(Patrick T. Fallon/Bloomberg)

THE QUESTION Diabetes can lead to nerve damage, kidney failure, vision problems and heart disease. Should cognitive decline be added to that list?

THIS STUDY analyzed data on 13,351 middle-aged and older adults (average age 57 at the start of the study), including 1,779 people (13.3 percent) with diabetes. Over a 20-year period, scores on standardized tests of cognitive function, given periodically, fell among nearly everyone, but the decline was 19 percent greater among those with diabetes. People whose diabetes was poorly controlled experienced the greatest decline. Those with pre-diabetes experienced greater cognitive decline than people who did not have diabetes. Also, the longer people had diabetes, the greater their decline. Having diabetes was found to age the brain about five years faster than normal, giving a 55-year-old with diabetes the cognitive ability of a 60-year-old without the disease.

WHO MAY BE AFFECTED? People with diabetes in middle age. In the United States, more than 29 million people, nearly 10 percent of the population, have diabetes, though not all have been diagnosed. Diabetes develops when the body either does not make enough insulin or cannot use insulin properly, resulting in too-high levels of glucose (sugar) in the blood. Diet and exercise are primary treatment and prevention methods, but many people with diabetes also need to take medication or insulin to control their condition.

CAVEATS Testing for diabetes was done once, at the start of the study. Some people had a stroke during the study period; the researchers noted that excluding their data slightly affected the results. The study found an association between diabetes and cognitive decline but did not prove that diabetes caused the decline.

FIND THIS STUDY Dec. 1 issue of Annals of Internal Medicine (www.annals.org).

LEARN MORE ABOUT diabetes at www.diabetes.org and www.mayoclinic.org.

The research described in Quick Study comes from credible, peer-reviewed journals. Nonetheless, conclusive evidence about a treatment's effectiveness is rarely found in a single study. Anyone considering changing or beginning treatment of any kind should consult with a physician.