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the gut must breakdown nutrients properly to specifically prepare it for the organs to process.
so, with kidney complications the absence of specific gut flora causes improper protein digestion leading the waste product urea to elevate, stressing the kidneys with the overload and eventually backing up and accumulating in the blood.

Happy gut, happy kidneys? Restoration of gut microbiome ameliorates acute and chronic kidney disease

"...aberrancy of gut microbiota-derived metabolites could affect the outcomes of acute kidney injury (AKI) and, in part, explain the presence of circulating uremic toxins in chronic kidney disease (CKD) 

Yu SM, He JC. Happy gut, happy kidneys? Restoration of gut microbiome ameliorates acute and chronic kidney disease. Cell Metab. 2021 Oct 5;33(10):1901-1903. doi: 10.1016/j.cmet.2021.08.019. PMID: 34614404; PMCID: PMC8994854.

https://pmc.ncbi.nlm.nih.gov/articles/PMC8994854/