This clinical trial aims to learn about the alterations in insulin resistance and metabolic flexibility following a transition to an obesogenic lifestyle in fit young men and women. The main questions it aims to answer are:

1. Does adding excess carbohydrates when transitioning to a sedentary lifestyle promote insulin resistance and impaired 24hr glucose regulation in healthy men and women?
2. Does adding excess carbohydrates when transitioning to a sedentary lifestyle lower the body's ability to break down fats and carbohydrates in healthy men and women?
3. Does the added physical activity blunt shifts in carbohydrate and fat oxidation in healthy men and women?

Excess adiposity remains a critical health issue in the United States. Obesity and severe obesity are projected to reach approximately 34% and 9% by 2030, respectively. However, recent 2021 NHANES data reveals that our obesity and severe obesity population has already surpassed these estimates reaching 41.9% and 9.2% by 2022, respectively. While early childhood obesity has a prevalence of about 22%, which can lead to obesity during adulthood, young adulthood (20-39 years old) is another critical time where young adults without obesity or severe obesity will accumulate excess adiposity as part of this transition into middle-aged adulthood (40-59 years). In particular, young adults often transition from higher levels of physical activity (i.e., sports participation in high school, increased walking to class on college campuses, increased free time for physical activity) to lower levels of physical activity (e.g., full-time employment) and limited time to prepare healthy meals. Although obesity models tend to be complex, with multiple contributors to the development of obesity, easily accessible and rapidly digestible carbohydrates with high glycemic indexes have contributed significantly to the rise in obesity and cardiometabolic diseases in the United States.

Previous animal models have demonstrated that high carbohydrate or high-fat diets and increased sedentary activity lead to excess adiposity and insulin resistance in animal models. Animal models help us to examine mechanistic contributors to obesity and adverse cardiometabolic risks. A recently developed obesogenic lifestyle model provides an excellent model for studying the transition to an obesogenic lifestyle in healthy young adults. The obesogenic lifestyle model uses an acute exposure to a sedentary lifestyle (~5,000 steps/day) and increased carbohydrate intake (~2 liters of soda/day) for a 10-day period. Using this obesogenic lifestyle model, researchers found that the acute obesogenic lifestyle model increased insulin resistance (measured by HOMA-IR) in both men and women, but only men had declines in vascular insulin sensitivity. The reduction in vascular sensitivity is considered an early precursor for the development of metabolic dysregulation and cardiovascular disease. Nonetheless, it remains unclear whether insulin resistance and vascular insulin sensitivity were due to a lack of physical activity or increased carbohydrate intake. Further, the model must be independently validated to confirm its ability to induce insulin resistance to create a sustainable model for repeated studies.

From a behavioral aspect, the designed obesogenic lifestyle model provides an opportunity to study increases in insulin resistance when individuals transition during young adulthood into a lifestyle that induces barriers to maintaining physical activity and impairs diet quality. Importantly, this young adult population remains underrepresented in the literature compared to studies on obese or physically inactive adults. Therefore, the model has ecological relevance. The model also provides an opportunity for earlier interventions to be developed to mitigate the harmful consequences that may be offset with simple interventions that promote physical activity. Therefore, the global hypothesis of this research study is that the obesogenic lifestyle model will be a suitable model for studying the early onset of insulin resistance as it will increase insulin resistance (HOMA-IR) and impair glucose regulation in recreationally active young men and women.

Inclusion Criteria:

• 18-30 years of age
• Recreationally active completing 75-150 minutes of moderate-to-vigorous intensity exercise (>2 days/week).
• Fair cardiorespiratory fitness levels (Men: VO2>38.4 ml/kg/min; Women: VO2>32.6 ml/kg/min).

Exclusion Criteria:

• Hypertension (resting or diagnosed)
• Impaired fasting blood glucose (>100mg/dL)
• Diagnosed cardiovascular disease
• Diagnosed diabetes
• Diagnosed cancer
• Diagnosed chronic kidney disease
• Diagnosed musculoskeletal disorders that prevents the individual from exercising on a bike.