Hippuric acid in urine reflects a combination of what you eat, how diverse your gut bacteria are, how well your liver and kidneys are working, and whether you have been exposed to the industrial solvent toluene. Everyone excretes some hippuric acid every day, because it is a normal product of the body’s handling of benzoic acid, which comes from fruits, vegetables, and other dietary sources. The interesting part is that the amount can shift meaningfully depending on your health, your diet, and your environment, which is why researchers and occupational health professionals pay attention to it for very different reasons.
How Your Body Makes Hippuric Acid
Hippuric acid is what you get when the liver (and to a lesser extent the kidneys) takes benzoic acid and attaches a molecule of the amino acid glycine to it. This reaction happens in the mitochondria of liver and kidney cells and is sometimes called glycine conjugation. The resulting compound, hippuric acid, is water-soluble and gets filtered out through the kidneys into urine. It is one of the most abundant organic acids in normal human urine.
Benzoic acid itself comes from several places. Your gut bacteria break down polyphenols from plant foods into simpler compounds, and benzoic acid is one of the end products. Some foods contain benzoic acid directly, and sodium benzoate, a widely used food preservative, converts to benzoic acid in the body. So the amount of hippuric acid you excrete depends on how much benzoic acid your body is producing or absorbing, how active your gut microbes are, and how efficiently your liver conjugates it with glycine.
Diet Is the Biggest Everyday Driver
For most people who are not occupationally exposed to solvents, what you eat is the main reason your hippuric acid levels go up or down. Fruits, vegetables, tea, and coffee are rich in polyphenols, and when gut bacteria metabolize those polyphenols, benzoic acid is a major byproduct. A study of adults placed on a high fruit-and-vegetable diet found that urinary hippuric acid correlated with polyphenol intake from vegetables, with particularly strong associations for polyphenols from leaf and stalk vegetables.1PubMed Central. The Relationship between Dietary Polyphenol Intakes and Urinary Polyphenol Concentrations in Adults Prescribed a High Vegetable and Fruit Diet In children, higher diet quality scores have been linked to higher urinary hippurate, while diets heavy in ultra-processed food show the opposite pattern.2PubMed Central. Urinary metabolic biomarkers of diet quality in European children are associated with metabolic health
This makes hippuric acid a surprisingly practical dietary biomarker. Asking people what they eat is famously unreliable, but measuring a urinary metabolite gives researchers an objective signal. If you eat a lot of plant-based foods, your hippuric acid will tend to be higher. If your diet is dominated by processed foods with little fruit or vegetable content, it will tend to be lower. Researchers have proposed using hippuric acid as a quick way to verify whether someone is actually following dietary advice, rather than just reporting that they are.
Sodium Benzoate in Processed Foods
Sodium benzoate is added to soft drinks, condiments, and other processed foods to prevent microbial growth. Since it converts directly to benzoic acid in the body, consuming foods or drinks containing it temporarily raises urinary hippuric acid. A study of workers exposed to toluene found that drinking a sodium benzoate-containing beverage raised their urinary hippuric acid within about ninety minutes, though the effect faded by three hours.3Annals of Occupational and Environmental Medicine. Effect of Sodium Benzoate on Change of Urinary Hippuric Acid and Methyl Hippuric Acid among Workers Coexposed to Toluene and Xylene Animal research has confirmed that when benzoic acid is added to the diet, most of it ends up excreted as hippuric acid in urine.4PubMed. Effects of dietary benzoic acid and sodium-benzoate on performance, nitrogen and mineral balance and hippuric acid excretion of piglets
This matters for interpretation. If someone had a soda preserved with sodium benzoate shortly before providing a urine sample, their hippuric acid could spike in a way that has nothing to do with their usual diet, their gut microbiome, or any solvent exposure. The effect is real but brief. Occupational health professionals who use hippuric acid to monitor toluene exposure have to account for this kind of dietary interference, and it is one reason the test has limitations at low exposure levels.
A Window into Gut Microbiome Diversity
One of the more interesting developments in hippuric acid research is its connection to gut microbial diversity. Hippuric acid is sometimes described as a “co-metabolite,” meaning it requires both the host (your liver) and your gut bacteria to produce it. The bacteria break down plant polyphenols into benzoate, and the liver finishes the job by attaching glycine.5PubMed. Hippurate: the natural history of a mammalian-microbial cometabolite People with a more diverse gut microbiome tend to excrete more hippuric acid, even after accounting for differences in diet.6Scientific Reports. Hippurate as a metabolomic marker of gut microbiome diversity: Modulation by diet and relationship to metabolic syndrome
This relationship makes hippuric acid a kind of functional readout for what your gut bacteria are actually doing, rather than just which species are present. Two people might harbor similar bacterial species, but if one person’s microbiome is more metabolically active in processing polyphenols, that person’s hippuric acid will be higher. Researchers interested in the microbiome have started treating hippuric acid as a convenient summary measure, because collecting and analyzing a urine sample is far cheaper and easier than performing a full metagenomic analysis of someone’s stool.
