People with depression have a 83% greater risk of developing dementia, a study has suggested.
Those with both depression and type 2 diabetes had a 117% higher risk, the research carried out in the United States found, while t hose with type 2 diabetes alone had a 20% greater chance.
A team at the University of Washington in Seattle carried out the research by examining the records of 2.4 million Danish citizens aged 50 plus and initially free from dementia, over six years.
They found that during that period, 2.4% (59,663 people) developed dementia with the average age at diagnosis nearly 81.
Of those individuals who developed dementia, 26.4% (15,729 people) had depression alone and 10.8% (6,466 people) had type 2 diabetes alone, while 6.7% (4,022 people) had both conditions.
The effect on risk for dementia appeared to be even greater among those study participants younger than 65, the study found.
Dr Doug Brown, director of research and development at Alzheimer's Society said: " The more we learn about dementia the more we learn how other conditions may play a part in increasing risk of developing the condition.
"This study adds to a growing body of evidence that poorly managed type 2 diabetes and depression may increase risk and suggests that together, the two conditions could interact in a way that contributes to even greater risk.
"What's not clear cut is why. This is a complex area that we need to see more research into so that we can fully understand this link and what this means for us all."
He said it was important that such research did not make people with diabetes or depression assume they are going to develop dementia.
"If you're concerned, contact your GP but in the meantime the best way to reduce risk of developing dementia is to eat a healthy balanced diet, take plenty of exercise, maintain a healthy weight and not smoke," he said.
Dr Tara Spires-Jones, of the Centre for Cognitive and Neural Systems at the University of Edinburgh, said that while "very interesting", the observational study does not prove that there is a direct link between the conditions and dementia.
She added: "This study of 2.4 million people highlights the need for more fundamental research into the relationships between depression, diabetes, and dementia.
"More studies will be needed to determine whether depression, diabetes, or the combination of the two are biologically important in causing dementia, or whether other related factors such as diet, genes, and healthy lifestyle are the culprits.
The research is published in JAMA Psychiatry.