Kiwicha in Gastrointestinal Digestion: Structure, Bioaccessibility, Processing and Health-Relevant Functionality

Kiwicha (Amaranthus caudatus L.) is an Andean pseudocereal recognized for its nutritional composition and growing potential as a functional food ingredient. Beyond its high-quality proteins, dietary fiber, unsaturated lipids, and antioxidant compounds, increasing evidence suggests that the potential health-related effects of kiwicha may depend, at least partly, on the bioaccessibility of these components after gastrointestinal digestion rather than on their native composition alone. This narrative review examines how the structural organization of the kiwicha food matrix influences gastrointestinal digestion, nutrient bioaccessibility, and physiological functionality. It summarizes current evidence regarding protein, starch, lipid, and phenolic compound digestibility, emphasizing the release of bioactive peptides, antioxidant compounds, and other potentially bioactive metabolites during digestion. Furthermore, the review also discusses how processing technologies, including germination, extrusion, cooking, roasting, and fermentation, modify the food matrix, may increase nutrient bioaccessibility, and influence the functional performance of kiwicha-based foods. Critically, the available evidence indicates that processing-induced improvements in digestibility or bioaccessibility should not be interpreted directly as evidence of enhanced physiological functionality. Most studies remain based on in vitro digestion and biochemical assays, whereas evidence integrating intestinal absorption, systemic bioavailability, and human responses is scarce. Therefore, the main conclusion of this review is that the nutritional and potential functional relevance of kiwicha should be understood as a continuum linking food-matrix structure, processing-induced structural changes, gastrointestinal release, bioaccessibility, intestinal absorption, and ultimately physiological response.