Muscle Response to Different Amounts of Dietary Protein During Leg Immobilization

Healthy, recreationally active men and women will complete this randomized, double-blind, parallel study. Muscle disuse will be implemented for five days using a unilateral leg immobilization model with one leg assigned to immobilization (DISUSE) and the contralateral leg used as a control (ACTIVE). Immobilization will be implemented with a rigid knee brace fixed at ~60° of flexion and participants will ambulate using crutches. Diets will be standardized during the immobilization phase. Participants will complete a protein feeding study at the end of immobilization consisting of primed, continuous stable isotope infusions, serial blood draws, and muscle biopsies. MPS will be assessed in both the DISUSE and ACTIVE legs in response to standard (20 g) or high (40 g) doses of whey protein. Findings from this work will directly inform the development of targeted nutritional countermeasures for overcoming disuse-induced anabolic resistance and preserving muscle mass after MSKI to optimize physical performance recovery.

Inclusion criteria

• Men and women aged 18-39 years (17-39 years if military personnel)
• Body mass index (BMI) between 18.5-32 kg/m2
• Routinely participate in aerobic and/or resistance exercise at least 2 days per week.
• Willing to refrain from alcohol, smoking, smokeless nicotine products (includes e-cigarettes, vaping, chewing tobacco), and dietary supplements (i.e., vitamin D, probiotics) 24 hours before and during immobilization and final testing day.
• Willing to only consume caffeine products provided by study staff during the study.
• Supervisor approval for federal civilian employees and non-SRV active-duty military personnel stationed at NSSC.
• Biological females must have normal menstrual cycles between 26-32 days in duration; 5 menstrual cycles within the past 6 months; or on continuous hormonal contraception (i.e., IUD or oral contraceptives without placebo).

Exclusion criteria

• Musculoskeletal injuries that may interfere with the safe use of crutches.
• Personal or family history of thrombosis, or prior diagnosis of deep vein thrombosis (DVT) or pulmonary embolism (PE).
• Metabolic or cardiovascular abnormalities, gastrointestinal disorders, neuromuscular disorders, lower-limb amputation, or muscle/bone wasting disorders (e.g., diabetes, cardiovascular disease, Crohn's disease, etc.).
• Taking medication that affects macronutrient utilization (i.e., statins, corticosteroids, weight loss medications such as Ozempic, etc.).
• Significantly abnormal blood clotting as determined by USARIEM Office of Medical Oversight (OMSO) or home duty station medical support (HMS).
• Allergy to lidocaine (or similar local anesthetic).
• Present condition of alcoholism, anabolic steroid use, or other substance abuse issues as determined by OMSO or HMS.
• Blood donation within 8-wk of beginning the study.
• Pregnant, trying to become pregnant, and/or breastfeeding (results of urine pregnancy test and self-report for breastfeeding will be obtained before body composition testing).
• Unwilling or unable to consume study diets or foods provided due to personal preference and/or food allergies.
• Unwilling or unable to adhere to study physical restrictions (i.e., no structured physical activity or recreational activity beyond activities of daily living) 24 hours before and during immobilization, and the final testing day.
• Unwilling or unable to keep the knee brace on and walk with crutches during the immobilization phase.