SCFAs measurement
Frozen faecal samples were mixed with 1 mL of ice-cold saline, homogenized in a ball mill (4 min, 40 Hz), and then ultrasonically processed for 5 min in ice water, a process which was repeated three times. The supernatant was obtained by centrifugation (5,000 rpm, 20 min, 4 °C) and added to extracting solution containing 2-methylpentanoic acid (25 mg/L in methyl tert-butyl ether) as an internal standard. The mixture was then fully mixed by vortexing and oscillation and centrifuged for 15 min (10,000 rpm, 4 °C). After centrifugation, the supernatant was transferred into a fresh vial for gas chromatography‒mass spectrometry analysis. Gas chromatography‒mass spectrometry analysis was performed using an Agilent gas chromatograph coupled with an Agilent 5977B mass spectrometer. Helium was used as the carrier gas, and the initial temperature was set to 80 °C (1 min), subsequently increased to 200 °C (in 5 min), and then maintained (1 min) at 240 °C. The injection and ion source temperatures were 240 and 230 °C, respectively. The electron impact energy was -70 Eva. The mass spectrometry data were acquired in Scan/SIM mode.