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"Our results have important public health implications given the increasing numbers of people developing diabetes and the need for more powerful interventions," study author Dr. Elina Ronnemaa, of Uppsala University, said in a statement.
The study included nearly 2,300 Swedish men who had glucose testing at age 50 to check for diabetes, a metabolic disease caused by abnormal insulin levels. The men were then followed for 32 years. The results: 102 were diagnosed with Alzheimer's disease, 57 with vascular dementia, and 235 with other types of dementia or cognitive impairment, the researchers said.
The study found that the men with low insulin levels at age 50 were nearly one-and-a-half times more likely to develop Alzheimer's disease than men who didn't have insulin problems. The risk of Alzheimer's increased, regardless of blood pressure, cholesterol, body-mass index and education.
"Our results suggest a link between insulin problems and the origins of Alzheimer's disease and emphasize the importance of insulin in normal brain function," Ronnemaa said. "It's possible that insulin problems damage blood vessels in the brain, which leads to memory problems and Alzheimer's disease, but more research is needed to identify the exact mechanisms."
The findings were published in the April 9 online issue of Neurology.
Another study, published in the April 8 print issue of Neurology, found that depression appears to more than double the risk of developing Alzheimer's.
Depression has been linked to an increased risk of Alzheimer's, and many researchers have suspected that risk is tied to changes in the brain caused by depression. The theory has been that depression shrinks specific areas, leaving the brain vulnerable to the development of Alzheimer's. However, this study found that's not the case.
"We don't know yet whether depression contributes to the development of Alzheimer's disease or whether another unknown factor causes both depression and dementia," lead researcher Dr. Monique M.B. Breteler, from the Erasmus University Medical Center in Rotterdam, the Netherlands, said in a statement. "We'll need to do more studies to understand the relationship between depression and dementia."
In the study, Breteler and her colleagues collected data on 486 people, aged 60 to 90, who did not have dementia. Among these individuals, 134 had had at least one episode of depression.
During an average six years of follow-up, 33 people developed Alzheimer's disease. The researchers found that those who had had an episode of depression were 2.5 times more likely to develop Alzheimer's compared with people who had never had depression. For people whose depression occurred before they were 60, the risk for developing Alzheimer's was fourfold greater than people who had never had depression, the researchers reported.
One goal of Breteler's research was to determine if depression causes changes in the brain that increase the risk of Alzheimer's disease, as had been suspected by other scientists.
Researchers have proposed that depression leads to a loss of cells in the areas of the brain called the hippocampus and the amygdala, thereby increasing the risk for Alzheimer's. However, Breteler's group didn't find any difference in the size of those areas of the brain in people with or without depression.
In addition, Breteler's team found that people who had symptoms of depression at the start of the study were no more likely to develop Alzheimer's than those who did not have the symptoms.
Even though the study found a connection between depression and Alzheimer's, experts continue to debate whether such a connection really exists.
"There are quite a few papers about the association between depression and Alzheimer's, with conflicting results," said Yaakov Stern, a professor of clinical neuropsychology at Columbia University in New York City. "No one really knows if there is a connection between depression and Alzheimer's."
Another expert wondered whether treating depression could prevent dementia.
"Depression may be a risk factor for developing dementia. I believe that's true," said Dr. Gary Kennedy, director of geriatric psychiatry at Montefiore Medical Center in New York City. "Depression is also one of the side effects of dementia.
"So there is an overlap relationship that makes it very difficult to look at the more interesting question of whether, if we treat depression, could we prevent dementia," Kennedy said.
Even though the clinical implications for the association between depression and dementia aren't clear, Kennedy believes that older people with depression should have their depression treated. "There's even more reason to make sure depression is aggressively treated," he said.
SOURCES: April 9, 2008, news release, American Academy of Neurology; Yaakov Stern, Ph.D., professor, clinical neuropsychology, Columbia University, New York City; Gary Kennedy, M.D., director, geriatric psychiatry, Montefiore Medical Center, New York City; April 8, 2008, Neurology
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