Effects of Exercise & Tart Cherry Ingestion on Heat Acclimation and Gut Microbiome

10 recreationally trained cyclists will be recruited and will complete the study in counter-balanced randomized order (5 will start with placebo and 5 will start with tart cherry juice, then have a 4 week washout and complete the other intervention). Participants will complete 15 total sessions in the exercise physiology lab including: a VO2max test, 4 heat stress tests (pre- and post-acclimation) and 5 consecutive days of heat acclimation sessions (twice).

The VO2max test will be an incremental cycling test, starting at 80 Watts and increasing 25 Watts per minute until fatigue. The maximal Wattage will be used to determine exercise intensity during the heat stress test and heat acclimation sessions.

The heat stress test and heat acclimation sessions will occur in a controlled environment of 38C and 25% humidity. Airflow will be provided with a fan at 1 m/s. Prior to entering the controlled environment, participants will provide a mid-stream urine sample to assess hydration status via urine specific gravity (USG) and osmolality (Vapro, ELITech), then empty their bladder, measure their nude body mass on a scale (in a private bathroom in the lab), and self-insert a rectal thermometer 10cm past the anal sphincter. If the participant is dehydrated, they will be instructed to drink 500 ml of water and wait to start exercise until their hydration level is normal (USG<1.020, urine osmolality 500 800 mOsm). Researchers will then affix skin temperature sensors to the participants chest, arm, thigh, and calf and a heart rate monitor to their chest. Participants will the enter the heat chamber and sit on the bicycle for 5 minutes to get acclimated to the environment and to obtain resting levels of thermal sensation and perceived effort. Following these 5 minutes, the participant will commence exercise (described below). Participants will be provided water kept in the controlled environment (to avoid cooling) to consume ad libitum throughout all exercise sessions. Following the exercise session, participants will exit the controlled environment and void their bladder, collecting all urine in a cylinder and towel dry and assess their nude body mass. These will be used along with volume of water consumed during exercise to assess body mass changes and sweat rate.

Heat stress test:

The heat stress test will consist of cycling at 40% of their maximal Watt output for 45 minutes, followed by a 5k time trial, where they will cover the 5k in as fast a time as possible.

Heat acclimation sessions Heat acclimation sessions will be 60 minutes and will utilize an isothermic heat acclimation modality where individuals will exercise at a self-selected cadence to reach a core temperature of 38.5C. Once 38.5C has been reached, they will stop exercising and rest. Typical time to reach a core body temperature of 38.5C is 30-50 minutes, so the individual will exercise for this time, and rest for the remainder of the 60 minutes. During this rest time, body temperature will continue to rise because of metabolic heat and heat produced from exercises. However if their temperature reduces below 38.5C, they will begin exercise again to maintain temperature above 38.5C. These sessions will occur daily for 5 days.

Supplementation Participants will begin supplementation 24 hours prior to their first heat stress test and throughout their heat acclimation sessions until the final heat stress test has been completed (7 days of supplementation total). Supplementation will consist of consuming an 8 oz bottle of tart cherry juice or placebo twice daily, once in the morning and once in the evening, at least 2 hours before bed. Following a 4 week washout period, participants will complete the opposite condition (placebo or polyphenol).

The investigators will assess the impact of hyperthermia on the gut microbiome before and after heat acclimation. Participants will provide a stool sample at pre-supplementation and 24 hours post each of the heat stress tests (pre- and post-acclimation) for each of the interventions (tart cherry and placebo). Participants will collect a stool sample using a toilet hat and aliquot three small samples. They will be provided with a toilet hat, a pair of gloves, a sterile spatula, and three sterile eppendorf tubes. They will be asked to put approximately a nickle sized sample in each of these tubes. These will be placed in the freezer and brought to the lab on ice (they will be provided with a biohazard cooler bag and ice pack for transport), where they will be stored at -80C until analysis.