a, Yeast depletion through feeding only sucrose lowers THF signals in the gut while feeding exogenous THF increases them. b, Quantification of THF signals in the gut of flies fed with normal food (yeast food), yeast depletion (sucrose only) and exogenous THF (sucrose + THF). c, LC-MS-based measurement of THF shows that Oregon R maintains the THF amount in the gut during ageing, but wOregon R does not. d, Feeding folates does not promote ISC proliferation in young Oregon R. e, Feeding THF promotes ISC proliferation in old wOregon R flies. f, Endogenous expression of White using GFP knock-in flies. White shows vesicular localization in principal cells of the Malpighian tubules. g, RT-qPCR of white or brown transcript from the carcass of Oregon R shows that there is no transcriptional change of these genes during ageing. Statistical significance was determined using one-way ANOVA with Dunnett’s post-test (b, d) and two-tailed unpaired t-test (c, e). The experiments were repeated independently at least twice with similar results (a, f). Scale bars, 10 μm.