The British Journal of Sports Medicine (
http://bjsm.bmjjournals.com/)
Hyperinsulinemia, hyperaminoacidemia and post-exercise muscle anabolism: the search for the optimal recovery drink
Anssi H. Manninen
Advanced Research Press, Inc., 690 Route 25A, Setauket, New York, 11733, USA
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Abstract
Dietary supplements and other ergogenic aids are popular among athletes. Recent studies have demonstrated that nutritional mixtures containing protein hydrolysates, added leucine, and high-glycemic carbohydrates strongly augment insulin secretion compared with the high-glycemic carbohydrates only trial. When post-exercise hyperinsulinemia is supported by protein hydrolysate and leucine ingestion-induced hyperaminoacidemia, net protein deposition in muscle should occur. Thus, post-exercise recovery drinks containing these nutrients may lead to increased skeletal muscle hypertrophy and strength in conjuction with appropriate resistance training. However, the long-term effects on body composition and exercise performance remains to be determined.
Keywords: amino acids, protein hydrolysate, leucine, sports nutrition, ergogenic aids
Introduction
“The importance of nutrition following training-induced homeostatic disruption can be traced to our most ancient of writings. Esau, the first born of Isaac, in what is estimated to be 1800 B.C. appears to have had incredible genetics. His training sessions, however, were not found in the gym, but rather in the field as he hunted the most ferocious of beasts. So vital was the post-workout meal to the father of the Edomites that he sold his birthright to his brother Jacob for it!” [1]
Appropriate resistance exercise leads to significant skeletal muscle hypertrophy, which can occur through an increase in protein synthesis, a decrease in protein degradation, or both. While stimulus (i.e., resistance exercise) is important for muscle hypertrophy, nutrient availability appears to be a critical factor regulating the degree of hypertrophy. Obviously, the muscle´s hormonal milieu also has a major impact on protein synthesis.
It is now apparent that both increased insulin and increased availability of amino acids are important to maximize the muscle protein anabolism. If hyperinsulinemia is not supported by an exogenous amino acid supply, plasma as well as muscle free amino acid concentrations drop due to reduced splanchnic release [2]. On the other hand, if amino acid concentratios are maintained at normal or higher levels, net protein deposition in muscle will occur because of a stimulation of synthesis, and possibly because of simultaneous decrease in breakdown [2].
The importance of availability of amino acids for the stimulatory effects of insulin to be evident was highlighted by Bennet et al, who reported that insulin, given with sufficient amino acids, can stimulate leg and whole body protein balance by mechanisms including stimulation of protein synthesis and inhibition of protein breakdown [3]. This is in line with the recent data by Borsheim et al., who showed that protein balance over the muscle remains negative after resistance exercise when only carbohydrate (CHO) is ingested [4]. In sharp contrast, amino acid ingestion alone significantly increases muscle protein anabolism after resistance exercise [5]. However, consumption of both amino acids and CHO results in much greater effects on muscle protein anabolism [6], suggesting an interactive effect between insulin, amino acid availability, and resistance exercise. Also, it is well-established that the stimulatory effect of amino acids after exercise is greater than the effect of amino acids on muscle protein synthesis when given at rest [7]. Thus, nutrient timing is also an important consideration [8-11].
Given the fact that raising the plasma insulin level is key to stimulating muscle protein synthesis and limiting protein catabolism following exercise [12], it is not surprising that some athletes abuse insulin to increase skeletal muscle hypertrophy. Insulin injections reportedly can produce “rapid and noticeable [muscle] growth... almost immediately after starting insulin therapy” [13]. Most athletes choose to administer insulin immediately after a workout [13]; they apparently realize that it is the most anabolic time of the day to use this hormone. However, insulin abuse is extremely risky – one mistake in dosage or diet can be fatal. Fortunately, recent studies have focused on safe insulinotropic nutritional mixtures containing protein hydrolysates, certain added amino acids (especially leucine), and high-glycemic CHO (e.,g., dextrose, maltodextrine) [14-18].
