Technical Abstract: Elevated blood branched chain amino acids (BCAA) are often associated with insulin resistance and type 2 diabetes. One possibility is that under these conditions there is a reduced cellular utilization and/or lower complete oxidation of BCAAs. White adipose tissue (WAT) has become appreciated as an important player in whole-body BCAA metabolism. The current studies tested if expression of the mitochondrial BCAA oxidation check-point, the branched chain a-ketoacid dehydrogenase (BCKD) complex, is reduced in rodent and human WAT in obesity and regulated by metabolic signals. WAT BCKD protein was significantly reduced by 35-50% in monogenic obesity models (fa/fa rats and db/db mice) and in diet-induced obese mice. Transcripts for BCKD components were significantly lower in isolated subcutaneous (SC) adipocytes from obese vs. lean Pima Indians, and reduced in omental WAT of obese subjects with metabolic syndrome compared to equally-obese healthy controls. Treatment of maturing 3T3-L1 adipocytes with PPAR' agonists generally increased BCAA catabolism enzyme mRNAs, whereas the non-metabolizable glucose analogue 2-deoxy-D-glucose had the opposite effect. Novel proof-of-principle studies revealed an unexpected net efflux of BCAA from human SC WAT, which in absolute terms was 500% and 100% higher in obese insulin-resistant individuals compared to obese and lean insulin-sensitive individuals, respectively. However, person-to-person variability was high and differences did not achieve statistical significance. Overall, the results support the hypothesis that sub-optimal insulin action and/or perturbed metabolic signals in WAT, as would be seen with insulin resistance or type 2 diabetes, act to lower WAT BCKD expression and impair WAT BCAA utilization.