Plant-based dietary patterns are typically discussed in terms of nutrients of concern—iron, creatine, indispensable amino acids, and rapidly fermented carbohydrates—based on the assumption that reduced dietary exposure necessarily translates into reduced physiological availability. This review examines physiological adaptation as an alternative framework, drawing on human intervention and observational evidence across five biological systems: iron homeostasis, endogenous biosynthesis of creatine and carnosine, protein metabolism, fermentation of dietary fiber by the gut microbiota, and the microbial biotransformation of plant polyphenols. Sustained changes in dietary exposure are accompanied by coordinated homeostatic responses, including increased intestinal iron absorption, upregulated endogenous synthesis, improved nitrogen economy, functional remodeling of the gut microbiota, and enhanced polyphenol biotransformation, which contribute to maintaining physiological function. However, adaptive capacity is not unlimited: pregnancy, chronic inflammation, malabsorptive disease, and severe or prolonged dietary insufficiency can exceed adaptive capacity and increase the physiological relevance of dietary intake, planning, or supplementation. Recognizing physiological adaptation as a distinct, measurable phenomenon reframes plant-based diets less as inherently deficient patterns requiring correction and more as a physiological model for understanding how the human body responds to sustained dietary change, with implications for interpreting acute versus long-term evidence and for nutritional counseling.