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.

Inclusion Criteria:

• Males or females between the ages of 20 and 40 years
• A body mass index (BMI) of 20.0 - 29.9 kg/m2.
• Low risk for risk of injury or health complications as determined by physical activity readiness questionnaire (PARQ).

Exclusion Criteria:

• Are currently taking any antioxidant supplementation (daily multivitamin not included), prescribed non-steroidal anti-inflammatory drugs (NSAIDS), antibiotics, steroids, or probiotics.
• Are currently taking any medications for management of cholesterol, diabetes, or body weight/obesity, or are undergoing hormone therapy.
• Are unwilling or unable to comply with study procedures, at the discretion of the investigators.
• Are unwilling to have biospecimens (blood and urine) collected and stored (only blood) for future research.
• Pregnant women - A urine pregnancy test will be conducted prior to any testing procedures.
• A history of disordered eating, including anorexia nervosa, bulimia nervosa, binge eating disorder, or avoidant restrictive food intake disorder or deemed high-risk for disordered eating via questionnaire.
• A history of gastrointestinal diseases, such as galactosemia, gastroesophageal reflux disease (GERD), irritable bowel syndrome (IBS), Celiac disease, Crohn's disease, or lactose intolerance.
• Individuals with known allergies to wheat or gluten, milk, soy, tree nuts (e.g., almonds, walnuts, pecans), or peanuts.
• Deemed high risk for food insecurity via the Hunger Vital Sign screener.
• Have followed a ketogenic diet within the last three months or have been told to avoid a high-fat diet by a medical provider for risk of cardiovascular disease.
• Cardiorespiratory fitness (VO2max) below the 30th percentile for age and sex (measured at initial screening visit)

Individual data will be annonymized by study ID number and removed of any identifying information. Data will be shared upon reasonable request.

Upon publication, data will be shared upon reasonable request to the corresponding author.