Obesity is the most important risk factor for type 2 diabetes. Extremely obese people (those with a body mass index [BMI] of 40 or greater) are seven times more likely than normal-weight people to develop type 2 diabetes.
As obesity has become more common in the United States, so has type 2 diabetes. In 1991, 12% of Americans were obese and 5% had diabetes. By 2001, nearly 21% of Americans were obese and 8% had diabetes. The reason why obesity increases the risk of type 2 diabetes is not clearly understood, but several possible explanations exist.
Obesity and Insulin
Type 2 diabetes involves a two-step process that first affects the body’s response to, and later its production of, insulin. The decreased responsiveness to insulin, called insulin resistance, forces the pancreas to work harder and produce more insulin. The pancreas of a normal-weight person might put out 30 units of insulin a day. If that person becomes obese, those same 30 units are no longer sufficient to clear glucose from the bloodstream, and more insulin is required.
The pancreas starts to produce more insulin to keep up with the increased demand, but not everyone’s pancreas can handle the extra workload. Type 2 diabetes results when the pancreas cannot produce enough insulin to control blood glucose levels.
Why Some Overweight People Don’t Have Type 2 Diabetes
Having a high BMI, a measure of body fat based on both height and weight, increases the risk of having diabetes. People with a BMI of 25 to 29.9 are considered overweight, and those with a BMI of 30 or above are considered obese.
Although most people with type 2 diabetes (80%) are obese, most obese people don’t have diabetes—so other factors clearly are at work. Most likely, genes determine whether someone’s pancreas is hardy enough to produce the extra insulin needed if the person becomes overweight.
Another factor is the distribution of the extra weight. People with abdominal obesity (excess weight above the hips) are more likely to develop type 2 diabetes than those with extra weight in the hips and thighs.
Abdominal obesity and elevated blood glucose levels are two components of metabolic syndrome, a cluster of findings present in about one out of four Americans that increases the risk of diabetes, as well as the risk of coronary heart disease and stroke.
New Findings on Obesity and Type 2 Diabetes
New theories have emerged recently about the possible links between obesity and diabetes. For example, experts have found that fat cells secrete many hormone-like substances that circulate through the bloodstream and can affect other systems in the body. Recent research has focused on three substances produced by fat cells: resistin, leptin, and free fatty acids.
Resistin. In 2001, researchers published a study in Nature describing a previously unknown hormone called resistin that, in mice, appeared to increase with greater body weight. The researchers found that higher blood levels of resistin led to insulin resistance, and antibodies that blocked resistin improved insulin sensitivity. They discovered that several diabetes drugs already in use work in part by blocking the gene that makes resistin.
Now, researchers are investigating whether a drug that specifically targets resistin can help improve insulin sensitivity with fewer side effects.
Leptin. Leptin, first discovered in 1994, is a hormone that signals the brain that no more food is needed—it has been referred to as an appetite suppressant or “satiety signal.” Obese people have high blood levels of leptin, so researchers speculate that they may not be sensitive to the appetite and weight suppression that leptin ordinarily provides.
The role that leptin plays in appetite is not yet fully understood, but further research may lead to new targets for obesity and diabetes drugs.
Free fatty acids. Free fatty acids, which are formed from the breakdown of stored fat in fat cells, circulate through the body and may also contribute to insulin resistance and type 2 diabetes. Normally, muscle cells remove glucose from the bloodstream for use as energy. However, research suggests that accumulation of fatty acids in muscle cells of obese people may prevent glucose from entering these cells. The resulting rise in blood glucose levels is the main characteristic of diabetes.
Positive Results
The good news is that type 2 diabetes is very sensitive to weight loss and occasionally may disappear when obese people lose weight. Weight loss should be achieved through a combination of increased physical activity and reduced calorie intake. Improved insulin resistance can occur with a loss of as few as 5 lbs., but better results are achieved with a 7% to 15% decrease in body weight.
In 2002, the Diabetes Prevention Program study showed that people who followed a low-calorie, low-fat diet; exercised 30 minutes a day; and lost an average of 15 lbs. were 58% less likely to develop type 2 diabetes over a three-year period than people who took a placebo pill.
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Posted in Diabetes on November 26, 2005
Reviewed May 2007
