More and more Americans are becoming obese, but not all fat is equal, say researchers at the University of Texas Southwestern Medical Center.
Their study, recently published in the Journal of the American Medical Association, found that certain obese patients were more likely to develop Type 2 diabetes, but that a person’s overall weight was not the deciding factor.
The greatest risk to developing diabetes was having fat stored inside the abdomen, nestling around the internal organs.
“Visceral fat inside the abdomen is definitely bad as far as diabetes goes,” said Dr. James de Lemos, a UT Southwestern professor of internal medicine and the study’s senior investigator.
Figuring out which obese patients are at greatest risk for diabetes would allow doctors to target only the worst cases for weight-loss surgery or other medical interventions, de Lemos suggested.
The study focused on the obese participants of the on-going Dallas Heart Study, which has enrolled more than 6,000 people since 2000. Each underwent extensive baseline testing and will be monitored by UT Southwestern researchers for decades.
After seven years, a research team headed by Dr. Ian Neeland tested 732 of the study’s obese subjects to see if any of them had developed diabetes.
The study found nearly 40 percent were in some stage of the disease, including a “pre-diabetic condition,” and 11 percent had reached Type 2 diabetes.
The Dallas researchers also found early signs of cardiac disease among the diabetic-prone subjects.
“We found that individuals who developed pre-diabetes and diabetes had evidence of early cardiovascular disease years before the onset of Type 2 diabetes,” said Neeland, a UT Southwestern cardiology fellow and the study’s lead author.
Type 2 diabetes, the most common form of the disease, occurs when a person either cannot produce enough insulin or its cells ignore the insulin needed to digest food. The condition can be treated with diet, exercise and certain medications.
The study’s findings also offer a more complicated view of the role body fat plays in the disease’s development process.
“There are places a body can put fat that are potentially protective, like the hips and the buttocks,” de Lemos said. “We need to recognize the difference.”
Previous studies offered simple formulas for deciding whether an obese individual might be disease prone. People with so-called apple-shaped bodies, for example, were said to have a greater risk of developing heart disease. Those with pear-shaped bodies were believed to be less threatened.
However, the Dallas study suggested that some apple-shaped people might be OK if their abdominal fat was stored directly under the skin. This subcutaneous fat, or belly fat, was not linked to diabetes development in the local study.
“Subcutaneous fat might be inert or even protective,” de Lemos said.
The study also seems to challenge a more recent approach to accessing obesity’s risk, using the body-mass-index scale, or BMI. It works by assigning a number to an overweight individual based on his or her weight and height.
People in the Dallas study, who ranged in age from 30 to 65, had to have a BMI of 30 or greater. That would include someone who was 5 feet tall and 153 pounds, or a six-footer at 221 pounds, both with a 30 BMI.
The researchers were surprised to learn that the heaviest people in the study were not the ones most likely to become diabetic. In fact, many of the study’s subjects appeared to be healthy, despite their girth.
“It’s important to recognize that that being severely overweight or obese doesn’t necessarily mean an individual will develop cardiac disease or diabetes,” de Lemos said.
The study’s findings likely will be added to a growing list of obesity research that mentions “healthy obesity” as one aspect of the nation’s weight epidemic.
The one decisive finding, connecting abdominal fat to diabetes development, might also be hard to diagnose in a growing obesity population.
Each participant in the UT Southwestern study underwent an MRI as well as another test called a dual energy X-ray absorptiometry, or DEXA, to determine where their body fat was stored.
The cost and availability of such tests probably would prohibit their widespread use for body-fat imaging. But without them, any assessment would likely be guesswork.
“A doctor can’t tell whether fat is located inside the abdomen just by looking at an obese patient,” de Lemos said.
“So we need to develop some simpler tools that give us the same information as these expensive tests.”
What do we know about fat?
Until now, obesity has been defined by the size of a person’s total body fat, a calculation comparing total weight to height, or body mass index (BMI). Every obese person was considered at high risk of developing certain diseases, including diabetes.
Researchers are beginning to define a condition called “healthy obesity” based on an understanding of how excess fat is stored inside the body.
A healthy obese person stores fat lower in the body, usually in the thighs or buttocks, but also in a subcutaneous layer outside the abdomen. Such fat may protect against metabolic diseases related to obesity.
An unhealthy obese person is more likely to store fat inside the abdomen, where it interferes with vital organ functions.
These “fat depots,” or storage locations, are not necessarily visible to a physician attempting to treat someone for obesity. A diagnosis will require the development of new medical tests related to obesity. Currently, MRIs can provide a more detailed measurement of fat and how it is stored, although such tests are expensive and unavailable to some people.
Diagnosing a healthy vs. unhealthy obese patient may eventually determine who requires intensive lifestyle change or gets medical and surgical interventions designed to reduce their body fat.
Other studies must duplicate these conclusions before they are generally accepted by doctors who treat obese patients.