In this paper, it is proposed that post-exercise hyperinsulinemia supported by protein hydrolysate and leucine ingestion-induced hyperaminoacidemia increases net protein deposition in muscle, leading to increased skeletal muscle hypertrophy and strength in conjuction with appropriate resistance training. Firstly, this review provides some information on 1) amino acid-stimulated insulin secretion, 2) leucine´s effects on post-exercise muscle protein synthesis, and 3) protein hydrolysates. Then, the studies examing the effects of insulinotropic nutritional mixtures on insulin secretion, nitrogen utilization, and post-exercise muscle protein anabolism are reviewed. Finally, the effects of post-exercise hyperinsulinemia on fat oxidation and de novo lipogenesis are discussed.
Amino acid-stimulated insulin secretion
Formerly, it was believed that insulin secretion was controlled almost entirely by the blood glucose concentration. However, it later become apparent that amino acids also play very important role in controlling insulin secretion. Certain amino acids cause insulin release in humans even under conditions where the blood sugar changes little from its basal level [19]. However, changes of blood sugar levels markedly influence the responsiveness of beta cells to individual amino acids. Studies on isolated perfused rat pancreas and islets have demonstrated that physiological amino acid mixtures and even pharmacological concentrations of individual amino acids require the presence of permissive levels of glucose (2.5 to 5.0 mM) to be effective beta cell stimulants [19]. However, leucine is an exception [20]. Contrary to popular belief, oral arginine is not a effective insulin secretagogue [14].
Effects of leucine on post-exercise muscle protein synthesis
The key branched-chain amino acid leucine acts as a nutrient signal to stimulate muscle protein anabolism. Leucine affects muscle protein metabolism by decreasing the rate of protein degradation [21] most likely via increases in circulating insulin. In addition, leucine affects phosphorylation of key proteins involved in the regulation of protein synthesis, which has been shown to occur even in the absence of an increase in circulating insulin concentrations [22]. After exercise, recovery of muscle protein synthesis requires dietary protein or branched-chain amino acids to increase tissue levels of leucine [23]. The important bottom line is that insulin and leucine allow skeletal muscle to coordinate protein synthesis with physiological state and dietary intake [23]. For more detailed reviews, see the recent papers by Norton & Layman [23], Blomstrand et al. [24], and Garlick [25].
Protein hydrolysates
Protein hydrolysates are produced from purified protein sources by heating with acid or preferably, addition of proteolytic enzymes, followed by purification procedures [26]. Extreme bitterness is a negative attribute associated with most protein hydrolysates. Fortunately, specific debittering strategies have focused on the application of proline specific exo- and endopeptidases given the contribution of proline residues to hydrolysate bitterness [27]. Hydrolysis process mimics our own digestive actions; thus, some feel it is an ideal way to process dietary protein. Extensively hydrolyzed proteins containing mostly di- and tripeptides are absorbed more rapidly than free form amino acids and much more rapidly than intact (non-hydrolyzed) proteins [26,28,29]. The considerably greater absorption rate of amino acids from the di- and tripeptides than from the amino acid mixture appears to be the result of uptake by a system that has a greater transport capacity than amino acid carrier system, thus minimizing competition among its substrates [28]. Each protein hydrolysate is a complex mixture of peptides of different chain length together with free amino acids, which can be defined by a global value known as degree of hydrolysis (DH), which is the fraction of peptide bonds that have been cleaved in the starter protein [30]. However, two protein hydrolysates made by different methods (e.g., oligopeptides/significant free amino acids vs. mainly di- and tripeptides) may have a similar DH even though their absoption kinetics are likely to be quit different [31]. Consequently, it has been suggested that it is better to use the term “peptide chain length profile” [31].
