APP and Alzheimer disease: In APP/PS1 transgenic mice, both butyric and isobutyric acid levels were reduced in feces and brain, with a positive correlation between brain and fecal butyric acid levels as studied by Zhang et al. 86 Another study using stable isotope labeling and liquid chromatography-tandem mass spectrometry found lower propionic acid and higher lactic acid levels in APP/PS1 mice compared to wild-type mice.87 Syeda et al. observed that the discrepancies in SCFA concentrations in AD and wild-type mice were present at various ages and life stages.