Twenty-seven IBS and six healthy subjects were randomly allocated one of two 21-day provided diets , differing only in FODMAP content ( mean ( 95 % CI ) low 3.05 ( 1.86 to 4.25 ) g/day vs Australian 23.7 ( 16.9 to 30.6 ) g/day ) , and then crossed over to the other diet with 21-day washout period .