Resistant Starch in MASLD: Microbial Ecology, Response Heterogeneity and the Path to Precision Nutrition

Resistant starch (RS) links a modifiable dietary exposure to microbial metabolism and hepatic health in metabolic dysfunction-associated steatotic liver disease (MASLD). A four-month randomized trial of 40 g/day high-amylose maize RS type 2 (HAM-RS2) reduced liver fat, while linked cohort analyses identified substantial variation in response. International trials in other metabolic populations reported null hepatic-fat effects, bringing population and nutritional context into focus. This critical narrative review examines the ecological basis of response variation and its implications for precision nutrition. Three interacting determinants organize the evidence: the preparation consumed, the functional capacity of the recipient microbiota and the host environment in which fermentation occurs. Substrate accessibility, primary degradation and microbial cross-feeding shape the metabolites available to the host. Disease-specific transfer and perturbation experiments support microbial amino-acid and inflammatory pathways; short-chain fatty acid (SCFA) metabolism, bile-acid handling and lipid absorption provide additional candidate links to hepatic response. Quantitative comparisons place RS alongside effective dietary and lifestyle interventions. We connect this framework to a concrete HAM-RS2 research regimen and a staged path from response prediction to comparative treatment benefit. Independent replication, reproducible functional measurements and marker-stratified trials can establish whether measuring microbial ecology improves nutritional choices alongside standard MASLD care.