While the mental decline may be invisible to the individual, the fact that the drop-off starts accumulating in middle age puts diabetics at greater risk later on because of reduced "brain reserves," Dr. David Knopman, of the Mayo Clinic in Rochester, Minnesota, told Reuters Health.
"Like a bicycle tire that's been partially deflated -- you'll be OK riding around but if you develop another little leak you'll be much closer to a flat tire much faster," said Knopman, who was not involved in the Dutch study.
Astrid Nooyens and colleagues at the National Institute for Public Health and the Environment in the Netherlands examined the health records and mental acuity scores of more than 2,600 men and women between the ages of 45 and 70 who enrolled in a large ongoing study into lifestyle effects on health.
At the five-year mark, of the 139 participants with type 2 diabetes, 61 were diabetics at the beginning of the study and 78 developed the chronic disease within the next five years.
The study confirmed the findings of earlier research, by Knopman and others, of an association between diabetes and declines in such mental functions as the ability to think quickly and recall words, but this is the first project to test memory and demonstrate how quickly the drop-off can occur.
Over a five-year period, decline in overall mental functioning in people with type 2 diabetes, while small, was nearly 3 times more pronounced than in non-diabetics.
But it didn't take many years for the impact to be felt. Even those who developed diabetes after beginning the study saw twice as much of a decline as their non-diabetic counterparts.
Compared to the "healthy" participants, participants who had long-term diabetes registered the largest declines in mental function. Those who developed diabetes during the trial saw less pronounced declines than their long-term counterparts in most areas except information processing, where they appeared to do a little better than the "healthy" people.
Type 2 diabetes is characterized by high blood sugar levels caused by the body's inability to process sugar properly. The illness can usually be controlled through diet and exercise but may also require drugs.
The Nooyens group found that while memory continues to decline for those with diabetes, the drop-off in thinking speed appears to occur in the first five years and then level off. That led the authors to suggest that early treatment and control of blood sugar levels could help thinking speed, but probably not memory, they note in the journal Diabetes Care.
The researchers found that for a small group of people who had lived with diabetes for nearly seven year, blood sugar levels did not explain the entire decline in mental function. In those people, they suspect other conditions related to diabetes such as high blood pressure and obesity.
The study did not look at whether patients with well-controlled diabetes experienced less mental decline compared to their poorly controlled counterparts, although the authors point out that there are other reasons, such as heart disease, to control sugar levels as well.
They also note that the random blood tests of both the long- and short-term diabetics suggested what treatment they were getting was "insufficient."
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