A lot of supplements simply aren't absorbed and utilized well in the body, especially the synthetic versions.
'Tis true. I think it is very important to realize that research doesn't actually "prove" things, but instead sets out to rule out what is NOT true. So an appropriate experiment would be to give subjects a supplement to prove it has no effect on them. If it did have an effect, that simply opens the door to the next experiment, and so on. If studies don't do this, there's something wrong at a very basic level.
Disregard most anything that says such and so "has been proven to..." or anything like that.
[From a career scientist, educator & advocate for public understanding of science]