Protein Pancakes :)

This study was designed to determine whether the response of muscle protein metabolism to an EAC solution was different if consumed immediately before resistance exercise rather than immediately after resistance exercise. Ingestion of EAC changed net muscle protein balance from negative values, i.e., net release, to positive net uptake, in both trials. However, the total response to the consumption of EAC immediately before exercise was greater than the response when EAC was consumed immediately after exercise. Furthermore, it appears that the change from a catabolic state in the muscle to an anabolic state was primarily due to an increase in muscle protein synthesis.

In the present study, the effectiveness of the drink appeared to be greater when it was consumed immediately before exercise (PRE) compared with immediately after exercise (POST). Approximately 209 ± 42 mg of phenylalanine were taken up across the leg in the PRE trial, whereas only 81 ± 19 mg of phenylalanine were taken up during POST. Whereas the response of muscle protein metabolism increased dramatically and then declined within 1 h to basal levels after EAC consumption in the POST trial, the response was sustained in the PRE trial. Net balance increased during exercise, declined slightly, and then increased a second time after exercise when the drink was consumed before exercise. The length of the effect, plus higher blood flow during exercise in the PRE trial, resulted in significantly greater total uptake over the entire study period.

In this study, the primary end point was to examine the impact of the timing of EAC ingestion in relation to resistance exercise on net muscle protein synthesis and, as a result, the accretion of muscle. Thus the response over the entire 3-h study period is the most appropriate to compare between trials. On the other hand, it could be argued that the results are biased toward the PRE trial by calculating the data over the entire 3-h study period. During PRE, the entire 3 h follows the consumption of EAC, whereas during POST, only 2 of the 3 h follow EAC ingestion. As a result, we also calculated the uptake across the leg over only the final 2 h after exercise of each trial, i.e., the 2nd and 3rd h after EAC ingestion during PRE and the 1st and 2nd h after EAC ingestion during POST. Calculated this way, the gap between the trials narrowed, but the mean uptake across the leg was still 80% greater for PRE than for POST (244 ± 120 mg vs. 130 ± 45 mg, respectively), although the difference did not reach statistical significance (P = 0.09). If anything, comparing only the final 2 h of each trial biases the results toward favoring the POST trial, because the 1st h after consumption of EAC during PRE is ignored. Nonetheless, it is still evident that consuming EAC before exercise is more effective than after exercise.

Effectiveness of the timing of EAC ingestion is supported by comparing the amount of phenylalanine taken up by the leg to the amount ingested in each trial. During PRE, ∼21% of ingested phenylalanine was taken up by the leg, thus ∼42% by both legs. The proportion was much lower during POST, ∼8% across one leg or 16% for both legs. When EAC was consumed 1 h after exercise, ∼125 mg of phenylalanine were taken up across the leg (21), or about one-half of the value found when EAC was consumed before exercise. This represented ∼11% of the ingested phenylalanine for one leg, or 22% for both legs. When amino acids were infused over a 3-h period after exercise, ∼34% of the infused amino acids were taken up across both legs (6). Clearly, EAC consumption before exercise is more effective than after exercise.

These data do not allow us to determine definitively the reasons for the greater response of net muscle protein synthesis to consuming essential amino acids plus carbohydrates immediately before exercise rather than after exercise. However, it is likely that the greater delivery of amino acids to the muscle during PRE accounts for the greater net uptake than during POST. During exercise in the POST trial, net muscle protein balance, as well as phenylalanine Rd, an index of muscle protein synthesis, was unchanged, whereas in the PRE trial, phenylalanine Rd and NB were increased. Consuming a source of amino acids before exercise increases amino acid availability. Providing amino acids at a time when blood flow is elevated, such as during the exercise bout, maximizes delivery to the muscle. Previous studies have demonstrated that muscle protein synthesis is related to amino acid delivery to the leg (5, 6,27). Phenylalanine delivery during exercise in the PRE trial was increased 6.5-fold over resting levels and was more than twice that of POST. Furthermore, delivery remained elevated after exercise during PRE to a significantly greater extent above that during POST. Similarly, in our previous study, amino acid delivery was increased by EAC ingestion at both 1 and 3 h postexercise (21) to levels comparable to those obtained when EAC was consumed immediately after exercise. Thus consumption of amino acids before exercise results in greater amino acid delivery than when they are consumed at various time points after exercise, likely accounting for the greater response of net muscle protein synthesis demonstrated during the PRE trial.

Previously, we showed that hyperaminoacidemia elicited by intravenous infusion of mixed amino acids (6) and oral ingestion of both mixed and essential amino acids (27) resulted in net muscle protein synthesis after resistance exercise. In these studies, ∼40 g of amino acids were provided steadily over a 3-h period. We also demonstrated that nonessential amino acids are unnecessary to stimulate net muscle protein synthesis at rest (28) or after exercise (27). Subsequently, we examined the response of muscle protein metabolism to ingestion of a smaller amount of essential amino acids plus carbohydrates (21) identical to the one used in the present study. Similar levels of net muscle protein synthesis resulted when subjects consumed the bolus amino acid-carbohydrate solution at both 1 and 3 h after exercise (21). Taken together with the present results, it is clear that a relatively small amount of essential amino acids, combined with carbohydrates, is a potent stimulator of net muscle protein synthesis when given either before or at various times after resistance exercise.

