By looking after his gut microbiome, the trillions of microbes that live in the colon lining and impact overall health, he's also lowering his risk of colon cancer, which is rising in people under 50. Research has linked eating plant foods such as vegetables, nuts, and beans to a diverse microbiome, which is an indicator of good gut health, and a lower colon cancer risk. While ultra-processed foods, including processed red meats such as hot dogs and salami, have been linked to less diversity and a higher risk.
There are differences in Alzheimer's patients and their gut microbiota, and that's been verified by many groups now. Prior research has shown that when you make changes in the gut microbiota of Alzheimer model mice, you in turn have effects on the pathology of the hallmarks of Alzheimer's - amyloid plaques and tau accumulation.