This randomised double blind, placebo-controlled study examined the effects of post-exercise consumption of whey protein (PRO) or carbohydrate (CHO) on performance recovery, muscle damage and inflammatory cytokine responses following a high intensity interval swim (HIIS) in 10-17-year old male and female swimmers.

Study Procedures: All tests and measurements were performed during three visits at the given swim teams training/practice facility.

During Visit 1, participants were informed of all tests and procedures that will take place and were familiarised with all testing protocols. All anthropometric measurements were also performed during this meeting (see below). Visit one took ~ 60-90 minutes.

During visit 2, Participants arrived at the testing centre for 06:00h, provided a fasted venous blood sample (for biochemical markers - see below). Following the saliva and blood samples, a light breakfast (~300-400 kcal, depending on body mass) (e.g. banana, apple, plain granola bar, water etc.) was provided. Following breakfast (approximately 45-60 min),all participants performed a standardised warm-up in the pool (1000m - short course) followed by 5min of rest and the subsequent performance test in the form of a maximal 200m-front-crawl. Following the maximal 200m performance test, participants underwent a maximal high intensity interval swim protocol (HIIS). The HIIS consisted of 15 high-intensity swimming repetitions (5x100m, 5x50m and 5x25m at maximal intensity) with a work-to-rest ratio of 1:1. Immediately following the HIIS participants exited the pool and provided a second blood sample (blood samples were collected pre-, 30min, 2, 6 and 24h post-HIIS). This was then followed by the first supplement drink ~30-40. 1.5h after consuming the first supplement drink, participants returned to the pool to complete a second maximal 200m swim, followed by a blood test and received their second supplement drink. 1.5h following the supplement drink, a standardised vegetarian lunch was provided, 1.5h following lunch participant provided another blood sample and performed the final 200m swim of that day. Prior to departure participants were provided with a food record sheet to track all food consumption that evening and were instructed to return it to a study investigator the following day.

During visit 3, Participants returned to the testing location at 06:00h, provided a fasted Blood and saliva samples, were given the same breakfast as they received on day 2 and were administered a muscle soreness questionnaire. Following breakfast (approximately 45-60 min),all participants performed the same standardised warm-up as they did in day 2 (1000m - short course) followed by 5min of rest and the subsequent performance test in the form of a maximal 200m-front-crawl.

Groups and supplementation protocol - Participants matched into one of three groups: Protein (0.3g/kg - whey protein isolate), Carbohydrate (0.3g/kg - maltodextrin) or placebo control (flavoured water) (H2O), matched for age, sex and body mass. All groups consumed a total of two supplements, both the investigators and the participants themselves were blinded to the contents of the drink. All drinks were chocolate flavoured and served in black opaque protein shaker cups. For the protein and carbohydrate groups, each participant received an isoenergetic amount of supplement (2 x 0.3g/kg - whey protein isolate or maltodextrin) throughout the duration of the study at the given time-points outlined above.

Biochemical measurements:

Venous blood samples and a saliva sample will be collected on 5 separate occasions (pre-, 30min, 2, 6 and 24h post-HIIS). To control for circadian rhythm, the exercise session and related blood samples will be performed at the same time of day.

Venous blood samples were collected from the antecubital fossa of each participant using a standard venipuncture technique. Endocrine response such as anabolic/hormonal markers, inflammatory markers (cytokines - IL-6, IL-10, TNF-α) and indicators of muscle fatigue/damage (CK) were examined.

Inclusion Criteria:

• competitive swimmers (swim competitively for a minimum of 2y)
• male or female 11-17y
• free from any medical conditions

Exclusion Criteria:

• yes on medical screening questionnaire
• not classified as a competitive swimmer (swim competitively for <2y)
• outside the desired age range (<11 or >17)

One member of the research team whom did not have direct contact with participant during performance testing, matched participants for age, sex and body mass into one of three groups (protein, carbohydrate or placebo/water). This member of the research team also prepared and distributed all the research supplements in a sealed black opaque containers