I've been taking Vitamin D for a while, it is supposed to combat Alzheimer's but you have to take at least a certain mg per day and you need to take it together with Vitamin C too as they work together. My Dr didn't know that

Although there definitely is a general problem with doctors not generally keeping up to date with new research - there is no study (as of yet) to confirm this link
This sums it up
https://www.nia.nih.gov/health/alzheimers-and-dementia/what-do-we-know-about-diet-and-prevention-alzheimers-diseaseIn particular the paragraph
But despite early findings of possible benefits for brain health, no vitamin or supplement has been proven to prevent Alzheimer’s in people. Overall, evidence remains weak because many studies were too small or too brief to be conclusive.
Which is to say various vitamins and supplements might help - but statistically there isn't enough evidence to back that up yet - that would be why the average GP wouldn't necessarily know anything about it
25 years ago my other half had cancer, we went to Mexico for 'alternative' treatment. The clinic was run by Dr Contreras, one of the world leading experts in cancer treatment and 'alternative' therapies, he still runs it. One of the things that we learnt from him is that there are many, many studies carried out into cancer treatments, almost all paid for by the drug companies, however if they seem to be producing evidence that natural remedies ARE effective against cancer then the funding almost always gets pulled and the results are not published. On his advice my other half had chemo back in the UK, she had 4 drugs, 3 were artificial copies of natural remedies. If the drugs companies can't make money out of something they are unlikely to pay for research to prove that it works. I wonder if the same is true for vitamin D, C or any other natural supplement and it's possible prevention of dementia and alzheimers?