Comment of the efficacy of protein nutrition for enhancing aerobic exercise adaptation. Leigh Breen, University of Birmingham 18 March 2013 Over the last decade a growing number of researchers have sought to determine the benefits of protein nutrition for endurance athletes with regard to performance, recovery and adaptation. Due to conflicting findings, a clear consensus has yet to be reached. In a recent JISSN publication, Hill et al. (2013) investigated whether 2 weeks of a high carbohydrate provision supplemented with a whey protein isolate (CHO+WPI), enhanced the adaptive response to endurance training in young cyclists/triathletes, compared with an energy-matched high carbohydrate, `normal¿ protein diet (CHO). Protein intakes were 2.4 and 1.2 g/kg/d for CHO+WPI and CHO, respectively, with participants completing both dietary interventions spaced by 4 weeks. Following the 2 wk dietary intervention, cycling time-to-exhaustion (TTE) and intramuscular signaling for mitochondrial biogenesis were measured over 6 h of recovery, during which participants ingested their respective treatment. Despite a ~10% increase in TTE duration for CHO, the authors reported no difference in TTE between groups. The small sample size (n = 6) may have obscured the authors from demonstrating a difference in TTE. Discrepant with the TTE data, muscle PGC-1¿ mRNA expression was greater for CHO+WPI vs. CHO at 6h post-TTE, leading the authors to conclude that CHO+WPI might have important implications for endurance exercise adaptation at the skeletal muscle level. Recommending nutritional strategies thought to confer an adaptive advantage, based on a `snapshot¿ of a single mRNA target for mitochondrial biogenesis is somewhat misleading, particularly when proteins underpinning muscle mitochondrial adaptations (i.e. TFAM, COXIV, p53 etc.) are not analyzed or do not support the conclusions; as is the case in the study of Hill et al (2013). Given our relatively poor understanding of the temporal time-course of intramuscular signaling expression in response to exercise and/or nutrient stimuli, it is difficult to form solid conclusions from acute signaling data alone. Hill et al. (2013) infer that WPI supplementation over 2 weeks was sufficient to induce a greater acute PGC-1¿ expression. However, given the study design it is impossible to distinguish whether the reported additive effect of WPI on post-TTE PGC-1¿ expression was the result of i) 2 wk of prior WPI supplementation, ii) the acute dietary provision over 3 h post-TTE or iii) a combination of both. In order to assess the efficacy of acute provision of CHO vs. CHO+WPI for mitochondrial signalling, prior diet should have been standardized across groups, rather than manipulated. Likewise, in order to directly assess the efficacy of 2 weeks of supplementary WPI on mitochondrial signalling, post-TTE nutrition should have been standardized across groups, or removed altogether (and replaced with a non-caloric placebo). From a scientific standpoint the lack of information provided by Hill et al. (2013) pertaining to the training load completed prior to each TTE trial is of concern. Participants were instructed to continue their habitual training regimen throughout the 2 wk dietary intervention, apparently without any monitoring. Information on the mode, duration, intensity and frequency of the training over the 2 weeks is not provided. More importantly, there is no mention as to whether training variables were recorded and replicated for the crossover trial. Finally, there is no indication of the duration of exercise refrainment prior to the TTE trial. Thus, although divergent nutritional strategies were successfully implemented, there is less certainty that physiological conditioning was matched across trials; which may have adversely affected TTE and intramuscular signaling. In addition, there is no indication of the success of treatment blinding or ingestion of post-training supplements. Knowledge of the treatment order, particularly in well-trained individuals, may have influenced training output during the 2 wk intervention. Standardizing and monitoring training sessions would have resolved these methodological issues. In contrast with Hill et al. (2013), we recently demonstrated, in trained cyclists, that 25g of whey protein co-ingested with 50g of carbohydrate following a prolonged, moderate intensity (70% VO2peak) bout of cycling increased the synthetic rate of muscle myofibrillar (i.e. contractile), but not mitochondrial proteins (Breen et al., 2011) compared with the ingestion of carbohydrate alone. Unlike Hill et al. (2013) pre-exercise dietary intake in our study was similar between conditions, allowing us to determine the isolated effects of each post-exercise treatment on sub-fractional protein synthesis rates. We concluded that muscle mitochondrial adaptation to exercise might be limited by mitochondrial pool size (~5-10% of muscle protein), a notion that was subsequently highlighted by Moore & Stellingwerff (2012). For example, amino acids released via proteolysis during a demanding endurance exercise bout may be sufficient to maximize post-exercise mitochondrial protein synthesis rates, such that provision of protein nutrition does not heighten the response. Alternatively, protein provision following demanding endurance exercise may assist in the remodeling of `damaged¿ muscle proteins, which may explain the greater increase in myofibrillar (~65% of total muscle protein) protein synthesis we observed. In spite of our findings (Breen et al., 2011) and the points raised on the work of Hill et al. (2013), there is emerging evidence that protein supplementation may enhance endurance exercise adaptation. Moreover, recent evidence has shed light on the potential mechanism facilitating the additive effect of protein, which does not involve enhanced adaptation at the muscular level. Ferguson-Stegall et al. (2011) recently demonstrated that immediate post-exercise ingestion carbohydrate-plus-protein ingestion over 4.5 weeks of training resulted in greater gains in VO2 compared with CHO ingestion alone, albeit in previously untrained participants. In support of our work (Breen et al., 2011) and in contrast to Hill et al. (2013) the greater adaptation for carbohydrate-plus-protein could not be explained by changes in muscle oxidative enzymes (Ferguson-Stegall et al., 2011). An improved ability of the muscle to utilize oxygen is not the only determinant of improved aerobic capacity. Endurance exercise adaptation also stems from the ability of the cardiovascular system to transport oxygen to working skeletal muscle. Plasma volume expansion is a hallmark of cardiovascular adaptation to endurance exercise training (Sawka et al., 2000). Expansion of the plasma pool is directly linked with increased plasma albumin content, the effect of which causes water to be retained in the vasculature due to increases in osmotic pressure gradient (Convertino et al., 1980; Gillen et al., 1991). Hepatic albumin synthesis has been shown to increase in response to endurance exercise training (Yang et al., 1998; Nagashima et al., 2000). Intriguingly, recent evidence demonstrates that co-ingesting protein with carbohydrate in the post-exercise period augments the increase in plasma albumin content, compared with a non-caloric placebo (Okazaki et al., 2009; Goto et al., 2010). Thus, despite the absence of an acute or chronic muscle mitochondrial benefit with post-exercise protein provision, adaptations at the cardiovascular level may occur. Although preliminary evidence suggests post-exercise protein ingestion may enhance cardiovascular adaptations to endurance training, further studies are required to confirm the precise mechanisms. Dr Leigh Breen, Ph.D. The School of Sport and Exercise Sciences University of Birmingham Email: L.firstname.lastname@example.org Competing interests No competing interests to declare.