I have the same reservations, and not just about dietitians but also other scientific disciplines. Many dietitians are first and foremost businessmen selling various supplements. Whether the supplement is necessary, effective or even harmful is less important.

The dietitians I trust document their dietary advice week after week by curing various ailments and diseases through scientific treatments (and they never recommend specific products): https://www.ndr.de/fernsehen/sendungen/die-ernaehrungsdocs/r...