The correlation between being overweight and risk of developing Alzheimer's disease is nowhere near as strong as, say, between being overweight and risk of developing type 2 diabetes. Given the evidence for chronic inflammation to be important in the development of Alzheimer's disease, and the many ways in which excess visceral fat tissue promotes chronic inflammation, it is somewhat puzzling that Alzheimer's isn't more of a lifestyle disease, similar to the way in which type 2 diabetes derives from lifestyle choices. That said, there is a contribution to Alzheimer's risk, and carrying excess weight is unwise, for this and many other reasons.
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. 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 analyzed 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 in 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. 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."