It has long been known that sporting a “spare tyre” raises the risk of heart disease and type 2 diabetes.
New research now suggests carrying too much abdominal fat may leave us less mentally sharp.
Scientists from Iowa State University in Ames looked at more than 4,000 healthy adults.
They found those with more abdominal fat and less lean muscle had reduced “fluid intelligence” - reasoning, thinking and problem solving skills - six years later.
“Chronological age doesn't seem to be a factor in fluid intelligence decreasing over time,” study author Dr Auriel Willette said.
“It appears to be biological age, which, here, is the amount of fat and muscle.”
To reach their results, the scientists analysed thousands of “cognitively unimpaired” adults, with an average age of 64.
The participants, taken from the UK Biobank study, had their fluid intelligence assessed every two years for six years.
Results - published in the journal Brain, Behavior and Immunity - suggest abdominal fat is linked to cognitive decline, while lean muscle mass protects against it, in both sexes.
The findings remained true after adjusting for age, education and socioeconomic factors.
As to why this occurs, the scientists suspect the immune system may play a role.
A high BMI has previously been linked to reduced immune activity in the blood. This may trigger immune reactions in the brain that throw off memory and thinking.
BMI as a measurement, however, does not distinguish between different types of fat and muscle mass.
During the six year study, the participants also gave blood samples, which were analysed for immune cells.
The scientists concluded a reduction in lymphocyte cells and an increase in oesinophils were behind the fat-brain link in women.
In men, fluctuations to the immune cell basophils were found to play a role.
Off the back of their results, the scientists stress the importance of staying fit into old age.
“If you eat alright and do at least brisk walking some of the time, it might help you with mentally staying quick on your feet,” Dr Willette said.
According to the NHS, many adults aged 65 or over spend an average of 10 hours a day sitting or lying down.
It encourages them to aim for at least two-and-a-half hours of moderate exercise a week. This can be fast walking, water aerobics or even pushing a lawn mower.
The Iowa team hope their study will pave the way for new treatments that help ageing adults stay sharp, particularly if they are obese, inactive or losing muscle mass.