Ketone Supplements and Substrate Oxidation and Physical Performance

Following an overnight (10 hour) fast, two catheters will be placed into the lower arm (one in each arm). One arm will be used for infusion of 6,6-[2H2] glucose tracer and the other will be used for blood sampling under resting and exercise conditions. Following an initial blood sample collection to determine background enrichments, a primed, continuous infusion of 6,6-[2H2] glucose will begin (prime, 82.2 µmol∙kg-1; continuous rate, 0.78 µmol∙kg-1∙min-1, Figure 1). The 6,6-[2H2] glucose will be infused for 100 min under resting fasted conditions to ensure isotopic steady-state is achieved prior to initiating exercise. Volunteers will consume either an isocaloric KE+CHO (KE: 573 mg KE/kg body mass, CHO: 110 g) or CHO drink prior to and during 90-min of metabolically-matched (~60 ± 5 % of VO2peak), steady-state load carriage (~30% body mass) exercise. Drinks will be enriched with U-13C-glucose (Cambridge Isotope Laboratory, Andover, MA, USA) to increase the isotopic enrichment well above natural levels and optimize the measurement of exogenous carbohydrate oxidation. During exercise V̇O2, V̇CO2, and HR will be measured at approximately 0, 20, 45, 60, 75, and 85 min.

After the 90-min steady-state treadmill exercise, participants will complete a TTE performance test. For volunteer safety TTE performance tests will be conducted without the additional weight from the load carriage exercise. Volunteers will be given time to stretch and warm-up on the treadmill before the TTE performance test beings. Volunteers will then run on the treadmill at a fixed speed and grade that elicits 85% of their VO2peak. The TTE performance test will be determined as the time of volitional exhaustion. Following completion of the test, a self-selected cool-down will occur. Volunteers will complete a minimum of two practice exercise sessions to ensure they are familiar with the performance test.

The following day volunteers will return to the laboratory after an overnight fast. After consuming the study drink (KE+CHO or CHO) participants will complete a self-paced 4-mile load carriage (~30% body mass) time trial on a treadmill to assess aerobic performance. Time trial performance will be assessed as the total amount of time taken to complete 4 miles. The treadmill will be set at a constant 1% grade for the entire test. Following a warm-up period, volunteers will blindly modulate treadmill speed in order to complete the distance as quickly as possible. The only feedback given will be distance covered at half mile increments. At half mile increments volunteers' rate of perceived exertion will be determined using the Borg Scale. Heart rate will be monitored throughout the time trial. Heart rate will be recorded at half mile increments. No motivation will be provided during the time trial. Participants may consume water ad libitum during the time trial. Following completion of the test, a self-selected cool-down will occur. Volunteers will complete a minimum of two practice exercise sessions to ensure they are familiar with the performance test.