Ascorbic acid intake, oxalate production and kidney stone risk
What it found
Higher ascorbic acid intake was linked to more oxalate in urine and a greater chance of kidney stones in people.
The authors say the oxidized form of ascorbic acid breaks down into oxalate inside the body.
What they found
Studies in people
Kidney stones in men
In a study of about 50,000 male health workers, those taking over 218 mg a day had a 31% higher chance of forming stones than those taking under 105 mg. A Swedish study found that taking ascorbic acid supplements nearly doubled the stone risk.
Oxalate in urine
Short feeding trials in people showed that extra ascorbic acid raised oxalate levels in urine. In one trial, normal subjects saw a 21.8% rise with 2 g a day, while stone formers saw a 34.4% rise on the same amount.
Large doses and kidneys
Several case reports describe people developing a type of kidney damage called oxalate nephropathy after taking very large oral or intravenous doses of ascorbic acid. Most needed temporary dialysis, and two cases resulted in death.
Absorption drops at high doses
People absorb 70 to 90% of ascorbic acid at oral doses up to 200 mg a day. At 1250 mg a day, absorption falls to about 33%, and most of what is absorbed leaves the body in urine.
Animal and lab studies
How oxalate forms
When ascorbic acid loses electrons it becomes an oxidized form that can turn into an unstable molecule. Lab tests show this unstable molecule quickly splits into oxalate and sugars.
Other findings
Daily breakdown amounts
About 60 mg of ascorbic acid breaks down in the human body each day. This could produce up to 30 mg of oxalate daily, though the authors estimate the real amount from this source is closer to 10.5 mg.
What the authors conclude
- They state that evidence links ascorbic acid intake to higher urinary oxalate and a greater risk of calcium oxalate kidney stones.
- They call for studies using labeled ascorbic acid to measure exactly how much turns into oxalate inside the human body.
- They advise that people with kidney stones should be cautious about taking ascorbic acid supplements and that the public needs to know about the risks of very large doses.
How it was done
Scientists have debated for over 50 years whether ascorbic acid raises the risk of calcium oxalate kidney stones by turning into oxalate in the body. They wanted to gather what is known about how the body breaks down ascorbic acid and where the gaps are.
The authors gathered and examined past research on how ascorbic acid is absorbed, used and broken down in people and animals. They looked at large surveys of tens of thousands of adults and smaller feeding trials measuring oxalate in urine. They also reviewed case reports of people who developed kidney damage after taking very large amounts of ascorbic acid.
What it can’t tell you
- Surveys following people over time can show a link between ascorbic acid and kidney stones but cannot prove it causes them.
- Lab tests showing how ascorbic acid breaks down into oxalate cannot reveal exactly how much of this happens inside a living person.
- Case reports of kidney damage after large doses involve very unusual amounts and may not apply to typical supplement use.
Who paid
- Funding
- Funded by NIDDK NIH HHS (from the PubMed record).
- Conflicts
- Conflict of Interest: Authors declare no conflict of interest.
- Authors work at
- University of Alabama at Birmingham, USA
The paper
- Title
- Ascorbic acid intake and oxalate synthesis
- Type
- ReviewThe authors read earlier studies and describe what they found. It isn’t a new study.
- Evidence
- Studies in people · animals · lab
- Summarised from
- Full text
- Cite
- Knight J, Madduma-Liyanage K, Mobley JA, Assimos DG, Holmes RP (2016). Ascorbic acid intake and oxalate synthesis. Urolithiasis. doi:10.1007/s00240-016-0868-7Free full textPubMed 27002809DOI
Summary written 30 Sep 2026. Check it against the paper before it changes what you eat. How we summarise papers · Report an error