Oxalic Acid and Oxalates: Health Effects and Kidney Stones
What Is Oxalic Acid?
Oxalic acid is a colorless, crystalline substance that is poisonous to humans in its pure form or when it’s dissolved in a liquid to make a concentrated solution. Potentially dangerous levels of the chemical are found in products such as certain wood cleaners, metal cleaners, rust treatments, bleaches, disinfectants, and pesticides.
Oxalic acid is also found in many plants, but at much lower concentrations than in synthetic products. Most of these plants are completely safe for human consumption. Some contain enough oxalic acid (generally in the form of oxalate) to cause health problems in sensitive people. The best known of these problems is the production of calcium oxalate kidney stones.
What Are Oxalates?
Oxalic acid often exists as oxalate in plants. When an oxalic acid molecule loses its two hydrogen atoms, it forms an oxalate ion. Oxalate ions join with certain metal ions to form salts, such as calcium oxalate and magnesium oxalate.
Calcium oxalate has very low solubility in water and tends to form crystals instead of dissolving. In some people these crystals form stones in the kidneys. People at risk for forming the kidney stones are often advised to follow a low oxalate diet.
The information in this article is intended for general interest. Anyone with questions about the effects of oxalic acid or oxalates on health should consult a physician or health agency.
Oxalic Acid Poisoning From Commercial Products
Oxalic acid is also known as ethanedioic acid. Oxalic acid cleaners are used to remove rust, paint, varnish, and ink stains from surfaces. They are also used as disinfectants to clean toilets and as pesticides to kill varroa mites in beehives. The acid is an important reactant in certain chemical reactions and is used in some industries and chemistry laboratories.
Synthetic oxalic acid products are more likely to be poisonous than oxalic acid in plants. The concentrated acid is corrosive and can burn the skin and eyes, irritate the lungs, and make breathing difficult. It can also cause a headache, nausea, vomiting, dizziness, and convulsions. In addition, it can lower the calcium level in the body and cause kidney damage. The acid enters our bodies by ingestion, inhalation, and skin absorption.
It's very important to use oxalic acid and products containing it with great care, using all the safety precautions listed on the product label. The products should be kept in a secure location so that children and pets can't get to them. It's a good idea to use a less toxic substance if this is possible.
Oxalates in Plants
Plants are healthy foods and according to dietitians and nutritionists should form a large part of our diet. A lot of plants contain a significant amount of oxalic acid or oxalates, however. This isn't a problem for many people, but for others the chemicals may cause health problems.
An example of a plant containing a large amount of oxalic acid is rhubarb. Rhubarb stalks make a nice meal when cooked and sweetened. Rhubarb leaves shouldn't be eaten, however. They are so rich in oxalic acid that they are considered to be poisonous for everyone, whether or not they are sensitive to the chemical.
Determining accurate oxalate levels in plants is hard, as described below. This is why the lists of low oxalate foods from different sources vary. In general, though, plant foods that are considered to be high in oxalates include:
- Swiss chard
- sweet potatoes
- certain legumes or pulses, including soybeans and peanuts
- some nuts
More detailed lists are given in the links at the end of this article. As noted below, however, although the links go to health and medical sources, a health professional should be consulted in person before a low oxalate diet is started.
Since oxalates tend to be most common in some very healthy foods, they shouldn't be avoided unless this is recommended by a doctor. If someone is advised by a health professional to go on a low oxalate diet, they should ask for a list of foods to avoid. The list above is not a substitute for a doctor's or nutritionist's recommendations.
Determining the Amount of Oxalic Acid in Plants
Determining the amount of oxalate provided by a particular plant is quite difficult. It depends on variable conditions such as the environment in which the plant was grown, the age of the plant when it was harvested, and the way in which the plant is prepared for eating. The serving size is important, too.
In addition, the form of the oxalate in the plant is significant. The more soluble types, such as magnesium oxalate, may be absorbed into the body from the small intestine, while the less soluble ones, such as calcium oxalate, may stay in the intestine and be passed harmlessly out of the body. Foods from animals don't contain as much oxalate as foods from plants.
