Muscle atrophy and strength decrements can occur following disease (e.g., cachexia), with increasing age (e.g., sarcopenia), or from disuse related to injury or occupational environment (e.g., microgravity). High-load resistance training and adequate protein and amino acid intake are effective countermeasures. However, high-load training may not be feasible in all populations. Lower intensity resistance training with blood flow restriction (BFR) is an effective alternative and its benefits may be increased when combined with supplementation of protein and leucine. Two studies explored the relationships between nutrient intake and physical activity and the effects of short-term combination on muscle health in middle age. Methods: To compare muscle strength and size, 98 participants were divided by age and physical activity (young active adults = 23.0 ± 3.1 years, sedentary young adults = 26.3 ± 4.7 years, middle-aged active adults = 57.3 ± 4.0 years, middle-aged sedentary adults = 57.9 ± 4.4 years). Relationships between muscular health, nutrient intake, and physical activity were also examined. Next, 16 participants (age = 36.0 ± 10.1 yrs., BMI = 27.2 ± 5.0 kg/m2) completed BFR training with supplementation of 28g of leucine-rich protein or an isocaloric placebo to evaluate the effects on health and performance. Results: There were significant group effects for muscle strength (p = .003-.010) and size (p=.002). Physical activity, protein intake, and leucine were significantly and positively associated with knee flexor size and strength (R2 = 0.28-0.71, p<.05). Physical activity and protein intake were negatively associated with dorsiflexor strength (r2 = 0.48-0.58, p<0.05). Total leucine intake (g/day) increased by ~38% with supplementation. Training volume in all exercises, sit-to-stand repetitions, gait speed increased significantly with BFR exercise while resting heart rate significantly decreased (p<0.05). Conclusion: These results indicate that muscle size and strength are lower in middle age and increased physical activity, protein intake, and leucine intake may be able to preserve muscle size and strength in larger muscle groups of the lower body. Additionally, they show that BFR exercise can improve muscular and cardiovascular health in middle-aged adults, however, there was no short-term benefit to increasing protein and leucine intake above the recommended dietary allowance.