This is a very interesting manuscript addressing emerging questions regarding gut to bone mechanisms of regulation. Here ablation of intestinal K cells using a a GIP promoter driven diphtheria toxin mouse suggested the secretion of an additional gut derived factor that opposes the anabolic effects of GIP, resulting in little change in overall bone structure. Follow up in vivo studies identified another K-cell-derived peptide xenin as the opposing factor, but the mechanism by which xenin works appears to be indirect, supported by in vitro studies. In the Discussion, the authors suggest that VMH neurons could be targets of xenin since activation of these neurons via then neurotensin receptor was reported to reduce bone resorption, to be further investigated.
Critique
1. The authors may wish to acknowledge that administration of xenin to Swiss NIH mice appears to primarily inhibit anabolic pathways with much lower magnitude effects on catabolic pathways, as shown in Figure 3. This observation may help to ultimately inform the mechanism by which xenin counteracts effects of GIP.
2. Were serum levels of GIP and xenin measured in mice lacking K-cells? It would be important to know if circulating levels of both peptides were actually ablated in these mice compared to control mice.
3. Did female mice in which K-cells were ablated have the same limited bone phenotype as the males? The authors should comment on this in the Discussion.

1. Thank you especially for addressing the question of serum xenin/GIP levels.
2. A GIP feedback mechanism of gut-derived anti-incretins has been previously proposed that involves gut-derived dopamine suppression of GIP activity by Maffei et al. (Molecular Endocrinology 29:542–557, 2015), and Daley et al JBMR Plus 3(10) e1012 DOI: 10.1002/jbm4.10212 (2019). Have the authors considered gut-generated dopamine as an alternative feedback mechanism similar to what was proposed here for xenin? The authors may wish to provide a comment in the Discussion.