Regulatory mechanisms of high-polyphenol dietary intervention on exercise-induced immunosuppression during high-intensity training periods in adolescent athletes

INTERVENTION: Participants were randomly allocated in a 1:1 ratio to either the high‐polyphenol dietary intervention group or the placebo group. Randomisation was stratified by sex, sport discipline, and baseline immune function. The random allocation sequence was generated using a computer‐based randomisation program by an independent researcher who was not involved in participant recruitment, intervention delivery, outcome assessment, or data analysis. Allocation concealment was maintained using sequentially numbered, opaque, sealed envelopes. Participants, coaches, investigators, laboratory staff, and outcome assessors were blinded to group allocation throughout the trial. The intervention group received a high‐polyphenol dietary supplement containing 1200 mg/day in total: quercetin 300 mg/day, EGCG 400 mg/day, resveratrol 200 mg/day, and curcumin 300 mg/day. The supplement was administered as two 600 mg doses per day, taken at approximately 07:00 and 19:00 with meals, during the 12‐week high‐intensity training period. Participants were instructed to take the supplement with at least 10 g of dietary fat and to avoid calcium‐rich foods, iron supplements, and polyphenol‐rich beverages within two hours of administration. The placebo group received matched placebo capsules containing microcrystalline cellulose, identical in appearance, packaging, and administration schedule to the active supplement. Compliance was monitored using electronic medication event monitoring systems, bi‐weekly pill counts, and urinary polyphenol metabolite assessment at weeks 4, 8, and 12. The intervention was delivered during a 12‐week periodized high‐intensity training programme, including a preparatory phase, competition/peak‐load phase, and recovery phase. Training load was monitored using session rati CONDITION: Exercise‐induced immunosuppression in adolescent athletes during periods of high‐intensity training, particularly immune homeostasis disruption, reduced mucosal immune protection, inflammatory and oxidative stress responses, altered T‐cell balance, gut microbiome changes, and increased susceptibility to upper respiratory tract infections. ; Other INCLUSION CRITERIA: Adolescent athletes aged 15–17 years who met all of the following criteria: 1. At least 3 years of systematic training experience 2. Participation in provincial‐level competition within the previous 12 months 3. Weekly training volume of =10 hours 4. Tanner stage III–V pubertal development