Insulin immunohistochemistry staining was performed on pancreas sections to determine β cell mass as previously described (Shrestha et al., 2020 (link); Talchai et al., 2012 (link); Xuan et al., 2010 (link)). In brief, pancreata from Chow, HFD, and HFD-Chow groups (5–7 mice/group) were dissected, cleared of fat, weighed, fixed in 4% paraformaldehyde (P6148; Sigma-Aldrich) overnight at 4°C. The tissues were embedded in paraffin, and consecutively sectioned with 3-μm thickness. Five sections at least 160 μm apart were subjected to insulin immunohistochemistry staining and scanned using the Olympus-VS200 scanner for each pancreas. Total insulin positive area and total pancreas area on each section were measured by the cellSens software (Olympus). The β cell mass was obtained through multiplying the pancreas weight by the ratio of insulin-positive area/pancreas area.