Fat-Bellied Monkeys Suggest Why Stress Sucks

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Chris huh

A cynomolgus monkey in Lopburi, Thailand

It's no secret that stress isn't good for you. But what's less clear is how social stressors like a high-pressure job or a failing marriage affect your physical well-being.

Researchers at Wake Forest University who study stress in monkeys think they may have discovered a clue: fat. More specifically, the particular form of fat called visceral fat, which tends to build up in the abdomen (those dreaded beer bellies and love handles). Researchers believe this abdominal fat lodges deep within visceral organs, such as the heart, liver and blood vessels, and may be an indicator of increased heart-attack risk. In a study of 42 female monkeys, the scientists found that those with the most social stress — in the monkeys' case, that meant being at the bottom of the social hierarchy — packed away the most fat around the middle.

"For years now, there has been a recognition that the pattern in which people lay down fat is associated more with health than the absolute amount of fat," says study co-author Carol Shively, a pathologist at Wake Forest. "Fat cells that live in the visceral depot behave differently than cells that live in other areas of the body."

Recent evidence suggests that visceral fat cells are active, unlike the fat cells found elsewhere in the body just under the skin, known as subcutaneous fat. Those fat cells are essentially just storage sinks for calories. But visceral fat cells actively secrete hormones and other agents that affect the metabolism of sugar and the way the body burns calories. In people, visceral fat has been linked to metabolic changes, such as higher blood pressure and blood-sugar levels, that increase risk for diabetes and heart disease.

Shively and her colleagues also knew that people who produce excessive amounts of the stress hormone cortisol tend to have bulky waistlines; they have apple-shaped bodies, rather than pear-shaped ones. So the researchers wanted to examine all these factors — stress, abdominal fat and health risk — in one study. The problem, of course, is that measuring the relationship between stress and visceral fat in people in a controlled fashion isn't easy. So the team turned to monkeys. For nearly 2½ years, she and her team fed the animals a typical Western diet, with 40% of calories coming from fat, measured their cortisol levels and used CT scans to calculate the amount of visceral fat each monkey carried.

The monkeys were housed in groups of four, automatically prompting them to establish a linear hierarchy of dominance. The dominant monkey in each group experienced the least stress, according to researchers. "They were groomed more than the subordinates, and they would get relaxed. Their eyes would roll up, sort of like they were getting a massage," says Shively. Monkeys further down the power chain, however, appeared more stressed-out. They were more vigilant, constantly scanning their environment for potentially aggressive threats from the leader. They also spent more time alone, out of contact with the other monkeys.

CT scans showed that group leaders and the second most dominant monkeys had lower amounts of visceral fat than their subordinates, who carried the bulk of their body fat in their guts. In human populations, something similar happens: studies have linked lower social status to a higher incidence of metabolic syndrome — the condition whose symptoms include high blood pressure, high glucose levels and being overweight — which promotes heart disease.

Together with Shively's findings, says Dr. David Katz, director and co-founder of the Yale Prevention Research Center, the human data suggest a possible cause-and-effect link: stress may promote accumulation of visceral fat, which in turn causes metabolic changes in the body that contribute to heart disease and other health problems.

"This study shows that psychological stress, which we know can affect stress-hormone levels, can have a fairly rapid influence on where extra calories go," he says. "I'm generally quite cautious about animal research, but here I think we're seeing something that has direct relevance to human health as well."