The gut posses a staggering number of bacteria. There are more bacteria living in our colons than there are human cells in our bodies. These microbes produce compounds, some of which are absorbed and eventually cleared by the kidney. As such, there are a number of compounds derived from the gut flora that build up in renal failure.
Two examples of such compounds are indoxyl sulfate and p-cresol sulfate which are produced by colon microbes from tryptophan and phenylalanine/tyrosine, respectively. In the ongoing search for the medley of true uremic toxins (remember that urea, produced from protein metabolism in the liver is at worst only mildly toxic) compounds such as indoxyl sulfate and p-cresol sulfate are potentially toxic candidates.
In an interesting study recently published in JASN a group from Stanford examined the profile of plasma solutes in normal subjects and dialysis patients with and without colons. As expected, HPLC assays revealed that indoxyl sulfate and p-cresol sulfate were nearly absent in dialysis patients without colons in contrast to solutes like urea that were present in equal amounts in both the colon intact and absent groups.
In addition, mass spectroscopy revealed a number of other known and unidentifiable solutes in significantly higher concentration in the dialysis patients with colons when compared with normal subject and dialysis patients without colons. This suggests that we are just at the beginning of understanding the contribution of the colonic microflora to uremic toxicity.
As suggested at the end of the JASN article, a deeper understanding of our micorflora's contribution to uremic toxicity could allow us to explore the health benefits of reducing the production of toxic solutes by nutritional, probiotic, or pharmacologic means.