a, Beginning at 3 weeks of age, three groups of mice were fed special diets for 4 weeks. SPF mice were fed either a nutrient-rich or a minimal diet, and GF mice were fed the nutrient-rich diet. Colonic T reg cells were analysed, and absolute numbers of RORγ+Helios− in the FOXP3+CD4+TCRβ+ T reg cell population are shown. b, c, Three-week-old SPF mice were fed as in a, and T reg cells in different tissues were analysed after 4 weeks. Representative plots (b) and frequencies of RORγ+Helios− in the FOXP3+CD4+TCRβ+ T reg cell population (c) are shown. iLN, inguinal lymph node. d–f, SPF mice were fed a nutrient-rich or a minimal diet at birth and were either maintained on that diet or switched to the opposite diet at 3 weeks of age. Colonic T reg cells were analysed after 4 weeks. Representative plots of RORγ+Helios− in the FOXP3+CD4+TCRβ+ T reg cell population (d), and the frequencies of FOXP3+ in the CD4+TCRβ+ cell population (e) and RORγ+Helios− in the colonic FOXP3+CD4+TCRβ+ T reg cell population (f) are shown. g–i, LC–MS quantification of faecal acetate (g), propionate (h) and butyrate (i) from SPF mice fed a nutrient-rich or a minimal diet, and from GF mice fed a nutrient-rich diet. j, Three-week-old SPF mice were fed a nutrient-rich diet, a minimal diet, or a minimal diet supplemented with individual or mixed SCFAs in drinking water. Colonic T reg cells were analysed after 4 weeks. Frequencies of RORγ+Helios− in the FOXP3+CD4+TCRβ+ T reg cell population are shown. Data are representative of two independent experiments. n represents biologically independent animals. Data are mean ± s.e.m. (a, c and e–j). *P < 0.05, **P < 0.01, ***P < 0.001, one-way ANOVA followed by the Bonferroni post hoc test (a, e–i) or two-tailed Student’s t-test (c). Source Data