Kidneys and Urine

What the Gut Has to Do With Kidneys and Bladder

The kidneys exist to remove waste from the blood. A substantial share of that waste is not made by the body at all. It is manufactured by gut bacteria from food, absorbed, and sent to the kidneys to be cleared. That means the workload the kidneys face is partly set in the intestine. Further down the urinary tract, the connection is different again, because the bladder holds its own bacterial community and the gut is the upstream source for most of what ends up there.

Two different connections, one system

The first runs through chemistry. When gut bacteria ferment leftover protein rather than fibre, they produce compounds including indoxyl sulfate and p-cresyl sulfate. Healthy kidneys clear both. Failing kidneys don't, so they accumulate, and a review pooling 11 studies covering 1,572 patients found higher levels predicting death. The evidence that bacteria are the source is unusually clean, because dialysis patients whose colon has been surgically removed have almost undetectable levels.

The same principle explains kidney stones. Oxalate has to leave through the kidneys, and a bacterium called Oxalobacter formigenes consumes it in the intestine before it can be absorbed. It was present in 17% of people with recurrent stones and 38% of people without them, an association with roughly 70% lower odds after adjustment.

The second connection runs through the immune system. In IgA nephropathy, the commonest form of kidney inflammation worldwide, the faulty antibody damaging the kidney is made in immune tissue in the small intestine. The treatment that followed that discovery is a capsule engineered to dissolve only there, and in a trial of 364 patients it preserved kidney function across two years.

Lower down, recurrent urinary infections turned out not to be about carrying more E. coli. Women who get them repeatedly had less varied gut communities short of butyrate producers, while the bacteria travelled from gut to bladder in everyone.

Source: Meta-Analysis of the Associations of p-Cresyl Sulfate and Indoxyl Sulfate with Cardiovascular Events and All-Cause Mortality in Patients with Chronic Renal Failure

Source: Oxalobacter formigenes may reduce the risk of calcium oxalate kidney stones

Source: Longitudinal multi-omics analyses link gut microbiome dysbiosis with recurrent urinary tract infections in women

What this means for you

Kidney function is checked with a simple blood and urine test, and anyone with high blood pressure, diabetes or a family history should ask their doctor about being monitored, since kidney disease is usually silent until it is advanced. In addition, the research above shows gut bacteria manufacturing the waste products that predict death in kidney disease. They also consume the oxalate that forms stones, and shape susceptibility to repeated urinary infections. A varied, plant-rich diet, adjusted with a renal dietitian if kidney function is already reduced, supports the same gut bacteria this research ties to fermenting fibre rather than leftover protein. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.

Explore kidney and bladder conditions on this page

The panels on this page separate into two groups, and the evidence differs between them.

Kidney function, chronic kidney disease and protein in urine concern compounds gut bacteria manufacture and the kidneys have to clear. Kidney stones concern one specialist bacterium that consumes oxalate. Kidney inflammation concerns immune tissue in the small intestine, and is the condition here with an approved gut-targeted treatment.

The bladder conditions work differently. Urinary infections, interstitial cystitis, overactive bladder and bladder control each involve the bladder's own bacterial community, the gut as its upstream source, or the mechanical effect of the bowel sitting against the bladder.

Use the panels on this page to find the one relevant to you.