In addition, we can observe that, at the genus level, Lachnospiraceae nk4a136 bacteria significantly increased after r-irisin treatment, becoming one of the signature bacteria of the r-irisin treatment group, and this bacterium is also a SCFA-producing bacterium, which is decreased in the condition of obesity and IR and increased in the condition of a healthy lifestyle or benign intervention such as spermidine and resveratrol (Ma et al., 2020; Wang et al., 2020; Muralidharan et al., 2021). Here, FNDC5 is linked to obesity due to melanocortin 4 receptor deficiency.