**Background:** Triathlon is a demanding endurance sport involving swimming, cycling, and running, placing high physiological demands on athletes. Sports supplements (SS) are commonly used to complement diet and enhance performance, especially in long-duration events where self-sufficient nutrition is required. The Australian Institute of Sport (AIS) classifies SS into four groups (A through D) based on scientific evidence. While SS use is widespread in endurance sports, research specifically focused on triathletes has been lacking. This study aimed to describe the pattern of SS consumption among Spanish triathletes and analyze differences by sex and competitive level (regional, national, international).
**Methods:** This descriptive cross-sectional study recruited 232 Spanish federated triathletes (165 men, 67 women; mean age 34.79 ± 9.93 years) through non-probabilistic convenience sampling via triathlon federations and clubs. Participants had competed for at least two years and were injury/illness-free for six months prior. Data were collected using a validated 35-question questionnaire covering anthropometrics, sports practice, and supplement consumption (types, reasons, advisors, purchase locations, timing). Statistical analysis included ANOVA for continuous variables (with Bonferroni correction for multiple comparisons) and chi-square tests for categorical variables, with significance set at p < 0.05.
**Key Results:** Overall, 92.2% of triathletes consumed SS, with no significant differences by sex (93.3% men vs. 89.6% women; p = 0.329) or competitive level (91.6% regional, 92% national, 96.2% international; p = 0.727). The primary reason for consumption was improving performance (82.33%), followed by health care (31.47%). Most athletes consumed SS during both training and competitions (64.22%). The main advisor was a dietitian-nutritionist (D-N) (41.6%), followed by the coach (40.7%). SS were mostly purchased online (60.6%) and in specialized stores (45.9%).
Significant differences by competitive level were found for total SS consumed (F = 3.919; p = 0.021), total Group A supplements (F = 4.477; p = 0.012), and ergogenic aids within Group A (F = 5.801; p = 0.003). Pairwise comparisons showed significantly higher consumption at the national level compared to the regional level for total SS (p = 0.029), total Group A (p = 0.002), and ergogenic aids (p = 0.002). For the interaction between sex and competitive level, significant differences emerged only for ergogenic aids (F = 3.067; p = 0.049), with national-level women consuming more than regional-level women (p = 0.002).
The four most-consumed SS were bars (83.6%), sports drinks (74.1%), sports gels (61.2%), and caffeine (46.6%), all classified in AIS Group A. Group A had the highest mean consumption (6.05 ± 0.24 supplements), driven largely by sports foods (4.4 ± 0.17). Group B consumption averaged 0.53 ± 0.06, and Group C averaged 1.68 ± 0.15. By sex, women consumed significantly more iron (p = 0.001) and less caffeine (p = 0.0017) than men. By competitive level, international triathletes consumed more sports gels (p = 0.047), while national-level triathletes consumed more whey protein (p = 0.005) and glutamine (p = 0.019).
**Clinical Implications:** The high prevalence of SS use (92.2%) among triathletes underscores the importance of evidence-based guidance in this population. The finding that the four most-consumed supplements all belong to AIS Group A (strongest scientific evidence) is encouraging and likely reflects the relatively high proportion of athletes receiving advice from qualified dietitian-nutritionists (41.6%), particularly at national and international levels. However, the consumption of Group C supplements (little or no evidence) was nearly double that of Group B supplements, indicating a need for improved nutrition education. The higher iron consumption among female triathletes aligns with known anemia risks in female athletes and represents appropriate evidence-based practice. The predominance of online SS purchasing (60.6%) raises concerns about product quality and contamination risks, especially given the potential for inadvertent doping violations. These findings support the integration of qualified sports nutrition professionals into athlete support teams and highlight the need for educational interventions targeting supplement choices, particularly for regional-level athletes and those purchasing supplements online.