Substituting Refined Flour with Soy Flour Improves Postprandial Glycemic Responses in Staple Foods Without Reducing Consumer Acceptability

Background/Objectives: Soy flour has been proposed as a functional ingredient to improve the protein and fiber content of foods; however, its metabolic and sensory effects, particularly in individuals at elevated risk for metabolic disease, remain insufficiently characterized. This randomized, repeated-measures study examined whether substituting refined wheat or corn flour with soy flour influences postprandial glucose and insulin plasma concentrations, appetite ratings, and product acceptability in adults with overweight or obesity. Methods: Participants (N = 17) attended at least three separate visits during which they consumed, in random order, a food matrix with 0% (control), 10%, or 30% soy flour substitution. Food matrices included breads (n = 10), tortillas (n = 10), and arepas (n = 8); some participants completed more than one matrix. Postprandial plasma glucose and insulin concentrations were measured at baseline and at 15, 30, 60, 90, and 120 min post-ingestion. Subjective hunger, satiety, and product liking were assessed using a 10 cm visual analog scale. Results: Compared with the control condition, substituting 30% of refined flour with soy flour significantly reduced the area under the concentration–time curve for postprandial glucose for breads (p = 0.03) and arepas (p = 0.04), and reduced plasma glucose concentrations at 90–120 min for tortillas (p = 0.0009). In contrast, postprandial insulin concentrations and subjective hunger and satiety ratings did not differ across substitution levels or food matrices (all p > 0.05). Importantly, even 30% soy flour substitution maintained product liking. Conclusions: Incorporating up to 30% soy flour may improve postprandial glycemic responses without compromising overall liking, supporting its potential as a practical food reformulation strategy to improve metabolic health in populations at increased risk of metabolic disease.