BCAA content in proteins as a criterion for protein quality

The quantity of BCAAs and their ratio in proteins is an important indicator for proteins used in sport. From this standpoint, the various forms of whey protein (WP) are currently regarded as optimal. At the same time, the additional incorporation of BCAAs into ready-made forms of proteins from various sources is a modern manufacturing strategy, equating such formulas with WP to a greater or lesser extent.

Metabolism of BCAAs during protein ingestion

During digestion of proteins in the GI tract, they are known to break down into larger protein molecules, which then form smaller peptides in the small intestine under the influence of pancreatic juice proteases. As they pass through the small intestine, the peptides break down into "light" peptides (several amino acids in a chain), and in the final stage of digestion, under the action of peptidases, into individual amino acids. Amino acids and a number of "light" peptides are actively absorbed in the intestinal wall by specific transporters, circulate in the bloodstream and reach the liver. Oxidation of BCAAs in the liver leads to the formation of oxo-keto acids, a specific form of BCAA. This means that basic BCAAs do not undergo direct metabolism in the liver. The highest percentage of BCAAs is oxidized in muscle tissue, and a small percentage in fat tissue. Thus, BCAAs exhibit organ-specific properties with respect to skeletal muscle.

Metabolism of BCAA during exogenous oral intake

Transformation of BCAA in the intestine

During passage through the gastrointestinal tract (before entering the bloodstream) BCAAs are already involved in the metabolic processes of the small intestinal epithelium. Under the influence of two types of transaminases and long-chain alpha-keto acids dehydrogenase, which are present in the cells of the small intestinal mucosa, BCAAs are included in intracellular metabolic processes. It has been shown in experiments that about 30% of all leucine consumed is extracted from the intestinal contents during the first passage of food. Of this amount, 55% is transaminated and 45% goes into protein synthesis. In humans, 20-30% of exogenously administered leucine is utilized by the intestine during the first passage (G. Wu, 1998). The same approximate figures characterize the reduction of isoleucine and valine for absorption and entry into the bloodstream, due to uptake by the GI endothelium (30-40% of ingested amounts).

Absorption of BCAAs in the intestine

BCAAs can enter the body as part of different proteins. Accordingly, their amount in protein and the rate of their release during digestion largely determine the dynamics of absorption in the intestine and their entry into the circulatory system. On the other hand, when it comes to sports nutrition, BCAA sources include hydrolysates, isolates and concentrates of proteins (primarily whey proteins - WPH, WPI, WPC), as well as other transformed forms of proteins with peptides of varying size, and formulations where BCAA are already free amino acids. Accordingly, the use of a specific BCAA source presupposes knowledge of the pharmacokinetics of each specific product. A direct comparison of the value of a BCAA source solely by its quantity in a product, especially a comparison with BCAAs as a single amino acid complex, is unacceptable. Exogenous intake of BCAAs in their pure form (including their supplements to protein complexes) already has an advantage over protein intake, because protein digestion and release of BCAAs from it requires energy and substrate (enzyme) supply and time. In addition, the digestion process releases not only BCAAs but also other amino acids, which are to some extent competitive products for the transport of proteins in the intestinal wall. The transporters may be of a nonselective nature.

A few key points when evaluating the absorption of BCAAs in proteins: 1) BCAAs are absorbed faster than amino acids with shorter chain lengths; 2) essential amino acids are absorbed faster than substitutable amino acids.

