Poor sensitivity to insulin is typically associated with type 2 diabetes, in which liver, fat and muscle cells fail to respond to the hormone. But results such as de la Monte's have led some researchers to wonder whether Alzheimer's may sometimes be another version of diabetes - one that hits the brain. Some have even renamed it "type 3 diabetes".

115m people globally will get Alzheimer's by 2050

If they are right - and a growing body of evidence suggests they might be - the implications are deeply troubling. Since calorific foods are known to impair our body's response to insulin, we may be unwittingly poisoning our brains every time we chow down on burgers and fries. People with type 2 diabetes, who have already developed insulin resistance, may be particularly at risk. "The epidemic of type 2 diabetes, if it continues on its current trajectory, is likely to be followed by an epidemic of dementia," says Ewan McNay of the University at Albany in New York. "That's going to be a huge challenge to the medical and care systems."