Fatty and sugary foods could also be playing a part by boosting levels of harmful compounds called advanced glycation endproducts (Ages).
Scientists in the US found evidence that diet-related Ages might aid the development of dementia by suppressing a protective anti-ageing enzyme.
Previous research in animals and humans has linked abnormally low levels of the enzyme, Sirt1, to age-related brain and metabolic diseases. Ages are formed by sugar reacting with amino acids, the building blocks of proteins.
They are abundant in "browned" meat cooked at high temperatures. Barbecued and fried meat may contain especially high levels of Ages. The compounds can also be derived from cheese, eggs, white bread, pasta, and sugary treats such as pastries, cakes and biscuits.
Researchers writing in the journal Proceedings of the National Academy of Sciences fed mice Ages at typical western diet levels and tracked brain health.
Unlike animals not given a high-Age diet, they were found to have low levels of Sirt1 in their blood and brain tissue.
Crucially, they also accumulated deposits of amyloid-beta protein in their brains, a key biomarker of Alzheimer's.
In addition, the mice displayed signs of mental impairment as well as insulin resistance, a pre-diabetic condition.
A follow-up study of 93 volunteers aged 60 and over found that, like the mice, people with high amounts of Age compounds in their blood also lacked Sirt1.