Higher Levels and Better Metabolic Health
A growing body of evidence links higher urinary hippuric acid to better metabolic outcomes. A study published in Gut found that high urine hippurate concentration was a general marker of metabolic health. In participants eating high-fat diets, hippurate appeared to contribute to metabolic improvements, suggesting it might be more than just a bystander signal.7Gut. Human and preclinical studies of the host–gut microbiome co-metabolite hippurate as a marker and mediator of metabolic health Separately, research using restricted cubic spline analysis found that urinary hippuric acid above a certain threshold had a negative correlation with the risk of metabolic disorders, pointing toward the protective role of diets rich in fruits, coffee, and tea.8PubMed. Analyzing urinary hippuric acid as a metabolic health biomarker through a supramolecular architecture
It is worth being careful about cause and effect here. People with higher hippuric acid tend to eat better and have more diverse gut microbiomes, both of which are independently associated with better metabolic health. Whether hippuric acid itself does something beneficial, or whether it is simply a marker that tracks alongside genuinely protective factors, is still being sorted out. The Gut study offered some experimental evidence that hippurate itself contributes to metabolic improvements, but the question is far from settled.
Toluene Exposure and Occupational Monitoring
The oldest and most established clinical use of urinary hippuric acid is as a biomarker for occupational toluene exposure. Toluene is a solvent found in paints, adhesives, printing inks, and various industrial products. When you inhale toluene, your liver metabolizes it into benzoic acid, which then gets conjugated with glycine and excreted as hippuric acid in urine.9Scientific Reports. Rapid determination of hippuric acid as an exposure biomarker of toluene using a colorimetric assay and comparison with high-performance liquid chromatography In workers with significant toluene exposure, urinary hippuric acid levels at the end of a shift are reliably higher than pre-shift levels, and the correlation with airborne toluene concentrations can be strong. A study in printing plant workers found correlation coefficients above 0.8 between end-of-shift hippuric acid and daily toluene exposure.10PubMed. Hippuric acid and ortho-cresol as biological indicators of occupational exposure to toluene
The test is also used for less obvious exposure scenarios. Surgical smoke, generated during electrocautery and laser procedures, contains toluene and xylene. Researchers have used urinary hippuric acid (and its methylated cousin, methylhippuric acid) to evaluate whether operating room staff are being exposed to meaningful levels of these chemicals during surgery.11PubMed. Use of urinary hippuric acid and o-/p-/m-methyl hippuric acid to evaluate surgical smoke exposure in operating room healthcare personnel Batik workers, who use toluene-based dyes, are another group monitored this way.12Heliyon. Urinary hippuric acid level as a biological indicator of toluene exposure on batik workers
Why Toluene Monitoring Has Limits
Despite its long history, hippuric acid has a well-known weakness as a toluene exposure marker: the background noise is enormous. Because everyone produces hippuric acid from food, the baseline level varies widely from person to person and from day to day. A landmark study using isotope-labeled toluene made the problem vivid. Researchers had subjects inhale toluene tagged with deuterium atoms so they could distinguish hippuric acid coming from toluene versus hippuric acid coming from normal metabolism. They found that the cumulative excretion of unlabeled (dietary) hippuric acid exceeded the total toluene uptake within just four hours, and the person-to-person variation in background hippuric acid was large.13PubMed Central. Toxicokinetics of toluene and urinary excretion of hippuric acid after human exposure to 2H8-toluene
The practical takeaway is that at low toluene exposure levels, urinary hippuric acid is not a reliable way to tell whether someone has been exposed. The dietary signal simply drowns out the toluene signal. At higher exposures, the toluene-derived hippuric acid rises enough to be distinguishable from baseline, and the test works well. This is why modern occupational health guidelines sometimes use other biomarkers like o-cresol or S-benzylmercapturic acid for low-level toluene monitoring, reserving hippuric acid for situations where exposure is expected to be substantial.
Kidney Disease and Hippuric Acid Accumulation
On the flip side of the equation, when kidneys cannot clear hippuric acid efficiently, it accumulates in the blood. In chronic kidney disease, hippuric acid is classified as a uremic toxin, one of the compounds that builds up as kidney function declines and contributes to the symptoms and complications of uremia. A review of uremic toxins noted that hippuric acid, along with compounds like indoxyl sulfate and p-cresyl sulfate, plays a role in the cardiovascular damage that accompanies progressive kidney disease.14PubMed Central. Uremic Toxins in the Progression of Chronic Kidney Disease and Cardiovascular Disease: Mechanisms and Therapeutic Targets
This creates a somewhat paradoxical situation. In people with healthy kidneys, higher urinary hippuric acid is generally a good sign, reflecting a polyphenol-rich diet and a diverse gut microbiome. In people with failing kidneys, the inability to excrete hippuric acid means it accumulates in the blood where it can do harm. The same molecule is a marker of health in one context and a toxin in another, depending entirely on whether the kidneys are working well enough to clear it.
