Rationale: During exercise protein breakdown in muscles is larger than protein synthesis. This negative muscle protein balance leads to increased muscle damage and thereby to a reduced muscle tissue recovery. To achieve a positive muscle protein balance and reduce muscle damage, many athletes use proteins. In professional soccer players, considerable stress is placed on the musculoskeletal system. Recovery time is often too short for these players to restore homeostasis, which results in catabolic processes. This increases playing errors and will lead to lapses in concentration. Therefore, it is of great importance for a professional soccer team to be in optimal condition and a fast recovery after exercise is desirable to accomplish maximum performance. Supplementation with whey proteins is thought to support this.

Objective: To assess whether post-exercise supplementation with whey protein will lead to a better muscle recovery than supplementation with carbohydrates in Dutch soccer players between the age of 15 and 18 years old.

Study design: A double blind randomised controlled cross-over trial.

Study population: Healthy soccer players of the A and B selection of AJAX between the age of 15 and 18 years old.

Intervention: Supplement, containing either whey proteins or an isocaloric carbohydrate placebo. Each supplement will be administered for 2 weeks separated by a 7 day washout period. Treatment order will be randomly assigned.

Main study parameters: The main study parameter will be the Yo-Yo intermittent recovery test level 2 score and the vertical jump test. The scores will evaluate the soccer players' ability to repeatedly perform intense exercise and his potential to recover from this exercise.

Inclusion Criteria:

• performing soccer >1 year
• part of A & B selection of AJAX

Exclusion Criteria:

• presence of lactose intolerance
• presence of illness
• presence of injury