
Picking the best overall protein to use in nutritional supplements is not an easy choice. One has to consider cost, quality, availability of protein subtypes and constituents, research support, production characteristics and taste. At present, it is my view that whey protein using CFM or proteolytic enzyme hydrolysis techniques is probably the best choice. However, additional research is needed to understand how subtle differences in protein sources affect health and performance before definitive conclusions could be drawn.
Richard B. Kreider, PhD, EPC, FACSM, FASEP, has published more than 150 research articles and abstracts in scientific journals. He is professor and chair of the department of health, human performance and recreation at Baylor University, Waco, Texas.
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