Visceral or "deep" fat wraps around the inner organs and spells trouble for your health. How do you know if you have it? "If you have a large waist or belly, of course you have visceral fat," Whitmer says. Visceral fat drives up your risk for diabetes, heart disease, stroke, and even dementia.
Visceral fat is thought to play a larger role in insulin resistance -- which boosts risk of diabetes -- than other fat, Whitmer tells WebMD. It's not clear why, but it could explain or partially explain why visceral fat is a health risk.
Whitmer investigated the link between visceral fat and dementia. In a study, she evaluated the records of more than 6,500 members of Kaiser Permanente of Northern California, a large health maintenance organization, for an average of 36 years, from the time they were in their 40s until they were in their 70s.
The records included details on height, weight, and belly diameter -- a reflection of the amount of visceral fat. Those with the biggest bellies had a higher risk of dementia than those with smaller bellies. The link was true even for people with excess belly fat but overall of normal weight.
She doesn't know why belly fat and dementia are linked, but speculates that substances such as leptin, a hormone released by the belly fat, may have some adverse effect on the brain. Leptin plays a role in appetite regulation but also in learning and memory.
Belly fat has gotten a mostly deserved reputation as an unhealthy fat. "Understand that belly fat is both visceral and subcutaneous," says Kristen Gill Hairston, MD, MPH, an assistant professor of medicine at Wake Forest University School of Medicine, Winston-Salem, N.C. "We don't have a perfect way yet to determine which [of belly fat] is subcutaneous or visceral, except by CT scan, but that's not cost-effective."
But if you've got an oversize belly, figuring out how much is visceral and how much is subcutaneous isn't as important as recognizing a big belly is unhealthy, she says. How big is too big? Women with a waist circumference more than 35 inches and men with a waist circumference more than 40 inches are at increased disease risk.
Abdominal fat is viewed as a bigger health risk than hip or thigh fat, Whitmer and other experts say. And that could mean having a worse effect on insulin resistance, boosting the risk of diabetes, and a worse effect on blood lipids, boosting heart and stroke risks.