It is not possible to delineate the effectiveness of the separate components of the drink from this study. We have previously demonstrated that muscle protein synthesis is stimulated by essential amino acids alone (27, 28). Even single essential amino acids in a flooding dose may stimulate muscle protein synthesis (24). It is more difficult to assign a role to insulin in the change from net negative protein balance to positive protein balance. After exercise, insulin seems to be necessary for protein synthesis to occur (11, 12, 14), yet increased insulin does not stimulate muscle protein synthesis (7). However, elevated insulin after resistance exercise does diminish the increase of muscle protein breakdown in response to exercise (7). Consistent with this notion, during the present study, phenylalanine Ra, an index of muscle protein breakdown, did not increase after exercise in either trial. Thus stimulation of muscle protein synthesis by essential amino acids, in addition to inhibition of the normal postexercise rise in breakdown, likely accounts for the effectiveness of the EAC drink for stimulating net muscle protein synthesis after resistance exercise.

Determination of the response of the muscle in the present study is based primarily on uptake of phenylalanine across the leg. It is assumed that phenylalanine uptake corresponds to accretion of muscle protein. However, it is possible that all of the amino acids taken up by the muscle are not incorporated into protein, but instead some fraction of the uptake simply expands the muscle free intracellular pool. The amino acids could then be released at some time after the conclusion of the measurements, without ever being utilized for muscle protein synthesis. Thus it is possible that net uptake overestimated the extent of net muscle protein synthesis. However, even if we assume the unlikely circumstance that all of the phenylalanine remaining in the muscle intracellular pool at the conclusion of the study would be subsequently released, the amount does not appear to be a substantial proportion of that taken up by muscle, especially in the PRE trial. During PRE, 24 ± 3 mg of phenylalanine were taken up by muscle but not utilized for protein synthesis, in contrast to 42 ± 8 mg during POST. Thus the total amount of phenylalanine taken up by the leg and utilized for protein synthesis was ∼180 mg (∼86% of total uptake) during PRE and ∼39 mg (∼48% of total uptake) during POST. Clearly, even with this conservative estimate, a large proportion of the phenylalanine taken up by muscle was, in fact, utilized for muscle protein synthesis during the study, further supporting the notion that the EAC solution is an effective stimulator of muscle protein anabolism.

In the fasted state, muscle protein breakdown exceeds muscle protein synthesis, resulting in a net negative muscle protein balance. Net positive muscle protein balance can result only from an increase in muscle protein synthesis and/or a decrease in muscle protein breakdown. Resistance exercise alone has been shown to increase muscle protein synthesis, but breakdown is also increased, such that net muscle protein balance remains negative (5). Additionally, net muscle protein synthesis as a consequence of hyperaminoacidemia after resistance exercise is primarily due to increased muscle protein synthesis (6, 27). In our previous study, increased muscle protein synthesis was responsible for the change from a catabolic to an anabolic state after ingestion of EAC at both 1 and 3 h postexercise (21). Similarly, in the present study, it is likely that the increase in NB from negative to positive after EAC consumption in both trials was also primarily due to an increase in muscle protein synthesis. Mean Rd, i.e., uptake of amino acids from the plasma pool, increased dramatically (216 and 200% for PRE and POST, respectively) after ingestion of EAC. The fact that phenylalanine Ra, an indicator of muscle protein breakdown, did not change in response to EAC ingestion further supports the notion that the change of net muscle protein balance from positive to negative is primarily due to an increase in protein synthesis.

In the present study, our arteriovenous tracer methodology has quantified only the fate of blood-borne amino acids (25,29). Because the incorporation of amino acids from the EAC solution into muscle protein was of primary interest, Rdand Ra calculated using blood-borne amino acids seemed the most appropriate measures. In past studies we have utilized a three-compartment model of muscle protein metabolism to describe the effects of nutrition and exercise on muscle protein synthesis and breakdown (3, 5, 6, 14, 15, 27). However, in the present study, the combination of sampling in close proximity to exercise and a bolus ingestion of amino acids has made the use of that model problematic. That model requires an isotopic and physiological steady state, as well as a measurable gradient between blood and intracellular phenylalanine enrichment. Instead, we calculated Ra and Rd by use of data only from blood (25, 29). Whereas care must be taken in interpreting Ra and Rd values from this model (3, 30), it is the appropriate model to use in the present study. The importance of the plasma amino acids as a source for muscle protein synthesis is emphasized in this study. Therefore utilization of Rd was the appropriate parameter with which to compare the effects of the timing of ingestion of the EAC drink. Moreover, utilization of the blood-borne precursor for measurement of Rd allows us to relate these values to net muscle protein synthesis determined by phenylalanine uptake.

The ingestion of a relatively small amount of essential amino acids, combined with carbohydrates, is an effective stimulator of net muscle protein synthesis. The stimulation of net muscle protein synthesis when EAC is consumed before exercise is superior to that when EAC is consumed after exercise. The combination of increased amino acid levels at a time when blood flow is increased appears to offer the maximum stimulation of muscle protein synthesis by increasing amino acid delivery to the muscle and thus amino acid availability.