Efficacy and Safety of Protein Supplement Use by Military and Other Physically Active Populations Protein Supplementation for Military Personnel : A Review of the Mechanisms and Performance Outcomes 1 – 3

Protein supplement use is common among athletes, active adults, andmilitary personnel. This review provides a summary of the evidence base that either supports or refutes the ergogenic effects associated with different mechanisms that have been proposed to support protein supplementation. It was clear that if carbohydrate delivery was optimal either during or after an acute bout of exercise that additional protein will not increase exercise capacity. Evidence was also weak to substantiate use of protein supplements to slow the increase in brain serotonin and onset of central fatigue. It was also evident that additional research is warranted to test whether the benefits of protein supplements for enhancing recovery of fluid balance after exercise will affect subsequent work in the heat. In contrast, with repeated exercise, use of protein supplementation was associated with reductions in muscle soreness and often a faster recovery of muscle function due to reductions in protein degradation. There was also good supportive evidence for long-term benefits of protein supplementation for gains in muscle mass and strength through accelerated rates of protein synthesis, as long as the training stimulus was of sufficient intensity, frequency, and duration. However, studies have not examined the impact of protein supplements under the combined stress of a military environment that includes repeated bouts of exercise with little opportunity for feeding and recovery, lack of sleep, and exposure to extreme environments. Both additional laboratory and field research is warranted to help provide evidence-based guidance for the choice of protein supplements to enhance soldier performance. J. Nutr. 143: 1820S–1833S, 2013. Introduction Athletes and recreationally active adults commonly cite an increase in muscle mass, improved exercise recovery, and improved exercise capacity as reasons for use of protein supplements (1,2). Given the physical and cognitive demands experienced during military training and deployment (3,4), it is equally common that active duty personnel report frequent use of protein supplements (5). Marketing efforts assure consumers that protein supplements will act through $1 of the following mechanisms to produce the desired performance outcomes: 1) To reduce endogenous carbohydrate oxidation during prolonged exercise, thereby increasing exercise capacity; 2) To hasten the recovery of muscle glycogen stores reduced during previous exercise, thereby increasing exercise capacity during subsequent prolonged work; 3) To influence exercise capacity via a central mechanism that limits the increase in brain serotonin and thereby delays sensations of fatigue; 4) To enhance fluid retention during and/or after exercise in the heat to promote greater evaporative cooling during subsequent exercise; 5) To reduce rates of protein degradation after exercise, leading to less muscle soreness and a faster recovery of muscle function; 6) To hasten the accretion of myofibrillar and mitochondrial protein during training, thereby returning faster and greater increases in muscle strength and aerobic and anaerobic power. 1 Presented at the Efficacy and Safety of Protein Supplements for U.S. Armed Forces Personnel meeting, held at the U.S. Army Research Institute of Environmental Medicine, Natick, MA, 7–8 November 2012. The summit was sponsored by the Department of Defense, Center Alliance for Dietary Supplements Research. The views expressed in these papers are not necessarily those of the Supplement Coordinator or Guest Editors. The Supplement Coordinator for this supplement was Krista G. Austin, U.S. Army Research Institute of Environmental Medicine. Supplement Coordinator disclosures: Krista G. Austin had no conflicts to disclose. This supplement is the responsibility of the Guest Editor to whom the Editor of The Journal of Nutrition has delegated supervision of both technical conformity to the published regulations of The Journal of Nutrition and general oversight of the scientific merit of each article. The Guest Editor for this supplement was Kevin Schalinske. Guest Editor disclosure: Kevin Schalinske had no conflicts to disclose. Publication costs for this supplement were defrayed in part by the payment of page charges. This publication must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact. The opinions expressed in this publication are those of the authors and are not attributable to the sponsors or the publisher, Editor, or Editorial Board of The Journal of Nutrition. 2 T.M. M. was provided an honorarium by the U.S. Army Research Institute of Environmental Medicine to prepare this review. 3 Author disclosures: T. M. McLellan, no conflicts of interest. * To whom correspondence should be addressed. E-mail: drtom.mclellan@ gmail.com. ã 2013 American Society for Nutrition. 