but it is making her lose hers.
Although carrying excess weight anywhere appears to impair older women’s brains, carrying it on the hips may make matters worse, they say.
The Northwestern Medicine team found “apple-shaped” women fared better than “pears” on cognitive tests.
But depositing fat around the waist increases the risk of cancer, diabetes and heart disease, experts warn.
They said the findings, in the Journal of the American Geriatrics Society, highlighted the importance of maintaining a healthy weight for both body and mind.
Some of the health risks associated with obesity, such as vascular disease and inflammation, may explain why people who are overweight appear to be at higher risk of dementia.
However, the latest study suggests a bit of extra fat around the waist may actually protect brain functioning.
The researchers believe belly fat makes more of the female hormone oestrogen that naturally dips after the menopause.
Oestrogen is thought to help protect the brain from cognitive decline.
The study involved 8,745 post-menopausal women aged 65 to 79.
These women were asked to complete a memory test that doctors use to judge brain function. They were also weighed and measured, then scored on an obesity scale known as Body Mass Index or BMI. Over two-thirds of the women were overweight or obese.
The researchers found that for every one point increase in a woman’s BMI, her memory score dropped by one point.
And pear-shaped women – those with smaller waists but bigger hips – scored particularly poorly.
The researchers say this is likely to be related to the type of fat deposited around the hips versus the waist.
Scientists already know different kinds of fat release different hormones and have varying effects on insulin resistance, lipids and blood pressure.
Lead researcher Dr Diana Kerwin said: “We need to find out if one kind of fat is more detrimental than the other, and how it affects brain function.
“The fat may contribute to the formation of plaques associated with Alzheimer’s disease or a restricted blood flow to the brain.”
Rebecca Wood, chief executive of the Alzheimer’s Research Trust, said: “The pear-shape is incredibly common, and while this study doesn’t explain fully the link between body shape and brain function, it surely makes the case for watching the scales.