BACKGROUND: Butyrate delivery to the large bowel may positively modulate commensal microbiota and enhance immunity. OBJECTIVE: To determine the effects of increasing large bowel butyrate concentration through ingestion of butyrylated high amylose maize starch (HAMSB) on faecal biochemistry and microbiota, and markers of immunity in healthy active individuals. DESIGN: Male and female volunteers were assigned randomly to consume either two doses of 20 g HAMSB (n = 23; age 37.9 +/- 7.8 y; mean +/- SD) or a low amylose maize starch (LAMS) (n = 18; age 36.9 = 9.5 y) twice daily for 28 days. Samples were collected on days 0, 10 and 28 for assessment of faecal bacterial groups, faecal biochemistry, serum cytokines and salivary antimicrobial proteins. RESULTS: HAMSB led to relative increases in faecal free (45%; 12-86%; mean; 90% confidence interval; P = 0.02), bound (950%; 563-1564%; P < 0.01) and total butyrate (260%; 174-373%; P < 0.01) and faecal propionate (41%; 12-77%; P = 0.02) from day 0 to day 28 compared to LAMS. HAMSB was also associated with a relative 1.6-fold (1.2- to 2.0-fold; P < 0.01) and 2.5-fold (1.4- to 4.4-fold; P = 0.01) increase in plasma IL-10 and TNF-alpha but did not alter other indices of immunity. There were relative greater increases in faecal P. distasonis (81-fold (28- to 237-fold; P < 0.01) and F. prausnitzii (5.1-fold (2.1- to 12-fold; P < 0.01) in the HAMSB group. CONCLUSIONS: HAMSB supplementation in healthy active individuals promotes the growth of bacteria that may improve bowel health and has only limited effects on plasma cytokines.