This is the first study known that has examined the efficacy of phosphatidic acid on enhancing strength and muscle growth. The results of this study indicate that 8 weeks of supplementation with PA is likely to very likely beneficial in increasing lower body strength and lean body mass, respectively, compared to PL (Table 4). The effects of PA supplementation on upper body strength and muscle architecture were unclear. Recent evidence on rodent models have indicated that resistance exercise or an intermittent muscle stretch can activate mTORC1 by direct binding of PA to mTOR [11, 21]. It has been suggested that the mechanical action of muscle contraction can stimulate the growth promoting pathways within muscle . Considering that the mTOR signaling pathway was not examined in this study, we can only speculate on the mechanisms that may have contributed to the observed results.
The mechanical stimulus of resistance training has been demonstrated to be a potent stimulus for increasing protein synthesis [23, 24]. If protein or essential amino acids are ingested either before or following a workout, the effect on muscle protein synthesis appears to be magnified . Recent evidence has suggested that leucine, even in low dosages, may be very effective in stimulating muscle protein synthesis . In consideration of the potential effects that protein ingestion has on muscle recovery and remodeling, we felt it important to provide a standardized protein supplement to all subjects (both PA and PL) following each training session. With daily nutritional intake, including protein, similar between each group, the changes noted in this study (increases in lower body strength and lean body mass) likely reflect the ingestion of PA (Tables 3, 4 and 5). Since PA is thought to act on a parallel pathway to a protein stimulus (specifically leucine) on activating the mTOR pathway [8, 27], the greater benefit towards increases in lower body strength and lean body mass in PA suggests that the ingestion of this supplement may enhance lean tissue accruement and lower body strength to a greater extent than protein supplementation or resistance exercise only.
Differences between upper and lower body strength gains seen in this study may reflect the training experience of the subjects. Though all subjects had at least one year of resistance training experience, previous research on competitive strength power athletes has indicated that improvements in lower body strength may precede changes in upper body strength [28, 29]. This may reflect a greater experience in upper body training and a requirement for performing the squat exercise to appropriate depth and technique. None of the subjects in the study were working with a strength coach or personal trainer prior to their enrollment into the study. Evaluation of the training logs and performance testing were conducted by certified strength and conditioning specialists that reinforced proper technique and form during the testing. Considering the skill and technique necessary for performing the squat exercise, many competitive and recreational resistance trained athletes do not perform this exercise correctly . It is likely that the resistance training experience of the subjects resulted in a relative high level of performance in the bench press exercise. Although all subjects had performed the squat exercise prior to this study, their technical ability and skill for this exercise (i.e. bar placement, knee and foot alignment and lowering to parallel) varied widely. Since proper technique was stressed during the training and testing program it is possible that the subjects had a larger window of opportunity for strength gains based upon improved technique in the squat exercise compared to the bench press exercise. Thus, the strength improvements seen in the squat exercise could be partially attributed to a learning effect.
There were no clear benefits from PA ingestion in changes to muscle architecture of the vastus lateralis (Tables 3 and 5). The training program appeared to result in similar changes in muscle thickness for both groups, but did not result in any significant changes in pennation angle. The results observed in vastus lateralis thickness are similar to those reported by Blazevich and colleagues  following 5-weeks of training in competitive athletes, but greater than those reported by Santilla and colleagues  following 8-weeks of training in tactical athletes. However, the subjects in the latter study were also performing their basic military training that likely blunted maximal muscle growth. Comparisons between studies are also difficult to make due to the differences in subjects training status, the resistance training program and training duration. Although PA did appear to have a likely benefit on 1-RM squat changes, it did not have a similar effect on changes in vastus lateralis thickness. A recent study has questioned the importance of vastus lateralis changes on lower body strength performance as those investigators were unable to find any significant correlation between vastus lateralis thickness and lower body power performance . The lack of any significant changes in pennation angle for either group may also be related to resistance training experience, as experience does appear to impact the magnitude of change in pennation angle .
There are a number of limitations associated with this study. The scientific treatise that has emanated on phosphatidic acid and its role on muscle protein synthesis stimulated the desire to examine this further. Although the results of this study provide a degree of efficacy on this novel ingredient, it does not provide any support to the previously discussed mechanisms of action. However, the results of this study do provide some evidence on the proof of concept that PA may have a role in muscle strength and lean tissue accruement. Additional research is needed to add support to these results: a bioavailability study to investigate the absorption profile of orally administered PA, a muscle biopsy study to investigate the potential increase in muscle PA content, different target groups: trained, untrained, elderly subjects, dose finding studies to investigate if the effect of PA is dose dependent, the minimum effective dose and mechanistic studies. This will have important implications for athletes participating in strength/power sports, as well as mature adults attempting to maintain muscle strength and mass as they age.
In conclusion, the results of this study suggest that a combination of a daily 750 mg PA ingestion, combined with a 4-day per week resistance training program for 8-weeks appears to have a likely benefit on strength improvement, and a very likely benefit on lean tissue accruement in young, resistance trained individuals. Additional research is warranted to provide further elucidation on the mechanisms that govern PA and muscle protein synthesis, muscle growth and performance.