Uric Acid Kidney Stones: Causes, Symptoms & How to Prevent Them

Most people with gout worry about their joints. Far fewer worry about their kidneys — but the same uric acid that crystallizes in a big toe can also crystallize in the urinary tract. The result is a uric acid kidney stone, and while this stone type accounts for only about 8–10% of all kidney stones in the United States, it is especially common in people with gout, diabetes, and metabolic syndrome.

Here is the good news that most articles on this topic skip: uric acid stones are one of the few kidney stone types that can often be dissolved without surgery, and they are among the most preventable through hydration and diet. This guide explains what they are, why they form, what they feel like, and the evidence-backed steps that lower your risk.

What uric acid kidney stones actually are

Your kidneys filter uric acid out of your blood and flush it into your urine. Normally it stays dissolved and washes away. A uric acid stone forms when the urine becomes too acidic, too concentrated with uric acid, or both — and the uric acid drops out of solution, forms crystals, and the crystals clump into a stone. Uric acid stones are smooth, often pale yellow or reddish-brown, and can range from sand-like grains to stones several centimeters across.

Three urinary abnormalities drive their formation, and one dominates. Research published in the Journal of Nephrology identifies low urine pH as the single most important factor: when urine pH drops below the ionization constant of uric acid (pKa ≈ 5.5), the undissociated form of uric acid becomes much more abundant — and it is far less soluble. The other two contributors are excess uric acid in the urine (hyperuricosuria) and low urine volume. In plain terms: acidic urine, a lot of uric acid in it, and not enough water to dilute it.

This makes uric acid stones fundamentally different from the more common calcium stones. The chemistry that creates them also undoes them — and that is why they are among the few stones that medication can actually dissolve. If you are weighing whether your levels are high at all, read our guide to what high uric acid really means first.

Why they form: the three factors

1. Persistently acidic urine — the big one

Surprisingly, a high purine diet is not the main driver. Studies show the vast majority of uric acid stone formers have an abnormally acidic baseline urine plus too little ammonium buffer to neutralize it. The mechanism is increasingly linked to insulin resistance: impaired renal ammoniagenesis means less ammonia reaches the urine to soak up acid, so the urine stays acidic around the clock. This is why uric acid stones track so closely with metabolic syndrome, type 2 diabetes, and obesity — and why many people who form them never eat a particularly purine-heavy diet.

2. Too much uric acid in the urine

When blood uric acid runs high — as it does in gout — the kidneys pass more of it into the urine. Certain situations push this further: chemotherapy, chronic fluid loss from diarrhea, and some genetic disorders. About one in five people with gout also forms uric acid stones, and many stone formers also have gout, according to research reviewed in the Journal of Urology.

3. Too little water

Concentrated urine gives crystals the perfect meeting ground. Anything that chronically reduces fluid intake or increases fluid loss — living in a hot climate, heavy sweating, ongoing diarrhea, poor bowel absorption — raises the risk.

Put together, the typical person at risk has some combination of gout or metabolic syndrome, persistently acidic urine, and lower-than-ideal hydration. The Mayo Clinic’s overview of kidney stone causes lists the same profile: high-protein diets, organ meats, shellfish, diabetes or metabolic syndrome, and chronic diarrhea or malabsorption.

Tall glass of water with lemon and orange slices on a kitchen counter — staying hydrated helps prevent uric acid kidney stones

Who is most at risk

Uric acid stones are not evenly distributed. Research and clinical guidelines point to the same high-risk groups:

  • People with gout. The shared chemistry means stones and gout often travel together. If you have gout, a stone workup is worth discussing with your doctor even without symptoms.
  • People with diabetes or metabolic syndrome. Insulin resistance is the leading explanation for the persistent aciduria behind most uric acid stones.
  • People with obesity. Higher body weight is linked to higher stone risk across all stone types, and specifically to uric acid stones.
  • People with chronic diarrhea, bowel disease, or ostomies. Ongoing fluid and bicarbonate loss concentrates the urine and keeps it acidic.
  • People eating very high-protein, purine-dense diets. Animal-source purines raise urinary uric acid. In our red-zone list of foods to avoid with gout, every one of these is measured with real purine numbers.
  • People with a family history of stones. Kidney stone risk has a real genetic component.

What uric acid kidney stones feel like

A stone sitting quietly inside the kidney often causes no symptoms at all. Pain starts when the stone begins to move or blocks the flow of urine. According to the Mayo Clinic, the warning signs include:

  • Severe, sharp pain in the side and back, below the ribs
  • Pain that spreads to the lower abdomen and groin
  • Pain that comes in waves and varies in intensity
  • Pain or a burning feeling while urinating
  • Pink, red, or brown urine; cloudy or foul-smelling urine
  • A constant urge to urinate, urinating more often than usual, or passing only small amounts
  • Nausea and vomiting
  • Fever and chills if an infection is present

Small stones and sand-like crystals can pass unnoticed, and some people pass stones repeatedly without realizing it. The pain people describe as “the worst of my life” comes from a stone stuck in the ureter — the narrow tube to the bladder — which makes the kidney swell and the ureter spasm. If your flank pain ever resembles a gout flare in intensity but sits in the wrong place, take it seriously: check our gout symptoms guide to compare, then get checked.

How doctors diagnose them

A uric acid stone is invisible on a plain X-ray (radiolucent), so diagnosis usually relies on a CT scan — the most reliable method — or an ultrasound, which visualizes them well. Blood and urine tests reveal the chemistry behind them: a 24-hour urine collection measuring volume, pH, and uric acid excretion tells a doctor exactly which of the three factors is driving your stones, and guides therapy. If you pass a stone, save it and bring it to your doctor; knowing the stone type is the single most useful fact for preventing the next one.

