How Much Science is in “Science-Based”? Adherence to Scientific Standards and Ethics among Fitness and Nutrition Influencers on YouTube

Social media platforms have become a primary source of health-related information, particularly in the fields of fitness and nutrition. Influencers increasingly use labels such as "science-based" or "evidence-based" to lend credibility to their content. However, the extent to which such claims are substantiated remains largely unexamined. This is particularly concerning given the limited digital health literacy among large segments of the population and the potential for misinformation to influence health-related behaviour. This study systematically analyses the scientific quality, transparency, comprehensibility, and ethical responsibility of fitness and nutrition content on YouTube. A theory-driven evaluation framework comprising 14 criteria across four dimensions was developed, drawing on established instruments including DISCERN, the JAMA criteria, HONcode, PEMAT, and GRADE. The sample comprised 48 videos from eight German-speaking YouTube channels with substantial reach, selected based on subscriber counts and video engagement. Content was assessed independently by four raters using a five-point Likert scale, with interrater reliability confirmed through Fleiss's kappa (κ = 0.66–0.73). Results indicate that while influencers generally achieve high scores for comprehensibility (M = 3.59) and moderate scores for transparency (M = 3.38), scientific accuracy (M = 2.12) and ethical responsibility (M = 2.28) remain considerably weaker, with relatively high differences among influencers. A hierarchical cluster analysis identified three distinct communication profiles: (1) popularisers and performers, who prioritise accessibility over scientific accuracy; (2) transparent communicators, who incorporate formal transparency elements but lack systematic evidence classification; and (3) scientifically based communicators, represented by a single channel, who combine methodological accuracy with accessible presentation. The findings reveal a structural tension between reach-oriented communication and adherence to scientific standards. Personal authority frequently substitutes for methodological substantiation, and commercial interests are often inadequately disclosed. The study concludes that ensuring quality in digital health communication requires closer integration of consumer protection, ethical guidelines, and scientific standards, including uniform disclosure requirements and verifiable quality labels.