<p>This study investigates how eight popular diets (DASH Diet, Flexitarian Diet, Intermittent Fasting, Keto Diet, Low-FODMAP Diet, Mediterranean Diet, Paleo Diet, and Vegan Diet) are perceived across the modern communication ecosystem. We analyze discourse from academic literature (Web of Science, 1975–2024), mainstream news (The New York Times, 2000–2024), and social media (Reddit, 2005–2022) to understand information flow in a fragmented media landscape. Using time series, engagement, and sentiment analysis on nearly 800,000 documents, we map how diet narratives are constructed and disseminated. Our findings reveal distinct agendas on each platform. Academic and news sources prioritized established diets, while Reddit amplified newer, trend-driven ones. The Mediterranean Diet, for instance, dominated scholarly and news coverage but was minimally discussed on Reddit. Intermittent Fasting received attention across scientific literature, news media, and social media. Monthly bidirectional Granger causality tests revealed significant cross-platform effects only for Intermittent Fasting, with scientific publications predicting New York Times coverage and New York Times coverage predicting Reddit activity. No significant effects were observed for the other diets. Sentiment also diverged across platforms: scientific discourse was generally positive, whereas user-generated content was more mixed or negative, particularly for Veganism on Reddit. This study reveals a fragmented information ecology in which scientific consensus does not uniformly shape public discourse, challenging traditional two-step flow models. Our cross-platform framework provides a novel approach for tracking how health topics emerge and diffuse through networked interactions among scientific, media, and social platforms.</p>