Dairy Consumption During a High-intensity, High-volume Training Week in Young Athletes

The proposed study is a double-blind, cross-over two-week intervention, simulating the peaking phases (high-intensity and volume) in young competitive soccer players.

Study Procedures: All tests and measurements will be performed within six-weeks across five visits. The two training intervention weeks will coincide with the soccer club's annual periodization plan to capture their "peaking phase" (high intensity and volume) ~5-6-weeks apart.

Visit 1: One week prior to the initial training intervention week, participants will be informed of all tests and procedures that will take place and be familiarized with all testing protocols. In addition, all participant's anthropometric measurements will be performed. Participants will then be randomly assigned to receive either the Greek yogurt or the isoenergetic CHO control pudding). Visit one will be 60-90 minutes in duration.

Visit 2: One day prior to each training intervention week, participants will provide a morning fasted saliva sample as well as a venous blood sample (for biochemical markers - see below). Following the saliva and blood samples, a light breakfast (~400-500 kcal, depending on body mass) (e.g. banana, apple, plain granola bar, water etc.) will be provided. Following breakfast (approximately 60 min), performance testing will begin (see below). Visit two will be approximately 3h in duration.

During the two training intervention weeks, all participants will report to the team's training facility at the same time each day. Each training session will be led by the same member of the coaching staff and supervised by a qualified member of the research team. Each training session will contain a dynamic warm up (~10-15 min) followed by high-load soccer-specific training (~100 min) and a cool down (10-15 min), for a total session duration of ~120 min. Following each training session, sessional rating of perceived exertion (RPE) will be taken. There will be a total of five consecutive high-load training sessions performed in accordance with the team's structured periodized training plan. During the one-week training, participants will also be asked not to change their regular nutritional habits and to track their daily food consumption with a diet record. Participants and their parents will receive detailed instructions on how to complete the food records, along with handouts, as instructional tools and will be asked to return food record information during visit 3.

Visit 3: Participants will return to the lab 24h following the last training session and perform the identical procedures outlined in Visit 2.

Visit 4: Participants will return to the lab ~5-6 weeks following Visit 3. Participant's anthropometric measurements will be assessed again. Following this, the identical procedures outlined in Visit 2 will be performed.

During the second one-week training intervention, participants will be administered the supplement they did not receive in the first one-week training intervention. i.e., those who had Greek yogurt will receive the CHO control pudding and those who had the CHO control pudding will receive Greek yogurt. Each training session will be led by the same member of the coaching staff and supervised by the same qualified member of the research team. Each training session will again contain a dynamic warm up (~10-15 min) followed by a similar high-load soccer-specific training program (~100 min) and a similar cool down (10-15 min), for a total session duration of ~120 min. To ensure similar training intensity and volume, sessional rating of perceived exertion (RPE) will again be taken. There will be a total of five consecutive high-load training sessions performed in accordance with the team's structured periodized training plan. During the second one-week intervention, participants will again be asked not to change their nutrition habits and to track their daily food consumption with a diet record.

Visit 5: Participants will return to the lab 24h following the last training session and perform the identical procedures outlined in Visit 2.

Food/Drink supplement consumption:

During the study, all groups will be encouraged to maintain their habitual diets. Participants will be asked to consume 175g of 0% Plain Greek yogurt (~110 Kcals, 18g protein, 10g carbs) or isoenergetic CHO supplement (~110Kcal, 0g protein, 27.5g carbs) 3 times per day (at breakfast, immediately post-exercise and before bed) during the one-week training interventions. The yogurt may be sweetened or flavored, based on participant preference using a non-calorie sweetener. The CHO supplement group will consume an isoenergetic dose of a maltodextrin-based semi-solid pudding. This product will be made from pudding powder (1 part), maltodextrin (1 part) and water. This product has been designed to resemble the consistency and texture of Greek yogurt.

Measurements

Questionnaires: Participant Screening and Medical History, Training History, Food frequency Questionnaire (FFQ), Muscle Soreness Questionnaire.

Anthropometric and body composition measurements: height, seated height, body mass and body fat percentage (%) using bioelectrical impedance analysis (BIA, InBody520; Biospace Co., Ltd., Seoul, S. Korea).

Somatic maturity: maturity offset (years from the age of peak height velocity) will be calculated based on height, seated height and weight measurements.

Sessional RPE: will be taken at the end of each training session using a 1-10 RPE scale previously validated for use in children and adolescents. RPE information will be utilized to determine perception of intensity following each individual training session throughout the one-week intervention.

Exercise performance: will be examined prior to, as well as following each training intervention week. These tests are common in soccer and athletes are familiar with them. To limit confounding exercise influence on both biochemical and performance parameters, participants will be asked to refrain from other activities outside those prescribed to them during the training sessions.

Venous blood and saliva samples: will be collected on 4 separate occasions (24h prior and 24h following the last training session of each training intervention week). To control for circadian rhythm, the exercise session and related blood and saliva samples will be conducted at the same time of day. Saliva samples will be used to examine endocrine response. Venous blood samples will be used to examine IGF-I, irisin, inflammatory markers (cytokines - IL-6, IL-10 and TNF-α), c-reactive protein (CRP), oxidative stress (TBARS and ROS) and indicators of muscle fatigue/damage (CK), will be examined in each sample. Bone turnover will be assessed by circulating markers of bone formation (PINP, Osteocalcin, BAP, OPG) and of bone resorption (CTX, TRAP, RANKL).