Stress Raises Belly Fat, Heart Risks
Study Shows Monkeys Under Long-Term Stress Put on Belly Fat, Get Heart Disease
Aug. 6, 2009 - Monkeys fed an American diet get fat -- but those under chronic stress put on much more belly fat.
That extra belly fat is why the stressed monkeys are much more likely to suffer blocked arteries and metabolic syndrome, a constellation of risk factors for heart disease, suggest Carol A. Shively, PhD, and colleagues at Wake Forest University.
In previous studies, Shively's team showed that socially stressed monkeys -- those at the bottom of the pecking order in a monkey colony -- get blocked arteries far faster than other monkeys fed the same high-fat diet.
But why do stressed monkeys get more belly fat?
"We wanted to know more about how the stress outside of you gets turned into plaque inside of your arteries," Shively tells WebMD. "So we looked at why stress caused atherosclerosis in our monkeys."
Over a two-year period, Shively and colleagues collected a vast array of data on stressed and unstressed female cynomolgus monkeys. The studies included a CT scan to detect visceral fat -- abdominal fat that often (but not always) protrudes as a "beer belly" on the outside. On the inside, it wraps around the organs.
Even compared to other monkeys with the same body mass index and weight, CT scans showed that the stressed monkeys had a great deal more belly fat. And when the researchers looked at the animals' arteries, they found plaque clogging the arteries of the stressed monkeys.
"So it's not how much fat you have, but where it is located," Shively says.
During the years of the study, the low-status monkeys had high levels of a stress hormone called cortisol. Over time, high cortisol levels cause belly fat to accumulate. It also makes individual fat cells get larger.
This is "sick fat," says Harold Bays, MD, medical director of the Louisville Metabolic and Atherosclerosis Research Center. Bays reviewed the Shively study for WebMD.
"Your body fat can become diseased like any other body tissue," Bays says. "Your fat cells are getting bigger and your fat tissue is getting bigger and neither the cells nor the tissues work as well as they should. The fat is sick."
"The monkeys that have a lot of abdominal fat have the metabolic syndrome, just like people with a lot of abdominal fat," Shively says. "When you have lots more fat in visceral fat cells and all the characteristics of the metabolic syndrome, each of these things promotes atherosclerosis."
Stress Strips Females of Heart Protection
All of the monkeys in the Shively study were female. One way monkeys are like humans is that females are less likely to get heart disease than males. Yet stressed female monkeys that put on belly fat are at least as likely to get heart disease as are male monkeys.