1820S Manuscript received February 20, 2013. Initial review completed April 26, 2013. Revision accepted June 13, 2013. First published online September 11, 2013; doi:10.3945/jn.113. 176313. by gest on O cber 1, 2017 jn.nition. org D ow nladed fom This review summarizes the evidence-based support for each of these proposed mechanisms as it relates to the purported functional outcomes and their relevance to military personnel. Reduce Endogenous Carbohydrate Oxidation to Increase Exercise Capacity Because the addition of protein to a carbohydrate drink consumed during prolonged exercise should potentiate the plasma insulin concentration (6), Ivy et al. (7) initially proposed that exercise capacity would be improved by promoting a greater uptake and oxidation of exogenous carbohydrate rather than endogenous glycogen use. They reported a 36% increase in cycling time to exhaustion at 85% maximal oxygen consumption ( _ VO2max) 4 with ingestion of a protein and carbohydrate 200-mL drink every 20 min during intermittent cycling for 3 h compared with the ingestion of carbohydrate drink alone. Cycle time to exhaustion increased >100% for the protein and carbohydrate trial compared with placebo. However, rates of exogenous carbohydrate delivery were equivalent to 46.5 g h, which were less than optimal rates of $1 g min (8). van Essen and Gibala (9) contested the findings by Ivy et al. (7) and subsequent work by Saunders et al. (10), which suggested exercise capacity was improved when protein was added to a carbohydrate drink. The authors (9) provided welltrained cyclists with 1 g min of carbohydrate, alone or with additional protein, or placebo during a simulated 80-km time trial. They found that exercise times for the carbohydrate and carbohydrate+protein trials were identical and significantly faster compared with the placebo condition. These authors concluded that adding protein to a carbohydrate drink provided no ergogenic benefit to trained cyclists if the amount of carbohydrate consumed was at an optimal rate for exogenous carbohydrate oxidation. Several other studies have reported conflicting findings on the possible ergogenic effect of protein supplements when combined with a carbohydrate beverage during time trials or tests to exhaustion. Table 1 provides a summary of the findings from these studies, which are grouped according to suboptimal or optimal delivery rates of exogenous carbohydrate. It is clear that the additional energy content of protein supplements can provide an ergogenic effect during an acute bout of exercise when delivery of carbohydrate is less than optimal (7,10–14). However, it was also clear that when carbohydrate supplementation was delivered $1 g min, protein supplements provided no further ergogenic effect, regardless of the performance metric that was used (9,15–20). The exceptions to these latter findings were a study by Saunders et al. (21), who reported an improvement during the late stages of a 60-km time trial, although the interpretation of these data has been challenged (19,22), and improvements with carbohydrate+protein in participants unaccustomed to performing exercise tests to exhaustion (23). Several studies discussed the possibility that the addition of protein to carbohydrate during exercise altered muscle metabolism by slowing the rate of muscle glycogen degradation and maintaining energy turnover of Krebs cycle intermediates but failed to provide any evidence to support these suggestions (7,10–13,18,21,24). In contrast, the only study that directly assessed these proposed mechanisms failed to support this rationale for additional protein supplementation when delivery rates of carbohydrate were optimal (25). For military personnel, therefore, there is little scientific evidence to justify the use of protein supplements to enhance sustained physical efforts for prolonged periods if a sufficient exogenous carbohydrate source is available. However, in the absence of optimal rates of exogenous carbohydrate availability, protein supplements may provide an ergogenic advantage. The recent efforts to test protein supplementation with reduced caloric drinks (11–13) warrant further investigation to verify the proposed mechanism(s) responsible for the ergogenic response. Conditions may exist for military personnel where optimal exogenous delivery of carbohydrate is not an option. Under these situations, protein supplements may be advantageous. It must also be noted that in contrast to athletes, soldiers do not usually perform time trials or tests to exhaustion alone but rather work as a unit to accomplish their mission. As a result, one could question the relevance of the metrics used in the exercise sciences literature for the transfer of knowledge to the military community. Nevertheless, the provision of an exogenous carbohydrate supplement has been shown to enhance both physical and cognitive performance of soldiers during fieldtraining exercises (26–28), so the use of protein supplements might be considered if carbohydrate delivery is suboptimal. Hasten the Recovery of