This study investigates how dietary changes affect the composition of the human colonic microbiota. It involved 14 overweight men who followed a controlled diet over 10 weeks, including a control diet, a high resistant starch (RS) diet, a high non-starch polysaccharide (NSP) diet, and a reduced carbohydrate weight loss (WL) diet. Analysis of 16S rRNA sequences from stool samples revealed 320 phylotypes, with 26 of them accounting for more than 1% of sequences. The study found that certain bacterial groups, such as relatives of Ruminococcus bromii, increased significantly on the RS diet, while others, like Eubacterium rectale, decreased on the WL diet. These changes were rapid and reversible, indicating that the microbiota can respond quickly to dietary shifts. Individual differences were significant, with some volunteers showing minimal fermentation of RS, leading to lower levels of certain bacteria. The study highlights the impact of dietary non-digestible carbohydrates on the gut microbiota, emphasizing the role of individual microbiota composition in determining responses to diet. The findings suggest that dietary advice on non-digestible carbohydrates should consider individual variations in the gut microbiota.This study investigates how dietary changes affect the composition of the human colonic microbiota. It involved 14 overweight men who followed a controlled diet over 10 weeks, including a control diet, a high resistant starch (RS) diet, a high non-starch polysaccharide (NSP) diet, and a reduced carbohydrate weight loss (WL) diet. Analysis of 16S rRNA sequences from stool samples revealed 320 phylotypes, with 26 of them accounting for more than 1% of sequences. The study found that certain bacterial groups, such as relatives of Ruminococcus bromii, increased significantly on the RS diet, while others, like Eubacterium rectale, decreased on the WL diet. These changes were rapid and reversible, indicating that the microbiota can respond quickly to dietary shifts. Individual differences were significant, with some volunteers showing minimal fermentation of RS, leading to lower levels of certain bacteria. The study highlights the impact of dietary non-digestible carbohydrates on the gut microbiota, emphasizing the role of individual microbiota composition in determining responses to diet. The findings suggest that dietary advice on non-digestible carbohydrates should consider individual variations in the gut microbiota.