Background/Objectives: This systematic review examined whether oral vitamin D supplementation improves vitamin D status, health, and exercise outcomes in indoor-training aquatic athletes. Methods: We systematically reviewed randomized, placebo-controlled trials (>2 weeks) investigating vitamin D supplementation in competitive swimmers and divers. Six eligible trials (n = 246) were included and summarized descriptively. Results: Supplementation (2000–5000 IU/day for 12 weeks to 6 months) consistently increased serum 25(OH)D compared with placebo, with average increases up to 9.3 ng/mL. While higher doses occasionally improved muscle strength and lean mass, evidence showed no consistent benefits for swimming performance, immune function, or bone turnover. Additionally, higher body mass index (BMI) correlated with smaller 25(OH)D increases. Conclusions: Vitamin D effectively corrects deficiencies in aquatic athletes but lacks consistent ergogenic benefits. Therefore, in practice, supplementation should serve primarily as a targeted corrective measure for deficiency to support fundamental musculoskeletal health, rather than a generalized strategy for performance enhancement.

Age-related declines in physical capacity may partly reflect impaired mitochondrial bioenergetics and oxidative stress. This systematic review and meta-analysis evaluated the effects of oral coenzyme Q10 (CoQ10) supplementation on muscle health and physical capacity in adult populations with a study-level mean or median age ≥ 40 years. MEDLINE, Embase, Web of Science, Scopus, and CENTRAL were searched through July 2026 for randomized controlled trials lasting ≥2 weeks. Directly usable evidence formed primary random-effects models, whereas assumption-dependent estimates were included only in sensitivity analyses. Effect measures were expressed as mean differences (MDs) or Hedges’ g, as appropriate. Thirty-three trials involving 3317 randomized participants were included, and 21 contributed to meta-analysis. The relative VO₂peak/VO₂max estimate was small and imprecise (5 studies, n = 172; MD 0.73 mL·kg⁻¹·min⁻¹, 95% CI −0.07 to 1.54; I² = 0%; low certainty) and remained compatible with no benefit. Primary estimates were also uncertain for 6-min walking distances, handgrip strength, lower-limb strength, chair-rise performance, exercise duration, and muscle/lean-tissue quantity. Sensitivity analyses did not materially alter the overall interpretation. Evidence certainty was low or very low for all key outcomes. Current evidence does not establish clinically meaningful benefits of CoQ10 supplementation for aerobic capacity, muscle health, or physical function.