Sustaining Muscle, Cardiovascular Health, and the Environment: Is Plant-Based Protein the Key?

Background/Objectives: Protein intake is a key determinant of skeletal muscle health across the lifespan, yet optimal strategies must also account for cardiometabolic health and environmental sustainability. Differences in digestibility and amino acid composition between plant and animal-based proteins may influence their capacity to stimulate muscle protein synthesis (MPS), particularly in aging. Methods: This narrative review integrates evidence from acute tracer studies, randomized controlled trials, and long-term observational research comparing plant versus animal-based proteins for preserving muscle while supporting environmental goals and cardiometabolic health across populations. PubMed and Google Scholar were searched from inception to 11 December 2025 (plant-based protein OR animal-based protein AND sarcopenia OR muscle protein synthesis), with citation tracking. In total, 80 relevant findings were identified. Results: Acute tracer studies show that, gram-for-gram, animal-based proteins (particularly whey/dairy) stimulate greater myofibrillar protein synthesis due to higher leucine density, digestibility, and more rapid aminoacidemia—an effect that is more pronounced in older adults with anabolic resistance. In younger individuals, these differences are largely attenuated when total protein intake is sufficient. Importantly, the anabolic potential of plant-based proteins can be enhanced through higher dosing, amino acid or leucine fortification, and complementary protein blending (e.g., cereals with legumes or use of high-DIAAS isolates). Consistent with this, longer-term resistance training studies demonstrate comparable gains in muscle mass and strength between plant- and animal-based diets when protein intake (≥1.0–1.2 g/kg/day; ≥1.2–1.5 g/kg/day in illness), per-meal distribution (~0.4 g/kg with ~3–4 g leucine in older adults), and energy intake are optimized. Beyond muscle outcomes, higher plant-based protein intake is associated with favorable cardiometabolic profiles and lower environmental impact. Conclusions: An age-specific, mixed protein approach is recommended, emphasizing plant-based proteins in younger adults and higher-quality, leucine-rich proteins in older individuals. Defining optimal plant-to-animal-based protein ratios remains a key research priority.