Insulin Resistance in the Control of Body Fat Distribution: A New Hypothesis

Obesity causes insulin resistance, which is a prime etiological factor for type 2 diabetes, dyslipidemia, and cardiovascular disease. However, insulin resistance may be a normal physiological response to obesity that limits further fat deposition and which only has pathological effects at high levels. The current hypothesis suggests that in obesity the initial deposition of triglycerides occurs in subcutaneous adipose tissue and as this increases in size insulin resistance will rise and limit further subcutaneous lipid accumulation. Triglycerides will then be diverted to the visceral fat depot as well as to ectopic sites. This leads to a substantial rise in insulin resistance and the prevalence of its associated disorders. Evidence supporting this hypothesis includes studies showing that in lean subjects the prime determinant of insulin resistance is BMI, that is, subcutaneous fat whilst in overweight and obese subjects it is waist circumference and visceral adiposity. It has also been shown that the metabolic syndrome suddenly increases in prevalence at high levels of insulin resistance and we suggest that this is due to the diversion of lipids from the subcutaneous to the visceral depot. This system may have functioned in our evolutionary past to limit excessive adiposity by causing lipid deposition to occur at a site that has maximal effects on insulin resistance but involves minimal weight gain.