This study proposes to characterize skeletal muscle amino acid kinetics to an EAA challenge, i.e., an oral amino acid tolerance test (OATT), in order to determine the state of muscle health. Analogous to the oral glucose tolerance test (OGTT) used to characterize alterations in glucose metabolism, the proposed OATT represents a potential low-cost solution to classifying patients' skeletal muscle health. The extrapolation of this work is the development of a simple analytical tool that would provide clinicians the ability to discern alterations in muscle amino acid kinetics prior to a loss of function.

Sarcopenia increases the risk of a number of deleterious health conditions and represents a major financial cost to our healthcare system. Among older adults who are hospitalized, those with sarcopenia on admission are 5-fold more likely to incur higher hospital costs than those without. The operational definition of sarcopenia is defined as meeting the criteria for all of the following: low muscle strength, low muscle quantity or quality, and low physical performance. The diagnosis of sarcopenia requires techniques that are both expensive and operator-dependent. Simple measurements, such as BMI, do not necessarily identify sarcopenia. Importantly, current techniques can only identify sarcopenia after a physical/functional impairment has occurred. Skeletal muscle amino acid kinetics predict muscle health and functionality. Altered amino acid kinetics lead to decrements in muscle mass, quality, and performance. Muscle response to circulating essential amino acids (EAA) determines muscle amino acid kinetics. Thus, this study proposes to characterize skeletal muscle amino acid kinetics to an EAA challenge, i.e., an oral amino acid tolerance test (OATT), in order to determine the state of muscle health. Analogous to the oral glucose tolerance test (OGTT) used to characterize alterations in glucose metabolism, the proposed OATT represents a potential low-cost solution to classifying patients' skeletal muscle health. The extrapolation of this work is the development of a simple analytical tool that would provide clinicians the ability to discern alterations in muscle amino acid kinetics prior to a loss of function.

Inclusion Criteria:

• Ages 18 - 30 and 70-89 yrs

Exclusion Criteria:

• History of diabetes that requires insulin for control of blood glucose
• History of malignancy or chemo/radiation therapy in the 6 months prior to enrollment
• History of gastrointestinal bypass/reduction surgery (Lapband, gastric sleeve, etc.)
• Pregnant females
• Unwilling to wear the breath-collection mask
• Subjects who cannot refrain from using protein or amino acid supplements for 7 days prior to Visit 2
• Concomitant use of oral or injectable corticosteroids
• Concomitant use of testosterone, IGF-1, or similar anabolic agent
• Any other disease or condition that would place the subject at increased risk of harm if they were to participate, at the discretion of the study physician

From enrollment to the completion of the study day. Approximately 2 weeks.