An acute bout of resistance exercise stimulates muscle protein synthesis (MPS) rates for up to 24-48 hours, supporting muscle growth and repair. To optimize the anabolic effects of resistance exercise, the provision of dietary amino acids (i.e., proteins) is essential. Dietary protein intake provides the body with necessary amounts of essential and non-essential amino acids, which represent the building blocks for muscle proteins, enhancing anabolic muscle growth. The ingestion of dietary protein, such as whey protein, is well established to stimulate an increase in the rate of protein synthesis in skeletal muscle following resistance exercise. Research has demonstrated a dose-dependent relationship between protein intake and MPS rate, with 25 grams being the optimal dose to maximally stimulate MPS rates in younger adults with excess protein oxidized as a fuel source. Determining whether this maximally stimulated MPS response can be further heightened during post-exercise recovery using non-protein dietary factors is yet to be explored. Recently, it has been shown that novel orally ingested ketone body supplements can stimulate MPS rates in younger adults at rest. Ketone bodies (β-OHB) are lipid- derived molecules normally produced under conditions of glucose deprivation (i.e., fasting/starvation, or a low carbohydrate 'ketogenic' diet). However, these orally ingested ketone supplements rapidly increase blood ketone levels without the need for dietary restriction6. In vitro research showed that the combination of leucine and ketone bodies stimulated a 2-fold increase in MPS, compared to the leucine group alone, indicating synergistic effects of protein and ketone bodies on MPS. However, the effect of ketone supplementation, with and without dietary protein co-ingestion, on MPS rate during post-exercise recovery is yet to be investigated. If ketone bodies can amplify the anabolic response to dietary protein, they may provide a novel approach to maximizing muscle adaptation during post-exercise recovery. Therefore, the purpose of this study is to evaluate the effects of ketone monoester intake on postprandial muscle protein synthesis rates when consumed alone and when co-ingested with an optimal dose (25 g) of whey protein during recovery after resistance exercise compared to 1) an optimal dose of whey protein (25 g), and 2) a control flavored water. It is hypothesized that muscle protein synthesis rates will be stimulated following the ingestion of the ketone body beverage. Further, muscle protein synthesis rates will be further enhanced when the ketone-containing beverage and an optimal dose are taken together.

A parallel group design will be used for this randomized double-blind, placebo-controlled study in healthy adults to investigate the effects of ketone monoester on myofibrillar protein synthesis rates during post-exercise recovery. There are 4 groups in this trial, including one ketone group (0.36g/kg body weight) (KET), one protein group (25 g whey protein) (PRO), a combination of ketone and protein (KET+PRO), and a placebo group (flavoured water) (CON). A total of 48 participants will be enrolled in this trial (n=12 per group). The study will include a screening visit (visit 1), 10-repetition maximum (10-RM) testing (visit 2), where participants' 10-RM will be determined for the exercise protocol, and the experimental trial (visit 3). During the experimental trials, participants will arrive to the laboratory in a fasted state, and a prime dose of the L-\[ring-2H5\]-phenylalanine will be administered followed by a continuous infusion at a rate 0.05 μmol/kg of body weight/min. Then, participants will perform a unilateral lower-body resistance exercise, consisting of 8 sets of 10 reps of unilateral leg extension at 90% of their 10-RM with 90 seconds rest in between sets. Following exercise, the nutritional treatment will be administered. Arterialized blood will be collected at baseline and 13 postprandial timepoints across 9 hours for plasma amino acid, glucose, and insulin quantitation. Additionally, changes in capillary blood β-HB concentration will be assessed throughout the trial by collecting capillary blood samples at baseline and 10 postprandial timepoints. Finally, muscle biopsy samples from both the exercised leg and the rested leg will be collected prior to beverage intake and at the 5-hour mark of the postprandial post-recovery period to assess myofibrillar protein synthesis rates.