It seems that only di- and tripeptides, which remain after luminal and brush-border peptidase digestion, are absorbed intact [32]. Tetra- and higher peptides appear to require brior brush-border hydrolysis before their hydrolysis products can be absorbed [32]. While the starter protein and method of hydrolysis affect absorptive characteristics, the peptide-chain length is the most important variable. Protein hydrolysates produced from various sources showed increased amino acid absorption in humans when the propotion of di- and tripeptides was increased [32]. Thus, in order to maximize absoption rate, the ideal protein hydrolysate should contain mainly di- and tripeptides. Such a protein hydrolysate seems to produce the most immediate hyperaminoacidemia. In general, it is the kinetics of the absorption (rather than the net absorption of amino acids) that determines the greater nutritional value of the protein hydrolysates. The use of a protein hydrolysate in the post-exercise drinks is preferred because it results in a faster increase in plasma amino acid concentrations during a 2-h period than does intact protein [14], and in turn the levels of essential amino acids in the blood regulate muscle protein synthesis [33]. A practical advage is that one can ingest a protein hydrolysate-containing supplement immediately after exercise without getting bloated and not excessively suppressing appetite, so one can eat another meal sooner, possibly optimizing the post-exercise “anabolic window”. In addition, protein hydrolysate ingestion has a strong insulinotropic effect [14-18].
Clearly, hydrolyzed whey protein is the most popular protein hydrolysate among athletes. Whey protein has been singled out as the ultimate source of protein based on an excellent amino acid profile [26,34]. Whey may offer other benefits too [34-39]. Casein hydrolysate is also utilized in some commercial protein mixtures. It should be realized that the biological value of hydrolyzed collagen (also known as gelatin) is zero; thus, collagen supplementation as a protein source is not recommended. However, it has been suggested that hydrolyzed collagen may be useful in counteracting degenerative joint diseases [40,41]. Finally, some commerical products are enriched with wheat gluten hydrolysate (i.e., “glutamine peptides”). Wheat gluten has a unique amino acid profile: glutamyl residues accont for about 40% of the amino acids [42]. Glutamine is an important fuel for some cells of the immune system and may have specific immunostimulatory effects [43].
It is worth noting that “classical” model of protein metabolism, which views nitrogen intake in terms of the flux of free amino acids from dietary protein and their exhange between plasma and intracellular compartments and between free and protein-bound amino acids is misleading, because it ignores the flux of amino acids trough intermediate pools of small peptides [32].
Effects of insulinotropic nutritional mixtures on insulin secretion and nitrogen utilization
A study by Calbet and MacLean was implemented to determine the effects different protein-containing solutions have on insulin response and amino acid availability in healthy humans [44]. Four different 600 mL solutions were used. The glucose solution (control) contained only glucose (25 g/L), and the three additional solutions contained the same quantity of glucose plus protein (0.25 g/kg body mass) but proteins were derived from different sources: whey hydrolysate, pea hydrolysate, and a complete cow´s milk solution. This study indicated that:
1. Ingestion of glucose and protein hydrolysate results in a synergistic and fast increases in plasma insulin. In fact, protein hydrolysates stimulated an increase in plasma insulin that was two and four times greater than that produced by the intact milk protein solution and glucose solution, respectively.
2. Protein hydrolysates are absorbed at a faster rate from the small intestine than are intact milk proteins, as reflected by the rapid increase in the plasma concentration of branched-chain amino acids in peripheral blood.
3. Whey protein hydrolysate elicited the greatest availability of amino acids during the three-hour postprandial period. The authors attributed this difference to the rapid increase in plasma amino acids evoked during the first 40 minutes of the digestive period, during which the increase was about 37% greater after the ingestion of whey protein hydrolysate solution than that after ingestion of the intack milk protein solution.
It is likely that the high levels of plasma amino acids and increased insulin explains a superiority of protein hydrolysates over intact proteins in promoting better nitrogen utilization. The co-ingestion of carbohydrate appears to affect the absorption kinetics, as one study showed that whey and casein proteins and their respective hydrolysates administered alone produce similar rates of intestinal absorption of amino acids [45]. Alternatively, it is possible that this study used protein hydrolysates containing mainly oligopeptides.