The purine numbers that matter

Because hyperuricosuria is one of the three stone factors, the foods that load your system with purines deserve attention. These are measured values from our full 38,078-food purine database, in milligrams per 100 grams:

  • Dried anchovy — 1,109 mg/100g. In a league of its own; raw anchovy measures 411 mg and canned 321 mg.
  • Canned sardines — 399 mg/100g. Small oily fish are purine bombs in any preserved form.
  • Beef liver, raw — 197 mg/100g (USA measurement). All organ meats sit high.
  • Shrimp, farmed, raw — 182.4 mg/100g. Shellfish counts as much as meat.
  • Chicken breast, roasted — 178.5 mg/100g. Moderate, not low — the “safe white meat” framing doesn’t survive measurement.
  • Beer — regular 8.48 mg/100mL, dark 13.51 mg/100mL. The purine numbers look modest, but alcohol also slows the kidneys’ excretion of uric acid — double jeopardy for stone formers.

At the safe end, milk at 2% fat measures just 0.6 mg/100g, tofu (kinu) 20 mg/100g, and green tea 1.67 mg/100g. Note that our database does not have measured purine values for lemon juice, orange juice, or sugar-sweetened soda — so where citrus or soda appears in prevention advice, the reasoning comes from citrate and fructose research, not a purine table.

For the full day-to-day plan built from these numbers, see our gout diet plan — the same purine logic that protects joints protects kidneys.

How doctors treat them: education only

Treating a confirmed uric acid stone is a doctor’s job, but it helps to understand the toolbox, because it explains the prevention advice below:

  • Hydration comes first. Diluting the urine is the foundation of both treatment and prevention.
  • Urine alkalinization is the cornerstone. Prescribed potassium citrate or sodium bicarbonate raises urine pH into the 6.0–6.8 range, where uric acid converts to its far more soluble urate form — and can dissolve existing stones over weeks to months. The Mayo Clinic notes that these medicines may dissolve existing uric acid stones, and clinical reviews call alkalinization the cornerstone of therapy.
  • Uric acid–lowering medication such as allopurinol is used in people with excess urinary uric acid, especially those who also have gout.
  • Surgery is reserved for large stones, obstruction, or cases where medical therapy fails — shock-wave lithotripsy or ureteroscopy rather than open surgery.

No home remedy dissolves a stone. Alkalinizing prescriptions need pH monitoring, and uric-acid-lowering drugs need a doctor’s supervision. The section that follows covers only the prevention habits that are safe to manage yourself.

Prevention: what you can actually do

Drink enough to keep your urine pale

This is the single most important habit. Health authorities generally advise drinking enough fluid to produce at least 2 liters (about 2.1 quarts) of urine per day — more in hot weather or with heavy exercise. The practical test is the color: light yellow to clear means you are diluting your urine enough to make stone formation much harder. Water counts; so do most unsweetened fluids. If you currently drink little, adding one extra liter a day is the highest-value change you can make.

Eat to lower the purine load

A diet built on the best foods for gout — dairy, eggs, fruits, vegetables, whole grains — naturally keeps urinary uric acid down. Shrink the red-zone foods (organ meats, dried anchovies, sardines, shellfish) to rare, small portions, and be especially careful with beer, which hits stone formers twice: purines plus the alcohol-driven drop in uric acid excretion. Two or more sugar-sweetened sodas a day more than doubled gout risk in a 22-year study of nearly 79,000 women (relative risk 2.39), and the fructose pathway raises urinary uric acid too — so sugary drinks belong on the avoid list even though they aren’t purine-heavy.

Favor citrus and alkali-forming foods

Citrate in citrus fruits and juices raises urinary citrate and helps keep urine less acidic — the same chemistry behind prescription alkalinization, in food form. Lemon and orange juice are often recommended in stone-prevention diets for this reason. Our database has no measured purine values for citrus juices, but purines were never the point here; the citrate is. Fruits and vegetables in general tend to alkalinize urine, while heavy meat loads push it more acidic.

Cut sodium and keep a healthy weight

High sodium intake raises the calcium the kidneys must filter, and obesity is firmly tied to uric acid stones through the insulin-resistance mechanism. Neither is a gout-specific fix; both lower the general chemistry that favors stones.

Keep dairy and legumes on the menu

Low-fat dairy shows up repeatedly as protective — research links higher dairy intake to lower gout risk, and milk’s measured purine load is near zero (0.6 mg/100g at 2% fat). Legumes such as lentils carry moderate plant purines, but the landmark 12-year study of 47,150 men found purine-rich vegetables had no association with gout at all — plant purines behave differently from animal ones. See our gout FAQ for the common “are legumes safe?” confusion answered with real numbers.

When to see a doctor

Seek care promptly if you have severe pain that won’t let you sit still, pain with nausea and vomiting, pain with fever and chills, blood in your urine, or trouble passing urine — fever with stone symptoms can signal an infected, blocked kidney, which is an emergency. Otherwise, a doctor visit is wise if you have gout and have never been evaluated for stones, if you pass a stone or suspect you did, or if you want a 24-hour urine workup to learn your personal pH and uric acid numbers. Silent stones exist: some people discover them only on imaging for something else, and a small stone caught early is far easier to dissolve than a large one.

References

For education only — not medical advice. Discuss diet changes with your doctor.