A correlation between above-average homocysteine and Alzheimer’s allows for outliers. There can be Alzheimer’s patients with normal or even below-average homocysteine levels.

If the effects are unrelated to Alzheimer’s, then they have no impact on the conclusion, which is focused only on whether Alzheimer’s can be reduced by converting homocysteine to those substances.

This just suggests B vitamins and folic acid should be taken in other forms besides vitamin-mineral supplements. The author never suggested any specific form in which we take B vitamins and folic acid.

This suggests that there’s also a genetic component to Alzheimer’s. But there could be a genetic component to having higher homocysteine levels. In addition, it’s possible for there to be multiple, independent causes of Alzheimer’s.

This provides an alternate explanation for the correlation observed. If Alzheimer’s increases homocysteine, we’d expect people with Alzheimer’s to have higher levels of homocysteine, even if homocysteine doesn’t cause Alzheimer’s.