["Numerous studies have investigated the effects of isolated CLA supplementation on glucose homeostasis in humans and rodents. However, both the amount and relative abundance of CLA isomers in supplemental form are not representative of what is consumed from natural sources. No study to date has examined the effects of altered CLA isomer content within a natural food source. Our goal was to increase the content of the insulin desensitizing CLAt10,c12 isomer relative to the CLAc9,t11 isomer in cow's milk by inducing subacute rumenal acidosis (SARA), and subsequently investigate the effects of this milk fat on parameters related to glucose and insulin tolerance in rats.", 'We fed female rats (~2.5 to 3\xa0months of age) CLA t10,c12 -enriched (SARA) butter or non-SARA butter based diets for 4\xa0weeks in either low (10% of kcal from fat; 0.18% total CLA by weight) or high (60% of kcal from fat; 0.55% total CLA by weight) amounts. In an effort to extend these findings, we then fed rats high (60%\xa0kcal) amounts of SARA or non-SARA butter for a longer duration (8\xa0weeks) and assessed changes in whole body glucose, insulin and pyruvate tolerance in comparison to low fat and 60% lard conditions.', 'There was a main effect for increased fasting blood glucose and insulin in SARA vs. non-SARA butter groups after 4\xa0weeks of feeding (p\u2009<\u20090.05). However, blood glucose and insulin concentration, and maximal insulin-stimulated glucose uptake in skeletal muscle were similar in all groups. Following 8\xa0weeks of feeding, insulin tolerance was impaired by the SARA butter, but not glucose or pyruvate tolerance. The non-SARA butter did not impair tolerance to glucose, insulin or pyruvate.'] Does feeding butter with elevated content of trans-10 , cis-12 conjugated linoleic acid to lean rats impair glucose tolerance or muscle insulin response?