Carrying around extra pounds during middle age was associated with a higher risk of dementia later in life in a new study that followed twins in Sweden for 30 years.
The research was not set up to prove that dementia was caused by the added weight, but Dr. Weili Xu, the study's lead author from the Karolinksa Institutet in Stockholm, said that the evidence is pointing in that direction.
The findings, published in the journal Neurology, suggest that "control of body fat as early as middle life is important to prevent dementia later in life," she told Reuters Health.
Xu and her colleagues analyzed data from close to 9,000 Swedish twins.
When the participants were an average age of 43, they gave researchers information about their height and weight.
Thirty years later, the researchers examined the same individuals for signs of declining thinking and memory skills, then diagnosed some of them with Alzheimer's disease and other types of dementia.
Close to one in three of the participants were overweight or obese in middle age. And those that were had about an 80% higher chance of getting any kind of dementia than people of normal weight.
The more participants weighed in mid-life, the higher their chance of getting dementia or "questionable dementia" - meaning they had signs of thinking and reasoning problems, but not enough to be diagnosed with dementia.
In total, about 4% of everyone in the study was diagnosed with dementia, and another 1 to 2% with questionable dementia.
Despite the link between excess pounds in midlife and later dementia, when the researchers looked specifically at 137 twin pairs who were "discordant" - one twin had dementia and the other didn't - the tie to midlife overweight shrank considerably.
While Xu said that finding suggests that "there are some common genes that predispose (people) to both diseases (overweight and dementia)," it could also be that it was just more difficult to establish a solid link in such a small sample.
Whether genes predispose a person to being overweight in adulthood, or it's just bad eating habits, the likely explanation for the link to dementia, researchers say, is that fat tissue in the body releases hormones and other signaling cells that may affect the brain's functioning.
In addition, Xu said, extra weight has been shown to increase a person's risk for diabetes and heart and blood vessel diseases - and those conditions are related to a higher dementia risk. However, the link between weight and dementia remained even after the researchers took other diseases into account.
The findings are the latest evidence that preventing Alzheimer's disease and dementia starts long before their signs and symptoms typically show up, said Rachel Whitmer, an epidemiologist at Kaiser Permanente Division of Research in Oakland who was not involved in the study.
"People need to understand that what they do today could have an effect on them 30 or 40 years from now," Whitmer told Reuters Health.
When it comes to maintaining a healthy weight, she said, "what's good for the heart is good for the brain."