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Protein in Combination with Carbohydrate. It is known that during the recovery period after resistance exercise, muscle protein synthesis is stimulated (1). Also, current research supports the idea that protein or amino acid ingestion is required to enhance muscle protein synthesis after resistance exercise and thus encourage positive net protein balance (2, 3). Since carbohydrate ingestion has been witnessed to increase net protein balance by attenuating exercise-induced protein breakdown (4, 5), some researchers examined the effects of coingestion of protein and carbohydrates after resistance training (6, 7).
A study conducted in 2005 sought to determine differences in muscle synthesis and net protein balance after an acute resistance exercise while ingesting various mixtures of proteins with carbohydrates or carbohydrate alone (6). Koopman et al used eight subjects (n = 8) who consumed one of three supplemental drinks after resistance exercise (leg press, leg extension; 8 sets for 8 repetitions; 80% of 1 RM; 2 min rest), which took 45 min to complete. The supplemental drinks were CHO+PRO+Leu (0.3+0.2+0.1 g/kg/h, respective), CHO+PRO (0.3+0.2 g/kg/h, respective), or CHO (0.3 g/kg/h). The protein source was whey protein hydrolysate and carbohydrates (sucrose and maltodextrin). Each subject was treated to every trail on three separate occasions, with a separation of >7 d between each new trail. Every trail took approximately 8 h and continual boluses of the test drink was ingested after exercise to ensure consistent nutrient provision.
After treatment was complete, several measures were assessed. Although CHO+PRO+Leu, compared to CHO+PRO and CHO, presented the lowest glucose response, it elicited the greatest plasma insulin level ( +240% vs CHO; +77% vs CHO+PRO) (Fig. 1B), expressed as area under the curve (AUC). Plasma essential amino acid response (also measured using AUC) in CHO was negative while CHO+PRO+Leu and CHO+PRO were significantly positive (Fig. 3). Valine in CHO+PRO+Leu was the only amino acid with a negative response and was not addressed by the authors.
Over the 6 h period of recovery period, both CHO+PRO+Leu and CHO+PRO presented significantly lower whole body protein breakdown, higher protein synthesis and higher protein balance compared to CHO (Fig. 5). Whole body phenylalanine oxidation was lowest, and only significant in CHO+PRO+Leu. It was found that protein breakdown is negatively correlated with insulin and leucine (r = -0.67, -.647, respectively), while net protein balance was directly correlated (r = 0.678, 0.626, respectively). This means higher blood insulin and leucine concentration results in low protein breakdown and higher net protein balance. Mixed muscle protein fractional synthesis rate (FSR) was also calculated. It was found that CHO+PRO+Leu was the only trail that showed a significant increase compared to CHO (+40% FSR vs CHO) (Fig. 6).
Results from the Koopman et al study just presented (6) indicate that the addition of 0.2 g/kg/h of protein to 0.3 g/kg/h carbohydrate will elicit favourable responses in whole body protein breakdown, protein synthesis and net protein balance. However, 0.1 g/kg/h of leucine will elicit further benefits in measures of FSR and whole body protein oxidation. The benefit of leucine may be its ability act as a nutrient substrate as well as a signal for mTOR pathway, however, protein synthesis requires all amino acids to be available (8)
From the above, ingesting carbohydrates with protein after resistance exercise clearly is able to increase muscle synthetic rate. As a benefit for athletes, carbohydrate ingestion aids in glycogen resynthesis (9). However, in populations such as dieting physique athletes or obese individuals attempting to improve health, where caloric intake must be kept low yet muscle synthesis is beneficial, the addition of carbohydrates may be a detriment since it increases caloric intake. The following study from the same laboratory addresses this issue.
In 2007, Koopman et al produced another study which set out to determine whether carbohydrates would further increase muscle protein synthesis after exercise when high protein amounts are ingested concurrently (7). The researchers had ten participants (n = 10) perform an acute bout of full body resistance exercise. Exercises consisted of, for the upper body (3 x 10 @ 40% - 50% of 1RM, 1 min rest): chess press, shoulder press and front pulldown, and for the lower body (8 x 10 @ 75% of 1RM, 2 min rest): leg press and leg extension. Approximately 1 h was needed to complete this exercise protocol. Following which, participants received one of three of the treatment drinks during a 6 h recovery period: protein only (PRO = 0.3 g PRO/kg/h + 0 g CHO/kg/h), protein with low carbohydrate (PRO+LCHO = 0.3 g PRO/kg/h + 0.15 g CHO/kg/h), or protein with high carbohydrate (PRO+HCHO = 0.3 g PRO/kg/h + 0.6 g CHO/kg/h). The protein type used was hydrolysed casein and the carbohydrate type was a combination of glucose and maltodextrin.
There were several differences after provision of the dietary treatment. It was found that PRO+HCHO elicited the highest plasma glucose rate of appearance (Ra) but also the highest plasma glucose rate of disappearance (Rd) (Fig. 2). Despite this, the same group also saw the highest glycemic levels of all and greatest insulin response (Fig. 1). The PRO only group showed the lowest in all of the above mentioned measures (Fig. 1, 2).
Plasma amino acid concentrations of phenylalanine, tyrosine, leucine, valine and isoleucine increased after ingestion of the protein drinks (Fig. 3). However, the PRO+HCHO group saw significantly reduced plasma levels of the branched-chain amino acids (BCAA): valine, leucine, and isoleucine. In fact correlational calculations confirm an inverse relationship between insulin response and plasma levels of leucine, valine, and isoleucine (r = -0.61, -0.57, -0.53, respective). In figure 4, the time course of the three plasma amino acid enrichments can be seen and no differences were found among treatments. Muscle analyses also show no difference among groups in muscle free phenylalanine and tyrosine, while muscle free leucine, isoleucine and valine levels were significantly increased in PRO. This may be because the provision of glucose attenuates BCAA oxidation due to reduced delivery (10).
Since the greatest level of hyperinsulinemia was observed concurrently with sufficent hyperaminoacidemia (11) in PRO+HCHO, it may be expected that this group would have the highest net protein balance. This was not the case. Researchers observed no differences between PRO, PRO+LCHO, and PRO+HCHO with respect to whole body protein breakdown, whole body protein oxidation and synthesis rates. As a result, no differences were seen in net protein balance as well (Fig. 5A). There was also no difference observed in fractional synthesis rate (Fig. 5B).
Some research has pointed to the fact that ingesting carbohydrate causes insulin to increase in the blood, thus reducing muscle breakdown after exercise and consequently improving net protein balance (12). Since protein provision also helps improve net protein balance by stimulating muscle protein synthesis (13), it stands to reason that a combination of carbohydrate and protein would increase net protein balance. This notion is supported by several studies (6, 14, 15, 16, 17). However the study just recently visited (7) provided far more protein than other studies and provides evidence that muscle protein synthesis and net protein balance cannot be further enhanced through carbohydrate intake when protein adequately ingested. That is, protein intake of 0.3 g/kg/h can maximize muscle protein synthesis and net protein balance over a 6 h period without assistance from the insulinogenic effects of carbohydrate ingestion.