A study by Farnfield et al[4] demonstrated the dynamics of amino acid concentrations in human plasma after ingestion of different fractions of whey protein (WP). This is a direct indicator of the intake of BCAAs consumed as part of proteins, allowing comparison of different proteins for use in sports and clinical medicine. WPs are fractionated during production to form peptides of different sizes. Such changes in WP composition can affect the rate and volume of amino acid absorption in the GI tract and ultimately alter protein synthesis in the body. In the routine practice of sports physicians it is common to describe WP as a "fast" (quickly absorbed) protein (emphasizing the "slow" nature of casein amino acid absorption), but accurate data on the rate of amino acid (AA) absorption from WP when using its different forms are not given in most works. Therefore, the purpose of the work of M.M.Farnfield et al. was to evaluate the plasma amino acid "response" to oral administration of several of the most popular forms of WP: b-lactoglobulin-rich WP (BLG), whey-protein isolate (WPI) and hydrolyzed whey-protein isolate (H-WPI). Pharmacokinetic study was performed on 8 healthy adult subjects (4 women and 4 men, mean age 27 years, weight 72 kg, height 170 cm, body mass index BMI - 23.2 kg/cm2). As shown in Table 1, the amino acid composition of the consumed protein mixtures was almost identical for total BCAA as for isoleucine and valine. A slight excess of leucine concentration (about 10 %) over its concentration in other mixtures was noted in BLG.

All three protein drinks caused a significant increase in plasma amino acid concentrations compared to controls. The dynamics of changes in AA concentration were similar in all time intervals after WPI and BLG ingestion. However, total plasma leucine and BCAA levels were significantly higher with BLG between 45 and 120 minutes compared to H-WPI (Figures 1 and 2). The magnitude of changes in total leucine and BCAA concentrations with WPI ingestion, despite the overall changes over time, was average between BLG (relatively maximal concentration shifts) and H-WPI (relatively minimal concentration shifts) (Figures 1 and 2).

These studies have made it possible to draw a very important conclusion in practical terms: the predictive evaluation of the nutritional value and efficacy of a protein source of BCAA (and leucine in particular) should be based not only on the quantitative BCAA content of the protein, but also on the pharmacokinetics of BCAA after intake of this protein.

However, the problems with selecting a WP for an adequate supply of BCAAs do not end there. The starting WP from various producers differs considerably in quantitative composition indices, which is connected both to the characteristics of the dairy raw material (whey) and to the addition of additional amounts of BCAA to the final product (which is typical for several American WP-complexes). The work of C.C. Almeida and coauthors [5] very clearly show the differences in the amino acid composition of WP, produced in the United States and Brazil (Table 2).

 Concentrations (mg/100 g) of free essential AA (EAA) and free BCAAs in different forms of WP from the USA and Brazil[5]

Amino acids Whey-protein USA Whey-protein Brazil

Histidine 2.7 ± 1.9 11.6 ± 21.8*

Isoleucine 95.5 ± 232.3 7.8 ± 13.9*

Leucine 125.6 ± 305.9 11.3 ± 19.0*

Lysine 21.1 ± 18.5 47.2 ± 61.1*

Methionine 5.2 ± 8.1 5.1 ± 7.6

Phenylalanine 13.8 ± 17.5 16.5 ± 28.0

Threonine 3.1 ± 4.3 9.2 ± 16.7

Valine 110.9 ± 278.8 9.7 ± 17.0*

ΣEAA 378.1 ± 854.9 118.7 ± 183.0

ΣCAA 332.0 ± 816.7 28.9 ± 49.9*

Notes: ΣEAA, sum of essential AA; ΣBCAA, sum of leucine, isoleucine, and valine; *significant differences (P<0.01); Table shows averaged data for 10 WP for each country: in Brazil, 5 WPI and 5 WPC samples; in USA, 5 WPI and 5 WPI+WPC mixture (WPCI) samples. The rest of the explanation is in the text.

As can be seen from the table, the differences in the main AA from the BCAA group reach a whole order of magnitude, which is due not only to the quality of raw milk, but also to the targeted addition of BCAA in some end products produced in the USA to enhance the anabolic effect of BCAA on the synthesis of muscle proteins.

Another point that complicates the predictive evaluation: about 40% of US products had lower protein content than declared on the label, while products from Brazil showed a 70% match between declared and actual protein content. Similar results were obtained by Consumer Lab, an independent private lab[6] specializing in food quality assessment: of the 24 commercial forms of WP produced in the U.S., 31% did not meet the declared amount of protein and, therefore, amino acids.

