Body Fat and Insulin Resistance

How does body-fat content produce insulin resistance? First, it appears that it is particularly intra-abdominal fat (also termed visceral fat) that is the culprit here. Intra-abdominal fat is adipose tissue associated with the abdominal viscera. Subcutaneous fat is much less of a problem. One hypothesis suggests that a process that is central to the pathogenesis of insulin resistance is fat ectopia. In the simplest terms, adipose tissue can only hold a certain amount of fat, and if excessively loaded with fat, there is a spillover or redistribution of lipid to ectopic sites, including liver and skeletal muscle.

In support of this, hepatic steatosis is frequently observed in individuals with the metabolic syndrome. Nonalcoholic fatty liver disease has a prevalence of 57% to 74% in obese individuals. It is the most common cause of abnormal liver function tests in the United States. The ectopic triglyceride deposition in non-adipose tissue, such as liver and skeletal muscle, has deleterious effects. There is both tissue damage (lipotoxicity) and the development of insulin resistance.

Another aspect of the lipid ectopia hypothesis is that the beta cells themselves are damaged by the deposition of the fat. This results in a gradual failure to produce sufficient insulin, making the insulinopenia worse. The evidence that this hypothesis has some validity comes from rare cases of lipodystrophic diabetes. Congenital lipodystrophies are conditions where body fat is significantly reduced or almost absent. The dearth of normal fat-storage capacity leads to early fat ectopia with deposition of fat (triglycerides) in skeletal muscle and liver and the development of insulin resistance despite the absence of obesity.

Conversely, in the Prader-Willi syndrome, where significant obesity is a major feature, insulin resistance is uncommon. These individuals appear to have an expanded capacity to store fat, so their risk of fat ectopia and type 2 diabetes is less than average. Additional support of this hypothesis derives from studies of low-birth-weight infants. As adults, these individuals are predisposed to insulin resistance. It appears that they have reduced amounts of adipose tissue and, therefore, a reduced capacity to store fat. They are more likely to experience spillover or fat ectopia, according to the hypothesis outlined above.

Further evidence comes from the use of a class of drugs termed PPAR-gamma agonists (thiazolidinediones). These compounds stimulate the development of new adipose tissue, allowing the redistribution or normalization of fat stores. Fat leaves the ectopic tissues and re-enters the new adipose tissue. Thiazolidinediones are known to be effective in treating type 2 diabetes.