Cardiorespiratory fitness (CRF), also known as "exercise capacity", is the capacity of respiratory and circulatory systems to supply oxygen to skeletal muscle during exercise for the generation of energy. Determinants of CRF include lung capacity, capillary density, cardiac output, hemoglobin concentration, and mitochondrial function. The research group studies how CRF is related to fuel utilization, yielding a mechanistic understanding of the association between lower CRF and worsening metabolic health via mitochondrial function. The objective of this study is to measure fuel utilization in response to habitual diet for one week in adolescents and young adults, ages 14-22 years (n=30). Fuel utilization will be estimated by glucose measures using a continuous glucose monitor (CGM). Habitual dietary intake will be collected via a mobile phone application (BiteAI, Inc) that uses artificial intelligence to extract nutrient information from food photographs. Participants will undergo two standard of care (SOC) meal tolerance tests at home - a glucose tolerance test and a Ensure® mixed meal tolerance test. CRF will be estimated by measuring maximum oxygen consumption (VO2 max) during a graded treadmill test. The hypothesis is that a higher VO2 max will be associated with increased fuel utilization, measured by lower glucose response to the SOC meal tolerance tests. The proposed study is described in the following aims:

SPECIFIC AIM 1. Test for feasibility of completion of self-report dietary food records, completion of two at home meal tolerance tests that are standard of care for metabolic health screening, and completion of a seven-day continuous glucose monitor.

SPECIFIC AIM 2. Identify nutrients and foods that are associated with an elevated glucose response.

SPECIFIC AIM 3. Assess the influence of VO2 max on the glucose response to the SOC glucose tolerance test and Ensure® mixed meal tolerance test.

Inclusion Criteria:

• Adolescents and young adults, ages 14-22 years

Exclusion Criteria:

• Previous diagnosis of type 1 or 2 diabetes
• Previous diagnosis of hyperlipidemia or other metabolic disease
• Use of medications known to affect glucose metabolism (metformin, oral steroids, sulfonylureas, insulin).
• Allergies to milk.
• Inability to participant in the maximal exercise test on the treadmill.
• Individuals not having android or IOS phones
• Individuals who cannot speak and/or write in English.

Adolescents and young adults, ages 14-22 years