More recently, Kaastra et al. determined the extent to which the combined ingestion of high-glycemic carbohydrates (CHO) and a casein protein hydrolysate with or without additional free leucine can increase insulin levels during post-exercise recovery [17]. Fourteen male athletes were subjected to three randomized crossover trials in which they performed 2 h of exercise. Thereafter, subjects were studied for 3.5 h during which they ingested CHO only (0.8 g/kg/h), CHO + protein hydrolysate (0.8 and 0.4 g/kg/h, respectively), or CHO + protein hydrolysate + free leucine (0.8, 0.4, and 0.1 g/kg/h, respectively) in a double-blind fashion. The results revealed that plasma insulin responses were 108% and 190% greater in the CHO + protein hydrolysate and CHO + protein hydrolysate + leucine trial, respectively, compared with the CHO only trial. This study also indicated that addition of free phenylalanine, as applied in earlier studies [15,16], is not necessary to obtain such high post-exercise insulin responses.
Similarly, Manders et al. examined plasma insulin responses after co-ingestion of casein protein hydrolysate with and without additional free leucine with a single bolus of high-glycemic CHO [18]. Again, the subjects participated in 3 trials in which blood insulin responses were determined after the ingestion of beverages of different composition: CHO only (0.7 g/kg), CHO + protein hydrolysate (0.7 and 0.3 g/kg, respectively) or CHO + protein hydrolysate + free leucine (0.7, 0.3 and 0.1 g/kg, respectively). The result indicated that plasma insulin responses were 66 and 221% greater in the healthy subjects in the CHO + protein hydrolysate and CHO + protein hydrolysate + free leucine trials, respectively, compared with those in the CHO only trial. In other words, this study also demostrated that co-ingestion of a protein hydrolysate with additional leucine strongly augments insulin secretion after the consumption of a single bolus of CHO.
This is in line with the data by Calbet and Holst, who reported that whey and casein hydrolysates elicited about 50% more gastric secretion than intact protein solutions, which was accompanied by higher glucose-dependent insulinotropic polypeptide (GIP) plasma levels during the first 20 min of the gastric emptying process [45]. Besides well-known effects on pancreatic beta cells, GIP also has direct metabolic effects on other tissues and organs, such as muscle, liver and adipose tissue, with most of its functions tending to increase anabolism.
The notion that the protein hydrolysates have strong insulinotropic properties is also supported by the studies examing the effects of intact protein-containing post-exercise drinks. Ivy et al. compared effects CHO + intact protein (80 g of CHO, 28 g of protein, 6 g of fat), low-CHO (80 g of CHO, 6 g of fat), or high-CHO (108 g of CHO, 6 of fat) and reported that plasma insulin levels did not differ at any time among treatments [46]. However, Zawadzki et al. observed that plasma insulin levels for the CHO + intact protein treatment (112 and 40.7 g, respectively) were somewhat higher than those for the CHO treatment (112 g of CHO) [47].
A post-exercise drink containing a mixture of free amino acids also has a potent effect on insulin secretion [6]. However, a large dose of amino acids can cause gastrointestinal discomfort. This may have something to do with the drink´s osmolarity. A protein hydrolysate containing di- and tripeptides reduces osmolarity because equal solution weights of di- and tripeptides have one and one-third the osmolarity of free amino acids, respectively [32].
Effects of insulinotropic nutritional mixtures on post-exercise muscle anabolism
A sophisticated study by Koopman et al. investigated post-exercise muscle protein synthesis and whole body protein balance following the combined ingestion of high-glycemic CHO with or without whey protein hydrolysate and/or leucine [16]. Their nutritional protocol was rather rigorous; the subjects received a beverage volume of 3 ml/kg every 30 min to ensure a given dose of 0.3 g high-glycemic CHO/kg and 0.2 g/kg of a protein hydrolysate every hour, with or without the addition of 0.1 g/kg/h free leucine. Repeated boluses were taken every 30 minutes until t = 330 minutes after exercise. The results revealed that the whole body protein synthesis rates were highest in the CHO + protein hydrolysate + leucine trial: 95.6 ± 0.1 vs. 92.0 ± 0.4 and 94.2 ± 0.4% in the CHO and CHO + protein hydrolysate trials, respectively. Similarly, fractional synthetic rate (FSR) in the vastus lateralis muscle was significantly greater in the CHO + protein hydrolysate + leucine trial compared with the CHO trial (0.095 ± 0.006 vs. 0.061 ± 0.008%/h, respectively), with intermediate values observed in the CHO + protein hydrolysate trial (0.0820 ± 0.0104%/h).
