As the World Health Organization raised questions this summer about the risks of a popular artificial sweetener, a new hashtag began spreading on the social media accounts of health professionals: #safetyofaspartame. Steph Grasso, a registered dietitian from Oakton, Va., used the hashtag and told her 2.2 million followers on TikTok that the WHO warnings about artificial sweeteners were “clickbait” based on “low-quality science.” Another dietitian, Cara Harbstreet of Kansas City, reassured her...