In a study of middle-aged and older people with diabetes, a decline in thinking and memory that is often later linked to dementia happened faster in those who were depressed than those who were not.
Over less than four years, researchers in Canada and the United States saw significant differences between depressed and non-depressed patients with diabetes in the erosion of a wide range of cognitive abilities.
The researchers looked at data on nearly 3,000 people over the age of 55 with type 2 diabetes for nine years, on average, and risk factors for cardiovascular events.
Tests of cognitive abilities were given to all participants at the start of the study, and again at 20months and 40 months.
One test measured psychomotor speed, or how long it takes the brain to register a stimulus, process it and respond. Another looked at the ability to remember words over time. A third test measured executive functioning, or how the brain uses memories to plan actions, pay attention and inhibit inappropriate behaviour.
The researchers determined whether an individual was depressed using a nine-question form that each person filled himself.
More than 2,600 people completed the tests at all three time points. Of them, 62 per cent did not have scores indicating depression, 18 per cent were depressed at the start of the study, 16 to 17 per cent were depressed at 20 months and 40 months and 5 per cent had scores indicating depression at all three time points.
Researchers found that people with symptoms of depression at any point were more likely to be women, younger, whites, overweight or obese, and to have higher levels of blood sugar, total cholesterol and low-density lipoprotein ("bad" cholesterol), than others.
Yet, the researchers reported in Jama Psychiatry that depression was linked to greater cognitive decline regardless of other risk factors. These included age, gender, race, obesity, smoking, alcohol use, previous heart attacks and strokes and the types of diabetes and heart disease drugs used.
Dr Mark Sullivan, professor of psychiatry at the University of Washington and lead author of the study, said: "This study demonstrates that depression accelerates cognitive decline in patients with diabetes, over a short time frame, in all patient subgroups and in all cognitive domains assessed. Whether depression treatment will reverse this effect remains to be tested in a separate randomised trial."
Both depression and diabetes have been found to raise the risk of Alzheimer's disease and other forms of dementia later in life.
Depression itself is associated with increases in stress hormones, inflammation and other processes that could directly contribute to cognitive decline, Dr Sullivan said.
Diabetes can damage the brain and raise a person's risk for a heart attack or stroke.
The array of other diseases that often go with diabetes may also harm cognition and promote depression.
For instance, people with diabetes are at higher risk for vascular dementia, which develops when blood supply to parts of the brain is cut off, often during a series of unnoticed "mini-strokes" that cause brain cells to die. Depression might be an early symptom of vascular disease in the brain, which later develops into dementia.
More than one-quarter of Americans aged above 65 have diabetes. About 6.5 million in this age group are depressed.
In Singapore, 11.3 per cent of adults have diabetes, and depression affects 6.3 per cent of adults at some stage of their lives.