Furthermore, the investigators found plasma insulin responses to be negatively correlated with whole body protein degradation, whereas whole body protein synthesis was positively correlated with plasma insulin response. However, FSR did not correlate with the plasma insulin response, whereas mixed muscle protein FSR did correlate with the amount of leucine that was ingested. It is difficult to interpret these results given the massive supplementation. Nevertheless, the authors concluded that, ”the additional ingestion of free leucine in combination with protein and carbohydrate likely represents an effective strategy to increase muscle anabolism following resistance exercise.” Other recent studies have shown that relatively small doses of leucine can improve exercise performance [48] and enhance the acquisition of strength [49].
Although the Koopman study indicates that dietary supplementation-induced post-exercise hyperinsulinemia plus hyperaminoacidemia can have favourable effects on the acute phase response to resistance training, the effects of repeated supplementation on long term adaptations to resistance training are currently unclear. To shed some light on this issue, Bird et al. examined the effects of chronic high-glycemic CHO and/or essential amino acid supplementation on hormonal and muscular adaptations in untrained young men [50]. All subjects followed the same supervised, resistance training protocol two times per week for 12 weeks. Following resistance exercise, the subjects consumed either a high-glycemic CHO, a essential amino acid (6 g), a combined high-glycemic CHO + essential amino acid supplement, or a placebo containing only aspartame and citrus flavouring. The results revealed that CHO + essential amino acid supplementation enhances muscular and hormonal adaptations to a greater extent than either CHO or essential amino acids consumed independently. Specifically, CHO + essential amino acid ingestion demonstrated the greatest relative increase in type I muscle fiber cross sectional area. Changes in type II muscle fibers exhibited a similar trend.
While beyond a scoop of this paper, it is very likely that chronic reductions in the exercise-induced cortisol response associated with post-exercise CHO-amino acid ingestion also positively impact the skeletal muscle hypertrophic adaptation to resistance training via reductions in hormone-mediated protein degradation. For reviews, see the recent papers by Volek [51], Kraemer and Ratamess [52], and Crewther et al. [53].
You can have your protein shake and drink it too
Contrary to some belief, higher protein intake has no adverse effects on healthy kidneys [54,55], fluid status [56], or bone [57-60]. In fact, proteins appears to have positive effects on bone health, as they increase circulating insulin-like growth factor I (IGF-1), which plays an important role in bone formation [58]. For example, Ballard et al. reported that a protein supplement during a strength and conditioning program led to an increase in plasma concentrations of IGF-I in those subjects compared with the concentrations in a group of persons who also trained but consumed a isocaloric CHO supplement [60]. Also, serum bone alkaline phosphatase concentrations increased over time and tended to be higher in the protein group than in the CHO group, indicating increased bone formation.
In addition, IGF-I plays a critical role in development, growth, repair, and maintenance of skeletal muscle [61]. Thus, IGF-I may partially explain why many strenght-power athletes (especially bodybuilders) feel that a very high protein intake is beneficial for skeletal muscle hypertrophy. Indeed, studies indicate increased positive nitrogen balance when protein intake is increased [62]; however, more reseach is clearly needed before the mystery of protein requirements in those attempting to increase muscle mass is settled [62-65]. Traditionally, the term "protein requirement" have meant the amount of dietary protein that must be consumed to provide the amino acids needed for the synthesis of those proteins irreversibly catabolized in the course of the body's metabolism. It should be noted, however, that the strength-power athletes are not concerned with the minimum amount of protein necessary to sustain normal body functions, but rather, with their absolute gains in muscle mass and strength. Other potential benefits of higher protein intake should be considered too [66-70].
Interestingly, a recent placebo-controlled study by Flakoll et al. reported that post-exercise protein-CHO supplementation 1) reduces bacterial/viral infections, 2) decreases medical visits due to muscle or joint problems, 3) diminishes episodes of heat exhaustion, 4) reduces muscle soreness, and 5) improves rifle scores in US Marine recruits during basic training [71].
