The development of Alzheimer's disease is associated with higher levels of visceral belly fat in midlife, according to research that will be presented next week at the annual conference of the Radiological Society of North America (RSNA).

Fat around the internal organs located deep within the abdomen is called visceral fat. This hidden belly fat has been linked by researchers to alterations in the brain that can develop up to 15 years before the first signs of Alzheimer's disease, such as memory loss.

Over 6 million Americans suffer from Alzheimer's disease, according to the Alzheimer's Association. This figure is expected to increase to around 13 million by 2050. Alzheimer's disease affects one in five women and one in ten men at some point in their lives.

To try and identify Alzheimer's risks earlier, researchers assessed the association between brain MRI volumes, as well as amyloid and tau uptake on positron emission tomography (PET) scans, with body mass index (BMI), obesity, insulin resistance and abdominal adipose (fatty) tissue in a cognitively normal midlife population. Amyloid and tau are proteins thought to interfere with the communication between brain cells.

"Even though there have been other studies linking BMI with brain atrophy or even a higher dementia risk, no prior study has linked a specific type of fat to the actual Alzheimer's disease protein in cognitively normal people," said study author Mahsa Dolatshahi, M.D., M.P.H., a post-doctoral research fellow with Mallinckrodt Institute of Radiology (MIR) at Washington University School of Medicine in St. Louis. "Similar studies have not investigated the differential role of visceral and subcutaneous fat, especially in terms of Alzheimer's amyloid pathology, as early as midlife."

For this cross-sectional study, researchers analysed data from 54 cognitively healthy participants, ranging in age from 40 to 60 years old, with an average BMI of 32. The participants underwent glucose and insulin measurements, as well as glucose tolerance tests.

The volume of subcutaneous fat (fat under the skin) and visceral fat were measured using abdominal MRI. Brain MRI measured the cortical thickness of brain regions that are affected by Alzheimer's disease.

PET was used to examine disease pathology in a subset of 32 participants, focusing on amyloid plaques and tau tangles that accumulate in Alzheimer's disease.

The researchers found that a higher visceral to subcutaneous fat ratio was associated with higher amyloid PET tracer uptake in the precuneus cortex, the region known to be affected early by amyloid pathology in Alzheimer's disease.

This relationship was worse in men than in women. The researchers also found that higher visceral fat measurements are related to an increased burden of inflammation in the brain.

"Several pathways are suggested to play a role," Dr. Dolatshahi said. "Inflammatory secretions of visceral fat--as opposed to potentially protective effects of subcutaneous fat--may lead to inflammation in the brain, one of the main mechanisms contributing to Alzheimer's disease."

Senior author Cyrus A. Raji, M.D., Ph.D., associate professor of radiology and neurology, and director of neuromagnetic resonance imaging at MIR, noted that the findings have several key implications for earlier diagnosis and intervention.

"This study highlights a key mechanism by which hidden fat can increase the risk of Alzheimer's disease," he said. "It shows that such brain changes occur as early as age 50, on average--up to 15 years before the earliest memory loss symptoms of Alzheimer's occur."

Dr. Raji added that the results may point to visceral fat as a treatment target to modify the risk of future brain inflammation and dementia.

"By moving beyond body mass index in better characterising the anatomical distribution of body fat on MRI, we now have a uniquely better understanding of why this factor may increase the risk for Alzheimer's disease," he said.