Given the app’s young userbase, the rise of ‘thinspirational’ videos — seen by billions — is worrying — Towards the end of December, TikTok’s content soundtracked by “All I Want For Christmas” was beginning to wane, and videos focused on weight loss goals for the New Year started to appear. One of the most ubiquitous trends features users creating a “weight loss shake” comprised of cucumber, pineapple, and…