Urine that smells like vomit usually points to a buildup of short-chain organic acids, the same types of compounds that give vomit its distinctive sour odor. The most common and least worrisome cause is concentrated urine from dehydration, which amplifies acids that are normally too dilute to notice. Less often, infections, dietary factors, or metabolic conditions can shift the chemical profile of your urine enough to produce that unmistakable sour, acidic smell.
How Dehydration Changes Urine Smell
When you don’t drink enough water, your kidneys produce less urine, and every compound dissolved in it becomes more concentrated. Research on urine chemistry shows that short-chain and branched-chain organic acids, which are naturally present in small amounts, become more prominent as water volume drops. These acids are chemically similar to butyric acid and isovaleric acid, the compounds responsible for the smell of vomit and sweaty feet. At normal hydration levels, they’re too dilute to register. When your urine is dark yellow or amber, those same acids can become pungent enough to notice.
This is the most likely explanation if the smell appeared during hot weather, after exercise, during illness with vomiting or diarrhea, or simply on a day you forgot to drink water. Increasing your fluid intake for a day or two typically resolves it. If the smell persists despite good hydration, something else is going on.
Urinary Tract Infections
Bacterial infections in the urinary tract can produce foul or sour-smelling urine because bacteria metabolize compounds in urine and release their own acidic byproducts. Different bacterial strains produce different odors. Some create smells described as ammonia-like, while others generate acidic, lactic-acid-type odors that can easily read as vomit-like. One documented strain, Aerococcus urinae, produces urine with a smell specifically described as reminiscent of lactic acid, which has that same sour, stomach-acid quality.
UTIs usually come with other symptoms: burning or pain during urination, an urgent or frequent need to go, cloudy or discolored urine, and sometimes pelvic pressure or lower back pain. If the vomit smell is paired with any of these, a simple urine test can confirm or rule out an infection quickly.
Diet, Supplements, and Protein Intake
Your body breaks down dietary protein into amino acids, and byproducts of that process are filtered through your kidneys. A high-protein diet or protein-heavy supplements increase the load of amino acid metabolites in your urine, some of which are organic acids with strong odors. Branched-chain amino acids like leucine, isoleucine, and valine are processed into compounds that, at higher concentrations, can smell sour or acidic.
B vitamins, particularly in high-dose supplements or energy drinks, are also well known for changing urine odor and color. While B vitamins more commonly produce a sulfurous or pungent smell, the combination of B vitamins with amino acid supplements can create an unusual acidic scent. If you recently started a new supplement, protein powder, or significantly changed your diet, try stopping it for a few days to see if the smell resolves.
Certain foods also contribute. Asparagus is the classic example, but garlic, onions, cruciferous vegetables like broccoli and Brussels sprouts, and fermented foods can all produce sulfur-containing or acidic metabolites that change urine odor.
Ketosis and Blood Sugar Problems
When your body burns fat instead of glucose for energy, it produces ketone bodies: acetone, acetoacetate, and beta-hydroxybutyrate. These ketones spill into your urine and can create a strong, sour, or sickly-sweet smell. Depending on the concentration, different people describe it as fruity, nail-polish-like, or vomit-like.
Mild ketosis happens during low-carb or ketogenic diets, fasting, or skipping meals, and the urine smell it produces is usually harmless. Diabetic ketoacidosis is the dangerous version, occurring when blood sugar is uncontrolled in people with diabetes. It produces much higher concentrations of ketones alongside symptoms like extreme thirst, frequent urination, nausea, abdominal pain, and confusion. If you have diabetes or suspect you might, a persistent sour urine smell alongside these symptoms needs prompt medical attention.
Rare Metabolic Conditions
Several inherited metabolic disorders cause the body to accumulate specific organic acids because it lacks the enzymes to break them down properly. These are rare and mostly diagnosed in infancy, but milder forms occasionally go undetected into adulthood.
Isovaleric acidemia is the closest match to a vomit-like smell. It’s caused by mutations in the gene responsible for breaking down leucine, an amino acid found in protein. Without the right enzyme, a compound called isovaleric acid builds up in the body. Isovaleric acid is one of the chemicals that gives vomit (and sweaty feet) their characteristic smell. During acute episodes, the odor is described as distinctly like sweaty feet, which overlaps significantly with what many people identify as a vomit smell.
Other metabolic conditions produce different characteristic odors. Maple syrup urine disease causes a sweet, syrupy smell due to problems processing branched-chain amino acids. Tyrosinemia type 1 creates a “boiled cabbage” or “rotten mushroom” odor. These are unlikely causes for an adult noticing a new smell, but if the odor has been present since childhood or runs in your family, they’re worth mentioning to a doctor.
Connections Between the Gut and Bladder
In rare cases, an abnormal connection called a fistula can form between the intestine and the bladder, allowing digestive contents to leak into urine. This produces foul-smelling urine that can have a sour, vomit-like, or even fecal quality. These fistulas can develop as complications of inflammatory bowel disease, diverticulitis, abdominal surgery, or rarely from conditions like appendicitis.
The hallmarks are persistent urinary infections that keep coming back despite treatment, gas or air bubbles in the urine (called pneumaturia), recurrent abdominal pain, and urine that looks cloudy or contains visible particles. This is uncommon, but if you’re experiencing that combination of symptoms, it’s a specific diagnosis your doctor can investigate with imaging.
What Testing Looks Like
A standard urinalysis is the starting point for any persistent change in urine odor. It checks for bacteria, white blood cells (indicating infection), glucose and ketones (indicating diabetes or ketosis), and protein levels. The test is quick, noninvasive, and can be done at virtually any clinic or doctor’s office.
If the urinalysis is normal but the smell continues, your doctor may order blood work to check blood sugar, kidney function, or liver enzymes. For suspected metabolic conditions, specialized tests can measure organic acid levels in urine or blood. Most people with a temporary vomit-like urine smell never need to go beyond the basic urinalysis, because the cause turns out to be dehydration, diet, or a treatable infection.