Effects of Rapid Weight Loss on the Immune System in Combat Sports Athletes: A Systematic Review

Rapid weight loss (RWL) is a common strategy among combat sports athletes aiming for a competitive advantage. However, it imposes significant immunological stress that compromises both innate and adaptive immune defenses. This systematic review synthesizes current experimental and mechanistic evidence on the effects of RWL in combat sports, focusing on cellular immunity, neuroendocrine regulation, and inflammatory pathways. Acute RWL activates the hypothalamic–pituitary–adrenal axis, elevating plasma cortisol and suppressing lymphocyte proliferation, T-cell function, and natural killer cell cytotoxicity. Although neutrophil counts increase, their phagocytic and oxidative burst capacities decline, reflecting impaired host defense. Monocyte and macrophage systems shift toward proinflammatory phenotypes, while mucosal immunity is weakened by reductions in secretory immunoglobulin A, leading to increased upper respiratory tract infection risk. The magnitude and speed of weight loss are critical determinants of immune dysfunction, with reductions exceeding 5% of body mass producing particularly severe consequences. Evidence-based intervention strategies—including gradual weight management, nutritional optimization, and biomarker monitoring—are essential to mitigate immunosuppression and safeguard athlete health. This review highlights key gaps in combat sports-specific protocols and proposes integrated approaches to preserve immune competence and optimize performance.