BCAAs from other protein sources

Natural plant proteins are inferior to WP in their amino acid composition. However, during the production process they can be additionally enriched with essential (especially BCAA) AA so that their composition is largely similar to that of WP (example - Table 3).

Comparative composition of essential AA, BCAA and leucine of two different protein supplements (per 25 g of protein)

Amino acids Complex plant protein in g Whey protein in g

Essential AC 11 12.4

BCAA 7.5    

Leucine 2.5 3.0

Notes: complex plant protein is a combined protein blend based on pea protein. The rest of the explanation is in the text.

Similarly, the final form of modern animal proteins is enriched with BCAAs. An example of this is one of Dymatize's latest developments, Dymatize Nutrition Elite Primal. In addition to the amino acids from beef protein, Dymatize Nutrition experts have added a supplement of free branched-chain amino acids (BPAA) and creatine to Elite Primal (hydrolyzed BP peptides; beef protein isolate hydrolysate - H-BIP; beef albumin). This leads to an equalization of the physiological value of beef protein mixtures and whey protein mixtures.

Thus, one cannot speak unequivocally about the advantages and disadvantages of particular forms of protein mixtures in terms of amounts of BCAA and leucine solely on the basis of the source of production (dairy, meat, fish, pea, wheat, etc. protein). In modern sports nutrition science, the qualitative and quantitative composition of a particular protein product, primarily the BCAA data, as well as the pharmacokinetics of the amino acids of the mixture are important. The quantitative parameters of BCAA content in a good quality product are necessarily indicated on the label and in the enclosed Instructions.

Metabolism of BCAA in skeletal muscle cells

Biochemical processes in skeletal muscle cells involving BCAA are shown in Figure 4.

There are two enzymes required for the metabolic changes of BCAAs: mitochondrial dehydrogenase and branched-chain keto-acid dehydrogenase (BCKD complex). After BCAAs are converted into their keto form (by the aminotransferase VSAT), the resulting keto acids can be used by muscles in the Krebs cycle to produce ATP (fuel for muscles), or they can be transported to the liver for oxidation. After oxidation of the keto acids, the oxo acids formed can be used in the liver as an energy source. Ultimately, BCAAs make up about 35% of all muscle tissue, and the products of their metabolism in both muscle and liver contribute to energy supply.

General principles of BCAA involvement in muscle metabolism during exercise

A total of 6 amino acids are involved in energy formation in muscle tissue: alanine, aspartate, glutamate and three BCAAs,[7] but the role of BCAAs is the greatest. Muscle tissue contains 60% of the specific enzymes needed to oxidize amino acids for energy production, especially BCAAs. During exercise, the body uses BCAA as an energy source. The more intense and prolonged the exercise, the more BCAAs are used. It has been estimated that 3% to 18% of all workout energy is provided by BCAAs, but this percentage can vary greatly depending on the nature of the exercise load. Leucine is an especially high requirement. The proportion of free (readily available for energy) leucine in the total pool of free amino acids is 25 times higher than others. Muscle is especially concerned, since the pool of free amino acids in skeletal muscle is 75%. BCAAs can also be converted in muscle to L-alanine or L-glutamine. The latter two amino acids can be converted into glucose by the process of gluconeogenesis in the liver. Leucine also directly stimulates protein synthesis through its signaling role (it increases the intracellular supply of amino acids). BCAAs, when taken as free amino acids, are separated into a fraction that goes into the liver and intestines and a fraction that goes directly into the bloodstream. Free form BCAA supplements rapidly increase the plasma concentration of these essential amino acids. This should be kept in mind when a sports physician calculates the time, dose and form of BCAA supplementation for a specific training task: BCAA in bound form (in the diet or in a WP) will ensure a relatively slow but prolonged BCAA supply to the muscles; BCAA in pure form will have a rapid but short-term anabolic effect. This role becomes particularly important when glycogen stores in the muscles are reduced and/or when carbohydrate intake is restricted in general (e.g., a low-carbohydrate diet). BCAA supplements are effective when taken both before and after exercise. Although leucine has a leading role to play in these processes, most experts find it more effective when taken as part of a BCAA complex.