Clinical guidelines can be corrupted by financial interests.
Medicinal and nutritional as well as most everything. Researchers are effectively held ransom to the funding. It is what keeps their jobs.
Follow the money and the recommendations are often swayed by that money trail.
Most research funding comes from industry and negative studies tat do not support the drug or food are not going to see the light of day.
This British Medical Journal article examines the problem in some depth.
I wish the system was perfect but it is not.