E Resistance training (RT) produces muscle hypertrophy due to the accretion of muscle protein over the course of the training period. The progressive accrual of myofibrillar protein is the net result of an overall increase in muscle protein synthesis (MPS) that occurs with each bout of resistance exercise. The process of MPS is governed by an integrated network of intracellular events that can be regulated by both systemic and local effects. Resistance exercise is known to induce systemic effects such as the release of hormones such as testosterone, growth hormone (GH), and insulin-like growth factor 1 (IGF-1) that can affect MPS by up-regulating intracellular signaling pathways. One such pathway involves the orchestration of phosphatidylinositol-3 kinase (PI3K), protein kinase B (Akt), and mechanistic target of rapamycin (mTOR) and is known as the PI3K/Akt-mTOR pathway. This intracellular signaling pathway has been recognized as a stimulus for skeletal muscle protein synthesis (MPS), and the cumulative effects of increased MPS over time can lead to muscular hypertrophic adaptations (Bodine et al., 2001; Koopman et al., 2006; Sandri et al., 2008). Activity of the PI3K/Akt-mTOR pathway has been shown to be sensitive to various substances such as L-leucine (Dennis et al., 2011), ursolic acid (Ogasawara et al., 2013), and phosphatidic acid (PA) (Joy et al., 2014).
Regarding PA, it is an acid form of phosphatidate, a part of common phospholipids which are major components of cell membranes. PA is a simplistic form of diacyl-glycerophospholipids, and is a vital cell lipid acting as a biosynthetic precursor for the materialization (directly or indirectly) of all acylglycerol lipids in the cell (Foster et al. 2014). PA is formed from the hydrolysis of phosphatidylcholine by the enzyme, phospholipase-D (PLD). PLD is a prime regulator in the activation of mTOR signaling by a variety of stimuli (Yoon et al., 2015). PA binding to mTOR consequently results in the stimulation of mTORC1 kinase activity and exogenous PA has been shown to directly activate mTORC1 signaling, possibly due to its association with the FKBP12-rapamycin-binding (FRB) binding domain of mTOR (Joy et al., 2014). An additional stimuli for mTOR upregulation is muscle contractions which are associated with damage to the sarcolemma resulting in phospholipase D (PLD) to be dislodged from the z-line of muscle tissue and hydrolyzes phosphatidylcholine to yield PA, a lipid second messenger, and choline (Yamada et al., 2012). More specifically, eccentric contractions have demonstrated the ability to result in a significant elevation of intracellular PA which inhibited the synthesis of PA by PLD and blocked the eccentric contraction-induced increase in S6K1 phosphorylation (O'Neil et al., 2009). Yoon et al. (2015) provided evidence that the mTOR inhibitor domain-containing mTOR-interacting protein (DEPTOR) is displaced from mTORC1 by PA when generated by PLD. This leads to activation of mTORC1 and when taken together, this data provides indication that the increase in PA promotes the plausible activation of mTOR signaling.
Potentially, the combination of resistance training (RT) with exogenous PA supplementation could further stimulate an up-regulation of mTOR, thereby augmenting increases in MPS. Although, to date there are no known published studies examining the effectiveness of PA supplementation and RT on mTORC1 activity in human skeletal muscle. However, there are three known studies investigating PA supplementation (750 mg daily) in combination with RT and the subsequent effects on body composition and muscle mass and strength in humans (Escalante et al., 2016; Hoffman et al., 2012; Joy et al., 2014). As with many studies, however, discrepancies exist in various aspects of these three investigations and can likely be attributed to such issues as differences in the experimental designs of the studies such as RT program structure and RT session supervision, to name a few. In regards to lean mass, muscle cross sectional area (CSA), and strength, Hoffman found only significant increases with RT whereas Joy et al. (2014) and Escalante et al. (2016) founds significant increase with RT that were a result of daily PA supplementation. Interestingly, Escalante et al. (2016) found the most robust impact on strength and lean body mass improvements utilizing a similar design to the study of Joy et al. (2014), but in addition to 750 mg PA the supplement included Lleucine, HMB, and vitamin D which make it difficult to clearly discern if these results were primarily attributable to PA.
The purpose of this study was to compare the effects of an eight week resistance training (RT) program in conjunction with daily, orally-delivered PA supplementation on body composition and muscle mass and strength at doses of 375 mg and 250 mg, compared to placebo.
Using a double-blind, randomized, placebo-controlled design, thirty-two resistance trained males (thrice weekly >1 year prior to study) volunteered as participants. Participants were initially screened via email and following an explanation of all procedures, risks and benefits, each participant signed a university-approved informed consent document prior to starting the study. Approval to conduct the study was granted by the Institutional Review Board for the Protection of Human Subjects in Research of Baylor University. Additionally, all experimental procedures involved in the study conformed to the ethical consideration of the Declaration of Helsinki.
Participants were instructed to not use any anabolic dietary supplements or drugs known to increase muscle mass and/or performance. Screening for dietary supplements and anabolic steroids was accomplished by a health questionnaire, completed during participant screening. Participants reported to the Exercise and Biochemical Nutrition Laboratory (EBNL) on two separate occasions, at baseline (day 0) and after eight weeks of RT and supplementation (Day 57). Testing procedures included collection of dietary logs and testing for body composition testing, rectus femoris CSA (RF CSA) for muscle mass, and muscle strength. The participants' diets were not standardized and they were instructed not to change their normal dietary habits during the course of the study. Participants were required to record their dietary intake for 4 consecutive days prior to each of the two testing sessions at visits 1 (Day 0) and 2 (Day 57) to confirm adherence to their typical daily dietary regimen.
Participants were randomly-assigned to one of three treatment groups, 375 mg PA (PA375), 250 mg PA (PA250), or 375 mg of rice flour placebo (PLC). The PA supplement (Mediator[TM]) was obtained from Chemi Nutra (Austin, TX). Both PA and PL were in capsule form and identical in size, shape, color, and texture. Participants were provided the entire allotment of supplement capsules in which daily doses were individually bagged. Participants were required to consume three capsules of either the placebo or PA once per day 60 minutes prior to RT and with dinner on non-RT days. Supplementation compliance was monitored by participants returning empty containers and individual bags of their supplement on Day 57, and also by completing a weekly supplement compliance questionnaire.
Body composition testing
At each of the 2 testing sessions at visit 1 (Day 0) and 2 (Day 57), total body mass (kg) was determined on a standard dual beam balance scale (Detecto Bridgeview, IL). Fat mass and fat-free mass were determined using DEXA (Hologic Discovery Series W, Waltham, MA). Quality control calibration procedures were performed on a spine phantom (Hologic X-CALIBER Model DPA/QDR-1 anthropometric spine phantom) and a density step calibration phantom prior to each testing session. For the variable of lean mass using DEXA, the intraclass correlation coefficient (ICC), standard error of measurement (SEM), and minimal differences needed to be considered real (MD) were 0.95, 1.69, and 4.74, respectively. Total body water was determined with bioelectrical spectroscopy [(BIS) ImpediMed Ltd., Australia] using a low energy, high frequency current (500 micro amps at a frequency of 50 kHz).
Rectus femoris cross sectional area
Prior to muscle strength assessment, determination of muscle size involved measuring RF CSA using ultrasonography (Sonosite M-Turbo, Milwaukee, WI, USA) based on previously-established...