Jul 21, 2011 11:18 PM
July 21, 2011 -- Where a person stores body fat may be more important for heart disease risk than how big they are, a new study shows.
Doctors have long wondered why some obese people look metabolically normal; that is, they have normal cholesterol, blood sugar, and insulin levels, while others are insulin resistant, have high blood sugar, and high cholesterol -- a profile that puts them at high risk of heart disease and type 2 diabetes.
Visceral fat, or fat that's stored around the internal organs, has been thought to play a role in that difference. Waist size is a rough measure for the amount of visceral fat a person has.
But newer studies have suggested that visceral fat may not deserve all the blame.
"During the last 10 years, with advances in imaging techniques, we have been able to measure the fat content of non-adipose tissues like skeletal muscles and liver," says Faidon Magkos, PhD, a research assistant professor of medicine in the department of nutrition at Washington University in St. Louis.
"And there's an accumulating body of evidence over the last decade that shows that it really isn't visceral fat that's associated with dyslipidemia or insulin resistance or other cardiometabolic risk factors, but instead, it's fat deposited in skeletal muscle and liver," says Magkos, who studies metabolism but was not involved in the current research.
The study looked at two groups of 14 obese men with waist sizes over 37 inches.
A third group of 10 normal-weight men, with normal triglyceride levels, was used for comparison.
Researchers in Sweden and Finland used specialized imaging techniques to measure how much fat was stored inside the men's livers and how much fat was stored in the body cavity around their organs and also underneath their skin.
Previous research has suggested that fat that's stored around the organs, called visceral fat, may play a role in the development of heart disease.
But the study found that men with high triglyceride levels and men with normal levels had about the same amounts of visceral fat.
Where the men were really different, it turns out, was their livers.
Men with high triglycerides had nearly twice as much fat stored in their livers as did men with normal triglyceride levels, 13% and 6.9% fat, respectively. The livers of non-obese men were about 2.9% fat.
The liver contributes to blood triglycerides by secreting particles of very low density lipoprotein (VLDL) cholesterol, a molecule that ferries high amounts of triglycerides through the blood and around the body.
VLDLs are like "big buses of triglycerides, delivering them to the tissues," says Scott R. Sherron, MD, a cardiologist with the Texas Heart Institute in Houston, who was not involved in the research.
The researchers found that obese men with high triglycerides and fatty livers not only secreted more VLDL cholesterol, they also had more trouble clearing it from the blood than men who were obese with normal triglyceride levels.
"What this suggests is that the deposition of fat in the liver may change the liver in a way that makes the liver do bad things for the heart," Sherron tells WebMD.
Other experts say the paper is an important contribution because it shows that it's not just a problem of making too much triglyceride, but not being able to use or clear it properly, that contributes to heart risk.
"What is new is the demonstration that the high triglycerides in the blood of people who are obese and have fat in their liver is equally due to a decreased removal of triglycerides from the blood as it is to an increased release of fat from the liver into the bloodstream," says Frank Greenway, MD, professor and chief of the outpatient clinic at the Pennington Biomedical Research Institute in Baton Rouge, La.
Researchers say more work will be needed to understand why people with fatty livers can't properly use triglycerides, which are used as energy sources.
The study is published in the journal Arteriosclerosis, Thrombosis, and Vascular Biology.