Insulin sensitivity (SI) in horses is affected by diet, exercise and obesity and has been implicated in metabolic disease. The objectives of this research were to assess the impact of BW gain on SI utilizing two diets known to differentially impact glucose dynamics, evaluate the contribution of light exercise to overall SI and relate changes in SI to BCS to identify the threshold at which SI declines to a level consistent with an increased risk of metabolic disease. Fifteen mature Thoroughbred geldings (BW 516 ± 13 kg, BCS 4.3 ± 0.1) were fed to gain 90.8 kg on a diet high in fat and fiber (HF, n = 6) starch and sugar (HS, n = 9). To assess SI, frequently sampled i.v. glucose tolerance tests were performed before treatment initiation (CFMM), at the start (TXMM) and end (ENDMM) of weight gain and following a period of minimal exercise. Using the minimal model of glucose dynamics, data from each test was used to estimate SI, glucose effectiveness (Sg) and the acute insulin response to glucose (AIRg). Final BW was 608 ± 12 kg and BCS was 7 ± 0.1. Diet had no effect on SI, AIRg or glucose effectiveness at CFMM. Within HF, SI, Sg and AIRg were unchanged at CFMM, TXMM and ENDMM. SI decreased at TXMM in HS (P = 0.05) and remained unchanged through ENDMM. SI in HS was lower than HF at TXMM (P = 0.01) and ENDMM (P = 0.07). At ENDMM, AIRg was higher in HS than HF (P = 0.01) and glucose effectiveness was reduced in both diets (P < 0.05). Following the minimal exercise period, SI decreased in HF (P = 0.03). These results indicate that diet may be more influential on SI than weight gain in mature Thoroughbred geldings. The higher SI in HF appears to be partially dependent on some level of physical activity. Because a BCS increase of 3 scores was not associated with a reduction in SI, the BCS where the perceived risk of metabolic disease is increased likely lies above that achieved in this study (BCS 7).