Meat that is barbecued, grilled or fried may contribute to accelerated ageing and Alzheimer's, a study suggests.
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 their 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 human volunteers aged 60 and over found that, like the mice, people with high amounts of Age compounds in their blood also lacked Sirt1.
Over a nine month period, raised levels of a neurotoxic family of Ages called methyl-glyoxal derivatives (MG) were associated with progressive mental decline and insulin resistance.
The scientists, led by Professor Helen Vlassara, from Mount Sinai School of Medicine in New York, wrote: "Because Ages can be modified in humans, recognition that this under-appreciated risk factor plays a role in AD (age-related dementia) and MS (metabolic syndrome) may open unique therapeutic avenues.
"Given the major public health potential of these findings, larger clinical trials are warranted."
British experts said the findings were thought-provoking but should be treated with caution.
Professor Derek Hill, from University College London, said: " There is a great deal of public interest in the way that diet can cause, or prevent, serious diseases in older life. Some of the proposed 'bad guys' in the diet are Ages, which are present in especially high quantities in meat that is cooked by frying or grilling.
"The results are compelling. Because cures for Alzheimer's disease remain a distant hope, efforts to prevent it are extremely important. But this study should be seen as encouraging further work, rather than as providing definitive answers.
"This paper adds to the body of evidence suggesting that using preventative strategies might reduce the prevalence of Alzheimer's disease and other dementias in society. And that could have very positive impact on us all."
Tom Dening, Professor of Dementia Research at the University of Nottingham, said: "Scientists have speculated since the 1990s that Ages may be important in Alzheimer's disease, which is the most important cause of dementia. This paper adds to the picture.
"Foods high in protein and fat, such as meat, cheese, and egg yolk, are rich in Ages, and cooking at high temperatures, eg frying and barbecues, increases Ages.
"What isn't yet clear is how much these effects contribute to Alzheimer's disease compared to other factors, and we don't know whether dietary restriction of Ages would be helpful in prevention."
Epidemiologist Dr David Llewellyn, from the University of Exeter, said: " Caution should be exercised when considering the possible clinical significance of oral glycotoxins, and this paper does not confirm a causal relationship in humans.
"We should be mindful that general dietary patterns are already known to be associated with dementia, and other environmental and lifestyle factors may also account for this association. This study therefore highlights the need for further research investigating dietary interventions as a strategy for dementia prevention or treatment."
Dr Doug Brown, director of research and development at the A lzheimer's Society, said: " We are often told that burgers or fried chicken are bad for us and this study is not the first to link the chemicals in some cooked foods to Alzheimer's. However, this research adds to our understanding of how they might work and makes a strong case for further research."
Dr Simon Ridley, from Alzheimer's Research UK, said: " Diabetes has previously been linked to an increased risk of dementia, and this small study provides some new insight into some of the possible molecular processes that may link the two conditions.
"Although these findings add to some earlier evidence linking a decrease in the Sirt1 protein to Alzheimer's, the most common cause of dementia, it's important to note that the people in this study did not have dementia. This subject has so far not been well studied in people, and we don't yet know whether the amount of Ages in our diet might affect our risk of dementia."