It is not that they lead to obesity on their own, but over consumption of them might and the effect they have on the body (inflammation) might have other secondary effects (insulin resistance, higher cholesterol, fatigue).
To say it is "just a fitness fad" is dismissing a ton of good research:
Meta comment: parent and GP are an interesting contrast between the "settled science" drum-beat dismissiveness narrative and the science/study-based curiosity counter-narrative.
Here's to hoping that with the death of the dollar, the "settled science" practitioners lose their bullhorns.
To say it is "just a fitness fad" is dismissing a ton of good research:
https://openheart.bmj.com/content/5/2/e000898