Two years ago, limiting sugar was associated with better mental health.

“They may be on to something,” says Rachel Richardson, director of the evidence generation and methods directorate at Cochrane, an independent not-for-profit network that collects and summarizes medical research.

However, this “doesn’t represent a smoking gun,” she says, linking earlier diabetes to dementia.

One big issue is that it’s impossible to know how much sugar each of the people in the study was eating, only when they were born in terms of sugar levels.

“This evidence relates to people born in the 1940s and 1950s and has limited applicability to young children today – for example, early weaning, which can have a big impact on the early feeding of babies and children,” Richardson said.

Professor Ian Maidment, professor of clinical pharmacy at Aston University, said: “It’s an interesting idea, it helps inform the debate, but in my opinion we need more evidence.

Linking “something from 80 years ago” and projecting that onto health means there are other “loads” that play a role, he said.

Dr Sarah Rodrigues, Alzheimer’s Research UK, said: “It’s important to remember that this is an observational study.

“This means that it can identify a link between diet and dementia risk, but it cannot prove that eating less sugar directly prevents dementia.

“Eating a healthy, balanced diet is one of the best ways to support your mental health and it’s never too late to make positive changes.”