Sports Nutrition Knowledge and Dietary Supplement Beliefs Among Fitness Center Attendees: Evidence of a Knowledge–Belief Dissociation

Purpose: This study examined whether sports nutrition knowledge predicts dietary supplement beliefs among fitness center attendees and whether knowledge is reflected in actual nutrient intakes.Methods: A cross-sectional study was conducted with 193 adults (106 women, 87 men; mean age 23.80 ± 6.18 years) attending a university-affiliated fitness center in Istanbul, Türkiye. Data were collected using a sociodemographic questionnaire, the Sports Nutrition Knowledge Questionnaire, the Sports Dietary Supplement Belief Scale, and a 24-hour dietary recall evaluated against TÜBER 2022. Chi-square, ANOVA, multiple linear regression, hierarchical regression, binary logistic regression, and partial correlation were used.Results: Of participants, 77.2% had poor sports nutrition knowledge. Supplement beliefs did not differ by knowledge level in women (F = 0.918, p = 0.435) or men (F = 1.07, p = 0.365). Knowledge did not predict supplement beliefs in regression (β = −0.032, p = 0.684); hierarchical contribution was negligible (ΔR² = 0.003, p = 0.437). Partial correlation was near zero (r = −0.057, p = 0.432). Previous supplement use was the only predictor of moderate-or-higher knowledge (OR = 1.73, 95% CI: 1.15–2.61, p = 0.009). When compared with TÜBER 2022 reference values, daily cholesterol and sodium intakes exceeded recommended levels, whereas vitamins A, D, and thiamine were below recommended intakes in both sexes. Only vitamin A intake was predicted by knowledge (p = 0.025).Conclusion: Among university-affiliated fitness center attendees, sports nutrition knowledge and dietary supplement beliefs were largely independent. Fitness participation alone did not ensure dietary adequacy in this population. Interventions targeting this group should combine evidence-based nutrition education with belief-focused counseling approaches.