Metabolic flexibility is the ability to switch rapidly from one substrate (i.e. fuel source) to another (e.g. fats to carbohydrates) in response to ingested foods. Worse metabolic flexibility is linked to worse metabolic outcomes, but exercise is thought to be protective against nutrient excess. We will conduct a standard oral glucose tolerance test (OGTT) to assess insulin resistance and metabolic flexibility. Participants will be randomized to a 24-hour hypercaloric, high-fat diet (HFD) with and without an exercise bout. During the hypercaloric HFD, participants will eat extra calories with more calories from fat than normal, which is expected to increase insulin resistance. After each arm of the high-fat diet, we will repeat the OGTT. During each OGTT, expired gases will be analyzed noninvasively to determine metablic flexibility. Insulin resistance will be determined with blood drawn during each OGTT. By comparing metabolic flexiblity and insulin resistance before and after the normal diet and after the high-fat diet intervention, a better understanding of how insulin resistance affects metabolism and how exercise can be protective will be developed.