A protein called NFIL3 is at the center of a key metabolic transaction in the gut, in which gut bacteria harvest energy from food and transfer it into fat storage. Like many metabolic pathways, this process appears to be synched to a day-night light cycle through the molecules that make up the body's circadian clock, but the exact mechanisms of the transaction are poorly understood. The connections are important, however, since they may shed light on why humans with disrupted clocks -- those who work the night shift or often travel internationally, for example -- may be at increased risk for metabolic diseases including obesity and diabetes. Yuhao Wang and colleagues now show that gut microbes produce proteins that tune the circadian cycling of NFIL3 through immune cell signaling. NFIL3, in turn, controls the circadian fluctuations of a metabolic pathway that regulates fat absorption and export into the cells that line the intestine. The findings offer "a deeper understanding of why perturbing microbiota-clock interactions can lead to metabolic disease," the authors write.