The purpose of this study is to determine how anabolic sensitivity, in response to the ingestion of a liquid protein meal, differs across the lifespan, between biological sexes, and with varying physical activity levels.

Skeletal muscle is a dynamic tissue that responds to anabolic (i.e., growth) stimuli such as protein ingestion and exercise. Differences in age and physical activity levels will influence an individual's ability to respond to these anabolic stimuli; this concept is known as anabolic sensitivity. For instance, older and less physically active individuals will display lower anabolic sensitivity (i.e., anabolic resistance) to a bout of exercise or the consumption of protein in comparison to younger, more physically active individuals. Seeing that individuals with anabolic resistance are at a greater risk for losing muscle mass over time, it is important to understand the anabolic sensitivity/resistance of an individual to assess for the efficiency of growth across the health and lifespan, in addition to a potential screen for metabolic alterations that could manifest into changes in lean body and muscle mass.

Traditional methods to assess for skeletal muscle anabolism and protein metabolism requires in-person and invasive research methods (i.e., stable isotope infusions and muscle biopsies) in controlled laboratory or clinical settings. However, there are many risks associated with these invasive procedures and they are not always feasible in all populations (e.g., children, clinical populations, etc..). Our lab has recently developed a non-invasive 13C breath test which employs an oral stable isotope tracer. Our breath test allows us to assess for anabolic sensitivity in response to protein ingestion, by determining how much of the ingested protein was used to build (synthesize) new proteins in our body (i.e., anabolism) or used as a source of energy (through a process called oxidation). As such, the purpose of the present study is to determine how anabolic sensitivity, in response to the ingestion of a liquid protein meal, differs across the lifespan and with varying physical activity levels.

Inclusion Criteria:

• Healthy will be defined as screened by the PAR-Q+ (The Physical Activity Readiness Questionnaire for everyone)
• Chronological age for Healthy Young (age: 18-35 years) or Older (age: 60-80 years) adults
• Chronological age between 8 to 16 years, with maturity offset < -1 year from age of Peak Height Velocity (aPHV) for children and between -0.5 to +1.5 years from aPHV in adolescent participants
• BMI between normal to overweight (18.5 - 29.9 kg/m2)

Exclusion Criteria:

• Regular use of nonsteroidal anti-inflammatory dugs (with the exception of daily low-dose aspirin)
• use of anticoagulants
• use of a walker, cane, or assistive walking device
• infectious or gastrointestinal disease
• inability to comply with study protocol (e.g., unable to track diet)
• regular tobacco use
• self-reported illicit drug use (e.g., growth hormone, testosterone, etc.)
• diagnosed chronic illness (e.g., type 2 diabetes, heart disease, thyroid disease)
• pregnant
• hormonal replacement therapy

Convenience sample of individuals who qualify for the corresponding cohorts based on inclusion/exclusion criteria stated.