Metabolic phenotypes and the gut microbiota in response to dietary resistant starch type 2 in normal-weight subjects: a randomized crossover trial

Nineteen healthy, normal-weight Chinese adults aged 18–55 years (BMI < 24 kg/m2; 10 women, 9 men). Randomized, double-blind, placebo-controlled crossover trial; 3 withdrew before the first intervention; 19 completed; identical controlled diet provided throughout.

HAM-RS2: 40 g/day for 4 weeks, taken with meals; matched energy-matched control starch (0 g RS) for 4 weeks, taken with meals.

RS2 at 40 g/day for 4 weeks reduced visceral fat area and subcutaneous fat area on MRI in normal-weight adults on a controlled diet, with no change in body weight. LDL-C decreased (≈0.16 mmol/L), BUN decreased (≈0.44 mmol/L), and uric acid decreased. Early-phase insulin and C-peptide increased after meals; active GLP-1 rose at 30 minutes post-meal; serum acetate increased (p<0.001). No significant changes in fasting/postprandial glucose or most SCFAs; glucose infusion rate during clamp unchanged. Gut microbiota shifted: 15 genera decreased; Ruminococcaceae_UCG-005 increased; Akkermansia tended to increase. Baseline Streptococcus abundance predicted hormonal and abdominal fat responses. RS was well tolerated with no adverse events and normal liver/renal markers after 4 weeks. Conclusion: daily 40 g RS2 may safely reduce abdominal adiposity and improve metabolic hormone responses and gut microbiota in normal-weight adults, suggesting potential metabolic-health benefits.

Small sample size (n=19); short duration per arm (4 weeks); single-population (Chinese, normal weight) limits generalizability; reliance on self-reported diet adherence; 16S sequencing limitations for microbiota analyses.