A new US study on monkeys found that social stress is linked to increase in deposits of harmful fat in the abdomen which can speed up the build up of plaque in blood vessels, a major risk factor for heart disease which is the number one cause of death in humans worldwide.

The study was the work of principal investigator Dr Carol A. Shively, a professor of pathology at Wake Forest University School of Medicine in Winston-Salem, North Carolina and colleagues Drs Thomas C Register and Thomas B Clarkson, also at Wake Forest. Their paper appears as the cover story in the current issue of Obesity, the peer-reviewed journal of the Obesity Society.

Shively said in a media statement that overweight people tend to carry most of their excess fat in the abdomen and fat located here behaves differently to fat in the rest of the body.

“If there’s too much, it can have far more harmful effects on health than fat located in other areas,” she added.

In Western societies, obesity appears to go up as socioeconomic status goes down, and this trend is the same for heart disease. Shively said this could be because the people who have the least access to resources that buffer us from the stresses of life are the ones most likely to suffer the health consequences.

In their paper, Shively and colleagues explained that in previous work with monkeys they showed there were significant links between (1) social stress and the amount of fat that gets deposited in the viscera or abdominal cavity, and (2) the amount of fat deposited around the middle of the body and the build up of plaque in blood vesses (coronary artery atherosclerosis, or CAA).

They said however that direct relationships between plaque build up and abdominal fat have so far not been demonstrated either in people or animals, and that this was the first study to look at links between stress, visceral obesity and CAA at the same time.

For this study, they fed 41 female monkey a Western-style diet containing fat and cholesterol for 32 months. The monkeys were kept in social groups where a natural dominant-subordinate hierarchy could develop.

In social groups, subordinate monkeys at the bottom of the hierarchy tend to be targets of aggression from other more dominant monkeys higher up the hierarchy. They are also given much fewer opportunities to take part in grooming sessions, and this can increase their social stress.

The researchers monitored the monkeys’ social behaviour and ovarian function, and a number of other biological variables, including BMI, stress biomarkers, and the amount of fat in the abdomen and elsewhere in the body (ie the subcutaneous fat).

Shively and colleagues found compared to monkeys whose ratio of adbominal fat to subcutaneous fat was low, the monkeys whose ratio was high were also the subordinate ones, who were socially isolated, received more aggresssion and less grooming, had impaired ovarian function, and had more biomarkers of stress (desensitized to circulating glucocorticoids). They also had higher heart rates late in the day and more plaque in their blood vessels (CAA).

Poor ovarian function meant that the ovaries produced fewer protective hormones.

They concluded that poor ovarian function in female monkeys with a high ratio of abdominal fat to subcutaneous fat is a new discovery and suggests there is a need to study fat distribution and ovarian function in women.

They suggested that the stress of being at the bottom of the pecking order resulted in the monkeys’ release of stress hormones that encouraged their bodies to deposit fat in their abdomens or viscera.

We already know that visceral fat encourages the build up of plaque in blood vessels, and this leads to heart disease, so this study suggests a link from social stress through plaque build up to heart disease.

However, Shively said that what is interesting about this relationship is that the bodies of human and monkey females have a natural protection against heart disease: on average women develop heart disease about 10 years after men.

She said perhaps stress and build up of visceral fat erodes this natural protection from the ovaries:

“Suppressed ovarian function is a very serious condition in a woman,” said Shively.

“Women who are hormone-deficient will develop more atherosclerosis and be at greater risk of developing coronary heart disease and other diseases such as osteoporosis and cognitive impairment,” she added.

Women whose ovaries don’t make enough hormones may not be aware of it as there are often no symptoms: the condition doesn’t always mean fewer menstrual cycles for instance.

Shively said:

“We need to take a closer look at the ovarian function of obese women.

“They might not be producing enough hormones to maintain adequate health,” she warned.

She also said that the study appeared to reinforce the usual message about health: be careful about what you eat, take regular exercise and manage your stress.

“Social Stress, Visceral Obesity, and Coronary Artery Atherosclerosis in Female Primates.”
Carol A. Shively, Thomas C. Register and Thomas B. Clarkson.
Obesity (2009) 17 8, 1513-1520.

Source: Wake Forest University Baptist Medical Center.

Written by: Catharine Paddock, PhD