: Studies have reported an increased risk of type 2 diabetes among people with higher protein intake. Moreover, branched-chain amino acids (BCAA) are reported to be positively associated with insulin resistance (IR). However, it is not understood whether elevated levels of BCAA are causal to IR development, or if higher BCAA are a marker of IR. The objective of this study was to examine the effects of long-term protein and carbohydrate supplementation on plasma BCAA levels, and the relationship between plasma BCAA and IR in postmenopausal women. : Stored samples and data from 84 postmenopausal women who participated in a protein supplementation trial (SPOON) were included. Exclusion criteria consisted of protein intakes less than 0.6 g/kg or greater than 1.0 g/kg, a body mass index (BMI) greater than 32 kg/m or less than 19 kg/m diseases, and conditions and medications known to impact musculoskeletal health. Subjects were randomized to a whey protein (PRO: = 38) or maltodextrin supplement (CHO: = 46) for 18 months. Plasma BCAA, homeostatic model assessment of insulin resistance (HOMA-IR) and body composition were analyzed at baseline and 18 months. : At baseline, there were no significant associations between plasma BCAA and IR. There were also no significant changes in plasma BCAA or IR by study arm. However, there was a significant positive association between plasma BCAA and IR in both groups at 18 months (CHO: r = 0.35, = 0.02; PRO: r = 0.35, = 0.03). : Findings from this study warrant future research to examine other diet and lifestyle factors that may mediate the relationship between circulating BCAA and IR in postmenopausal women.