This study in healthy active young men investigated the acute effects of Phe supplementation combined with exercise on hormone secretion and substrate oxidation. The study showed that, compared with ingestion of a placebo, ingestion of the Phe supplement significantly increased the concentrations of glycerol and glucagon. The RER was also decreased significantly by Phe ingestion. These findings suggest that whole body lipid oxidation increased and that pre-exercise supplementation of Phe stimulated fat oxidation. To the author’s knowledge this is the first study to examine the effects of Phe on human fat oxidation combined with exercise. Although considerable evidence exists on the safety of Phe consumption in humans  there is no evidence on the functional effects related to fat oxidation combined with exercise. The data presented in this study lay the groundwork for further investigations on Phe supplementation in sport.
The main mechanism for the stimulation of fat oxidation following Phe administration may be via glucagon secretion. Glucagon is a key hormone involved in fat catabolism during exercise [14, 15]. Previous reports have noted that several widely divergent effect of glucagon appear to be mediated by a effector, adenosine 3′,5′-cyclic monophosphate (cAMP) . The best-known mechanism mediating lipolysis is the cAMP pathway, wherein increased levels of cAMP activate cAMP-dependent protein kinase A (PKA). In this pathway, hormone sensitive lipase (HSL) is phosphorylated by PKA and then translocates from the cytoplasm to the lipid droplet surface, where it interacts with perilipin A, the result of which is a subsequent release of free fatty acids [17, 18]. HSL is the most important lipase in lipolysis and is subject to hormonal regulation . In addition to HSL, adipose triglyceride lipase is expressed predominantly in adipose tissue and is considered the rate-limiting lipolytic enzyme in adipocytes [20, 21]. Therefore, an additional effect by Phe supplementation combined with exercise may stimulate the cAMP-dependent cascade pathway. Previous studies have suggested that glucagon secretion is regulated by gut hormones including glicentin, GLP-1, GIP, and GLP-2 and other peptides secreted by the gastrointestinal tract such as gastrin-releasing peptide (GRP), cholecystokinin (CCK), and secretin . Further studies are necessary to determine whether or not these hormones are involved in the stimulation of fat oxidation caused by Phe administration.
It has been reported that a single pre-ingestion dose of mixtures of either 17 specific AAs  or 3 AAs  enhanced lipolysis and hepatic ketogenesis during and after exercise by stimulating glucagon secretion. However, in the current study pre-exercise ingestion of Phe increased glycerol concentrations but not ketone body levels. The results suggest that pre-exercise ingestion of Phe markedly stimulate lipolysis but not hepatic ketogenesis. Taken together, these results indicate that ingestion of a specific AA stimulates fat oxidation efficiently during exercise.
In 1970 Harper et al.  published a review of the effects of disproportionate levels of AA intake, with the information subsequently being updated in 1984 by Benevenga and Steeles . There is concern that Phe supplementation may be associated with abnormal brain development known to occur in humans with phenylketonuria, a condition that results in a buildup of Phe and its metabolites in the blood . However, no adverse effects were noted in humans given either a single oral dose of up to 10 g , ~30 g i.v. , or 3–4 g orally as aspartame . No serious or study-related adverse events were observed in the current trial and therefore we conclude that Phe supplementation may be a safe method for accelerating fat oxidation during exercise.
This study has several strengths worth mentioning. First, the trial was a placebo-controlled, double-blind, crossover, randomized, controlled design and therefore the findings are highly reliable. Second, evaluation of the respiratory exchange ratio is regarded as the gold standard for evaluating whole body fat oxidation. Third, the study only administered 3 g of Phe prior to exercise, a low-dose of amino acid supplementation that could be used easily elsewhere.
In contrast, we must also note some limitations. First, the number of enrolled participants was only six of young male. Therefore, the data robustness may not be high. Second, we failed to measure blood insulin levels. Insulin signaling is a key factor for glucagon secretion , so further studies are needed to demonstrate the effect of Phe combined with exercise for insulin secretion. However, stimulation of sympathetic nervous system by exercise suppresses insulin secretion . Moreover, in our previous report, pre-ingestion of the amino acid mixture containing 1.5 g of Phe did not stimulate insulin secretion during 50% VO2max exercise . Therefore, we thought that the influence of insulin secretion on this fat oxidation effect mediated by Phe ingestion combined with exercise is small. Third, we did not present the data of energy expenditure, so it remains unclear this higher fat oxidation leads to obesity prevention or weight control. However, ingestion of Phe may increase fat mobilization, and this effect might to lead, at least in part, to an increase in fat oxidation during exercise. Further studies are needed to investigate whether this acute response induced by the administration of Phe is sustained if the supplement is ingested over several weeks.