Learn Turkish: Almamam gereken yasaklı takviyeler ve besinler var. - There are banned supplements and nutrients that I must not take.

It breaks down as al (take) + -ma (negative marker: not) + -ma (verbal noun: taking) + -m (my). Literally, it means "my not taking". When followed by gereken (the adjective form of "necessary"), the full phrase almamam gereken translates to "which it is necessary that I don't take" or simply "that I must not take".

I thought yasak already meant "forbidden". Why use yasaklı here?

While both mean "banned" or "forbidden", they have a slight nuance. Yasak is typically used for rules or actions (like "Smoking is forbidden"). Yasaklı (literally "having a ban") is used to describe specific physical items, websites, or people that are placed on a restricted list. For things like "banned substances" or "banned supplements", yasaklı is the natural choice.

Could I use the "must" suffix -malı here and say almamalı olduğum instead?

You could piece that together grammatically, but it sounds quite clunky and unnatural. When turning an obligation into a relative clause (e.g., changing "I must take it" to "the thing that I must take"), Turkish native speakers almost exclusively use this [verb]-mAm gereken structure rather than trying to force -malı into a modifier.

Does the phrase almamam gereken yasaklı describe just the supplements, or the nutrients too?