Effects of post-exercise hyperinsulinemia on fat oxidation and de novo lipogenesis
The chief lipid-related functions of insulin are inhibition of lipolysis and lipid oxidation (at 13 and 44 mU/mL, respectively) [72]. When insulin concentration drops below 13 mU/mL, lipolysis is powerfully and exponentially stimulated [72]. Volek et al. reported that a very-low-carbohydrate diet significantly decreased serum insulin (-34%), and that about 70% of the variability in fat loss was accounted for by the decrease in serum insulin concentrations [73]. Further, exogenous insulin promotes body fat accumulation [74], so one could speculate that insulitropic supplements have similar effects. However, I feel this is hardly a concern for the healthy athletes when these supplements are ingested immediately after rigorous exercise when the muscle cells are highly receptive to insulin [75] and “screaming” for new fuel. The physiological state of that of a sedentary individual and that of a well-trained athlete following exercise are polar opposites. AMP-activated protein kinase acts as a “metabolic switch” in multiple tissues after exercise; the net effect of its activation is to increase fatty acid oxidation and diminish glycerolipid synthesis [76].
To investigate the hormonal and metabolic adaptations occurring when high glycemic CHO are ingested after exercise, Krezentowski et al. compared the fate of a 100-g oral glucose load in healthy volunteers after an overnight fast at rest either without previous exercise or after a 3-h exercise performed on a treadmill at about 50% of the individual VO2max [77]. Indirect calorimetry indicated that glucose ingestion in post-exercise recovery was associated with decreased CHO oxidation and increased lipid oxidation when compared to control conditions. More recently, Folch et al. reported that de novo lipogenesis is totally suppressed following exercise, even when a very large CHO load is ingested, and that fat oxidation remained high in subjects who had exercised following both the small and large CHO meal [78]. Finally, Bird et al. observed that post-exercise ingestion of high-glycemic CHO do not inhibit resistance training-induced fat loss [50].
Certainly, this author is not suggesting that insulinotopic supplements should be used while watching TV. It is possible that hyperinsulenemic condition prevalent in obese, insulin-resistant individuals is responsible for the repartitioning of fatty acids away from oxidation and toward storage [79].
Anti-inflammatory effects of insulin
Recently, Dandona et al. proposed that insulin is the ideal anti-inflammatory agent for critically ill patients, because it normalizes plasma glucose concentrations (glucose is proinflammatory) while exerting its anti-inflammatory effect [80]. In addition, insulin suppresses generation of reactive oxygen species (ROS) and the expression of p47phox, a key component of NADPH oxidase, the enzyme that generates the superoxide radical [80]. Thus, insulin has antioxidant effects too. Since strenous exercise produces muscle inflammation [81] as well as increases generation of ROS [82], it is possible that post-exercise hyperinsulinemia offers additional benefits beyond muscle protein synthesis.
Conclusions
The studies reviewed here indicate that nutritional mixtures containing protein hydrolysates, added leucine, and high-glycemic carbohydrates strongly augment insulin secretion compared with the high-glycemic carbohydrates only trial. When post-exercise hyperinsulinemia is supported by protein hydrolysate and leucine ingestion-induced hyperaminoacidemia, net protein deposition in muscle should occur. Thus, post-exercise recovery drinks containing these nutrients may lead to increased skeletal muscle hypertrophy and strength in conjuction with appropriate resistance training. If so, such post-exercise supplements would be of considerable benefit not only to athletes but also to any invidividual who have lost muscle function due to disease (e.g., Duchenne muscular dystrophy). Future studies should evaluate their long-term effects on body composition and exercise performance.
What is already know on this topic
Both increased insulin and increased availability of amino acids are important to maximize the muscle protein anabolism.
What this study adds
This paper proposes that post-exercise hyperinsulinemia supported by protein hydrolysate and leucine ingestion-induced hyperaminoacidemia increases net protein deposition in muscle, leading to increased skeletal muscle hypertrophy and strength in conjuction with appropriate resistance training.
Competing interests
The author is a consultant to BioQuest Pharmaceuticals, Inc.
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