Could autonomic dysfunction, signaled by changes in heart rate variability (HRV), play a role in the development of metabolic syndrome (MetS) -- and in the increased coronary artery disease (CAD) mortality risk in persons with MetS? Emory research presented here today at the American Heart Association's Scientific Sessions in New Orleans suggests that's the case.
There has been a sharp rise of metabolic syndrome in the U.S. population -- statistics point to a nearly five-fold increase in the past 40 years and, according to the AHA, about 47 million U.S. adults now have MetS. People with the syndrome display a combination of several symptoms that may include increased abdominal fat, obesity, high blood sugar, raised levels of triglycerides and low levels of HDL ("good" cholesterol). A key underlying abnormality in MetS is known to be insulin resistance. Normally, receptors recognize insulin and allow it to transport glucose into cells where glucose is converted to energy. Insulin resistance occurs when receptors can't manage increased insulin levels and fail to allow glucose to enter cells, resulting in hyperglycemia and hyperinsulinemia.
"The causes of metabolic syndrome are uncertain, but several of its components have been linked with adrenocortical and autonomic disturbances. Our research looked at whether autonomic dysfunction, as measured by heart rate variability (HRV), is associated with insulin resistance," says Emory Heart Center scientist Viola Vaccarino, PhD, MD, who headed the research team.
The scientists studied 160 middle-aged male twins free of symptomatic CAD drawn from a national twin registry, the Vietnam Era Twin Registry based in Seattle, Washington. They assessed the function of the autonomic nervous system by measuring HRV with power spectral analysis , a non-invasive method of recording changes between heartbeats using 24 hour ECG Holter monitoring. An index of insulin resistance, the Homeostatic Model Assessment (HOMA), was derived from fasting glucose and insulin levels.
The average age of the research subjects was 53 and 33 percent were obese. Almost 10 percent had a history of diabetes. "We found those with greater insulin resistance had lower HRV scores," notes Dr. Vaccarino. "This association was not changed when we controlled for age, education and current smoking. Even excluding diabetic subjects did not change the results."
Dr. Vaccarino points out that the actual causes of MetS remain uncertain. "However, data are accumulating suggesting that autonomic and neuro-endocrine abnormalities typical of the stress response, such as excess cortisol, may play a role -- and our findings fit into this picture," she says. "If the role of stress in everyday life and its impact on the metabolism is clarified, this may have a huge impact in our understanding of what causes MetS as well as how to better prevent diabetes and heart disease."
In addition to Dr. Vaccarino, the research team included James D Bremner, MD; Carisa A. Maisano, MS.; Jerome L. Abramson, Ph.D.; Faiz A. Cheema, MD, and Nancy V. Murrah, RN of the Emory University School of Medicine, Atlanta, GA; Jack Goldberg, Vietnam Era Twin Registry, Seattle, Washington; and Rachel Lampert, MD, and Forrester Lee, MD, Yale University School of Medicine.