Being obese in middle age is a known risk factor, not only for heart disease, but for dementia.

But in recent years, there’s been some suggestion that the so-called “healthy” obese, those whose obesity is not accompanied by other risk factors like high glucose or high cholesterol, don’t have an increased risk of heart disease or other health conditions. What wasn’t known, however, was whether the healthy obese are also safe from the more rapid decline in cognitive function.

Now, researchers in Europe have answered this question. According to a new study, published in the journal Neurology, obesity in and of itself increases the speed of mental decline over time. There is no "healthy overweight" when it comes to preserving your mind.

The study included about 6,400 middle-aged adults, averaging about 50 years old at the start of the study. The researchers at the French National Institute of Health and Medical Research (INSERM), collected information on body mass index (BMI) and other metabolic risk factors as well as on memory and other cognitive skills at the beginning of the study. The participants were then retested for memory and cognitive skills three times over the next 10 years.

About 40 percent of the 582 participants who were obese were considered metabolically healthy. Participants were classified as metabolically unhealthy if they had two or more of the following risk factors: high blood pressure or taking medication for it; low HDL or "good" cholesterol; high blood sugar or taking diabetes medication; or high triglycerides (a type of fat found in the blood).

“Obesity, in those who were metabolically healthy and unhealthy, was associated with poor cognitive function at the start of the study and greater cognitive decline over 10 years,” said Archana Singh-Manoux, research director at INSERM and a co-author of the study.  The cognitive decline was greatest in those with both obesity and metabolic abnormality, but not statistically different from the decline in the “healthy” obese. The “unhealthy obese” experienced a 22.5 percent faster decline in cognitive test scores over the ten years than those who were normal weight and without metabolic abnormalities.

In addition, normal and overweight individuals who had two or more metabolic risk factors also showed poorer cognitive function, suggesting that these are independent risk factors for dementia as well.

The authors of the study explained that obesity may lead to vascular changes, which can affect the flow of blood to the brain or cause inflammatory factors that circulate to the brain. Fat cells also release hormones that may affect the brain.

Singh-Manoux said the study refutes the idea of a "metabolically healthy obesity."

“The prevalence of obesity is rising--400 million adults were obese in 2005 and this number is expected to rise to over 700 million by 2015," Singh-Manoux said. "Obesity is known to have adverse effects on health; it is associated with higher risk of mortality and chronic diseases. Our results add to this list of adverse health effects, showing poorer cognitive outcomes among the obese."