Crohn’s Disease and Altered Gut Metabolism
People with Crohn’s disease tend to have lower baseline urinary hippuric acid than healthy individuals. One study found significantly reduced hippuric acid levels in Crohn’s patients at baseline, but when participants were given an oral dose of benzoic acid to bypass the gut microbial step, their liver conjugation worked fine and peak hippuric acid excretion was similar to that of healthy controls.15PubMed Central. Differences in gut microbial metabolism are responsible for reduced hippurate synthesis in Crohn’s disease This pointed the finger squarely at the gut bacteria, not the liver. In Crohn’s disease, the microbiome is disrupted, producing less benzoic acid from dietary polyphenols, and so less hippuric acid makes it into urine.
Broader metabolomic studies have confirmed this pattern. Compared with healthy controls, patients with Crohn’s disease show decreased hippuric acid in both urine and blood, and Crohn’s patients tend to have lower hippuric acid than those with ulcerative colitis, reflecting the more severe microbial disruption in Crohn’s.16Inflammatory Bowel Diseases. Serum and Urine Metabolomic Profiling of Newly Diagnosed Treatment-Naïve Inflammatory Bowel Disease Patients Low hippuric acid is not diagnostic for Crohn’s on its own, but it adds to a metabolic profile that researchers are using to better understand how the disease alters body chemistry.
Do Smoking and Alcohol Change Your Levels?
Given how many things influence urinary hippuric acid, you might expect that common habits like drinking and smoking would matter too. The evidence here is mixed, which itself is informative. A large three-year occupational health study found that smoking was positively associated with urinary hippuric acid concentration, while alcohol consumption showed a negative association.17PubMed Central. Climatic effects on urinary hippuric acid concentrations: a 3-year occupational health study in a manufacturing workforce However, two other studies looking specifically at adults without occupational toluene exposure concluded that neither drinking nor smoking significantly affected basal hippuric acid levels.18Journal of Occupational Health. Possible Effects of Drinking and Smoking Habits on Hippuric Acid Levels in Urine of Adults with No Occupational Toluene Exposure19PubMed. Hippuric acid in urine: reference values
The discrepancy probably reflects differences in study design, population size, and how carefully diet was controlled. The safest interpretation is that smoking and alcohol are not major drivers of hippuric acid levels in most people, though in large enough datasets with enough statistical power, subtle effects might emerge. If you are getting your hippuric acid measured for occupational health reasons, what you ate in the past twelve hours matters far more than whether you had a beer the night before.
Hippuric Acid and the Brain
A newer and more speculative line of research connects hippuric acid to brain function through the gut-brain axis. A study in mice found that hippuric acid, produced through gut microbiome-dependent pathways, appeared to play a role in social behavior development. Germ-free mice, which have no gut bacteria and therefore produce very little hippuric acid, showed abnormal social behavior. When researchers administered hippuric acid to these mice, their social behavior normalized. The mechanism seemed to involve restoration of oxytocin expression in the brain, suggesting hippuric acid may activate a signaling pathway that links gut health to social functioning.20PubMed Central. Gut Microbiota Affects Mouse Social Behavior via Hippuric Acid Metabolism
This is a mouse study, and the leap from germ-free mice in a lab to human psychiatric health is enormous. But it fits into a broader emerging picture in which metabolites produced by gut bacteria influence brain chemistry in ways that were barely imagined a decade ago. Whether hippuric acid turns out to be a meaningful player in human neurobiology or just a convenient model compound for studying gut-brain communication remains to be seen.
How Hippuric Acid Gets Measured
If you are wondering whether a test for hippuric acid is something you would encounter in routine medical care, the answer for most people is no. Hippuric acid measurement is primarily an occupational health tool, used for workplace biomonitoring of solvent exposure. The gold standard has been high-performance liquid chromatography, which is accurate but requires lab equipment and trained technicians.21PubMed. Analytical techniques for monitoring of toluene and xylene exposure biomarkers hippuric acid and methylhippuric acid in human urine samples
Newer approaches are making measurement faster and cheaper. A colorimetric assay developed recently can detect hippuric acid in urine in about two minutes, with good accuracy across different urine backgrounds.22PubMed. A fast and accurate colorimetric assay for quantifying hippuric acid in human urine Immunochromatographic strips, similar in concept to a home pregnancy test, have also been developed for rapid toluene exposure screening, with a cutoff calibrated to detect occupationally relevant levels of hippuric acid.23Journal of Analytical Toxicology. Immunochromatographic Analysis of Hippuric Acid in Urine As hippuric acid gains traction in metabolic health research, the availability of quick, inexpensive measurement tools could eventually bring it closer to routine clinical use, perhaps as part of dietary or microbiome health assessments. For now, though, it lives squarely in the domain of occupational medicine and research metabolomics.