This study investigates whether increasing legume consumption within a Mediterranean diet framework improves cellular hydration, autonomic regulation, and functional performance in competitive swimmers. Thirty-nine adolescent and young adult swimmers were monitored over a five-month competitive season under standardized training conditions. Participants were allocated to three groups according to habitual legume intake: a control group (\<1 serving/week, no dietary change), a moderate-intake group (≈2 servings/week increased to 3/week), and a high-intake group (≈3-4 servings/week increased to 6/week). Dietary counseling focused exclusively on adjusting legume intake, while overall training load and other dietary components were maintained. Primary outcomes include bioelectrical impedance-derived phase angle and body water distribution indices (extracellular water, intracellular water, ECW/ICW ratio) as markers of cellular health and hydration. Secondary outcomes include heart rate variability, cardiac coherence as indices of autonomic balance, and critical swim speed as a performance-related measure. The study aims to clarify whether a simple, sustainable increase in legume intake can support athletes' cellular integrity, autonomic function, and functional status without altering standard training programs.