That's basically what the doctor said, though. I personally doubt those slow it down very much. I also think the science is still so poorly understood that certain supplements or foods could potentially even worsen things. (Just to throw out a totally made up scenario, maybe some compound found in some plant, and its metabolites, cause a small increase in monoamine release. This may be perceived as a mild stimulating effect in healthy people, but could hypothetically potentially speed up some feedback loop which leads to faster neuron death in people with certain neurodegenerative diseases.)

Exercise and healthy eating is probably still a good idea, but basically it just seems like hoping for the best without real knowledge of what may or may not be helping or hurting.