Visceral adipose tissue, rather than body mass index (BMI) or overall subcutaneous fat, was associated with the development of hypertension, researchers found.
In a multivariate analysis, BMI initially was associated with incident hypertension, with a relative risk of 1.24 (95% CI 1.12-1.36, P<0.0001) per standard deviation increase, according to Aslan T. Turer, MD, and colleagues from the University of Texas Southwestern Medical Center in Dallas.
However, after visceral adiposity and subcutaneous fat were included in the model, only visceral adipose tissue remained significant, with a relative risk of 1.22 (95% CI 1.07-1.39, P=0.004) for each standard deviation increase, the researchers reported in the Journal of the American College of Cardiology.
“There is growing evidence that [visceral adipose tissue] represents a pathological adipose tissue depot, which accumulates when subcutaneous depots are overwhelmed or otherwise unavailable for storage. Relative to [subcutaneous adipose tissue], visceral fat is more sensitive to lipolysis and secretes higher amounts of inflammatory cytokines,” Turer and colleagues wrote.
The central mystery that the investigators sought to address is that hypertension has been clearly linked with obesity, yet not all obese individuals develop high blood pressure.
To see if the distribution, rather than quantity, of body fat could be implicated, the researchers analyzed data from the Dallas Heart Study, in which 903 participants had an initial evaluation between 2000 and 2002.
What’s wrong with abdominal fat?
Body fat, or adipose tissue, was once regarded as little more than a storage depot for fat blobs waiting passively to be used for energy. But research suggests that fat cells - particularly abdominal fat cells - are biologically active. It’s more accurate to think of fat as an endocrine organ or gland, producing hormones and other substances that can profoundly affect our health. One such hormone is leptin, which is normally released after a meal and dampens appetite. Fat cells also produce the hormone adiponectin, which is thought to influence the response of cells to insulin. Although scientists are still deciphering the roles of individual hormones, it’s becoming clear that excess body fat, especially abdominal fat, disrupts the normal balance and functioning of these hormones.
Scientists are also learning that visceral fat pumps out immune system chemicals called cytokines - for example, tumor necrosis factor and interleukin-6 - that can increase the risk of cardiovascular disease by promoting insulin resistance and low-level chronic inflammation. These and other biochemicals, some not yet identified, are thought to have deleterious effects on cells’ sensitivity to insulin, blood pressure, and blood clotting.
One reason excess visceral fat is so harmful could be its location near the portal vein, which carries blood from the intestinal area to the liver. Substances released by visceral fat, including free fatty acids, enter the portal vein and travel to the liver, where they can influence the production of blood lipids. Visceral fat is directly linked with higher total cholesterol and LDL (bad) cholesterol, lower HDL (good) cholesterol, and insulin resistance.
Insulin resistance means that your body’s muscle and liver cells don’t respond adequately to normal levels of insulin, the pancreatic hormone that carries glucose into the body’s cells. Glucose levels in the blood rise, heightening the risk for diabetes. Together, insulin resistance, high blood glucose, excess abdominal fat, unfavorable cholesterol levels (including high triglycerides), and high blood pressure constitute the metabolic syndrome, a major risk factor for heart disease and stroke.
Excess fat at the waist has been linked to several other disorders as well. A European study of nearly 500,000 women and men found that, for women, a waist-to-hip ratio above 0.85 was associated with a 52% increase in colorectal cancer risk. A long-running community study on atherosclerosis conducted by researchers at Wake Forest University found that even among normal-weight people, those with higher waist-to-hip ratios had just as much difficulty as those with higher BMIs in carrying out various activities of daily living, such as getting in and out of bed and performing household chores.
A larger waist measurement also predicts the development of high blood pressure, regardless of total body fat, according to a 10-year study of Chinese adults published in the August 2006 American Journal of Hypertension. Finally, a study presented at the 2005 annual meeting of the Society for Neuroscience found that older people with bigger bellies had worse memory and less verbal fluency, even after taking diabetes into account.
This consisted of blood pressure measurements, imaging studies to quantify adipose tissue at various sites in the abdomen and lower body, and laboratory assessment of relevant biomarkers.
Follow-up studies were then done between 2007 and 2009.
During the intervening 7 years, 230 individuals - one-quarter of the cohort -were diagnosed with hypertension.
Those who became hypertensive were older (43 versus 39), more commonly had already developed diabetes (7% versus 2%) and more often were black (56% versus 33%).
At the end of the study period, 25 percent of patients developed hypertension. While higher BMI was associated with increased incidence of hypertension, when abdominal fat content, overall fat content and lower-body fat content were factored in, only abdominal fat remained independently associated with hypertension. The relationship between abdominal fat and hypertension did not change when factoring in gender, age or race.
The strongest correlation between abdominal fat and hypertension was observed with retroperitoneal fat, which is a type of visceral fat located behind the abdominal cavity and largely around the kidneys.
The study was published in the Journal of the American College of Cardiology.