Oxalates in the Diet and Nutrient Absorption
There are some concerns that eating plants rich in oxalates can decrease our mineral intake. For example, spinach is a healthy food and contains a lot of iron and some calcium too, but it’s also high in oxalate. The chemical binds to both iron and calcium, preventing the substances from being absorbed from our small intestine.
Some researchers say that provided we eat a wide variety of food and don't eat a huge amount of food that is rich in oxalates, we don't need to worry about a lower intake of nutrients. Spinach and many other plants are great additions to the diet for most of us.
In the urinary system diagram above, 2 = kidney, 3 = renal pelvis, 4 = ureter, 5 = urinary bladder and 6 = urethra. The other numbers represent nearby structures and blood vessels.
Calcium Oxalate Kidney Stones
The kidneys extract waste and excess water from blood to make urine. The urine is sent to the urinary bladder through tubes called ureters. When the bladder is full, urine is released from the body through the urethra.
About 80% of kidney stones are made of calcium oxalate. A kidney stone often causes no pain if it remains in the kidney. If it's very small, it may pass through a ureter and out of the body without difficulty. A larger stone trying to pass through a ureter may cause severe pain, however.
A stone can be removed by a variety of medical treatments. Once a person has experienced one kidney stone, the chance of developing another one is increased.
Urine is produced inside the kidney. It eventually drains into a tube called the ureter, which transports the urine to the urinary bladder. Stones generally form in the kidney but may enter or be produced in the ureter, as shown in the illustration above. Stones may also be produced in the urinary bladder.
Diet and Kidney Stones
In some people, there is a genetic basis for the tendency to develop kidney stones and the stones run in the family. This isn't always true, however. Diet plays an important role in the creation or prevention of a calcium oxalate kidney stone in people who are sensitive to oxalates.
- Dehydration increases the risk of kidney stone development, since it causes a concentrated urine to be produced. In a person who is well hydrated, water dilutes the calcium oxalate in the urine and make the development of a kidney stone less likely.
- A diet that is high in oxalates may increase the risk of kidney stones, since it increases the probability that the chemicals will be excreted into the urine.
- A diet high in salt promotes kidney stone formation by causing calcium to enter the urine. The calcium then binds with oxalate in the urine, potentially leading to the production of a stone.
- We make oxalate in our bodies. Researchers have discovered that taking high doses of vitamin C supplements increases our production of the chemical.
- Eating a lot of animal protein reduces the amount of citrate in the urine. Citrates and citric acid are inhibitors of kidney stone development. Citrate binds to calcium in the urine, preventing it from binding to oxalate. Doctors may prescribe citrate supplements to their patients who have a tendency to develop kidney stones.
Anyone with kidney stones, with symptoms suggesting the presence of a stone, or who is at risk for stone development should seek a doctor's advice.
Calcium in the Diet
Interestingly, doctors often say that patients who are susceptible to calcium oxalate kidney stones don't need to follow a diet that's low in calcium. Calcium in the body isn't a problem, as long as it doesn't collect in urine containing a high level of oxalate. In fact, calcium in food seems to be helpful to people who tend to develop kidney stones.
Some researchers recommend that calcium supplements be avoided or be taken with a meal. Some of the calcium binds with oxalate in food, forming insoluble calcium oxalate and preventing the oxalate from being absorbed from the digestive tract. The use of calcium supplements by someone susceptible to kidney stones should be discussed with a doctor.
Remember that if you form kidney stones made from a substance other than calcium oxalate, the dietary steps to prevent stone formation will be different. A doctor's advice should be sought when dealing with any type of kidney stones. Drinking a sufficient quantity of water and reducing salt intake are good steps for everyone to follow, though.
A Kidney Stone Poll
Have you ever had a kidney stone?
- "Vegetables, Grains, and Mineral Absorption" from Berkeley Wellness
- Kidney stone information from the Mayo Clinic
- The National Institutes of Health diet suggestion for kidney stone prevention
- Low oxalate diet from the University of Pittsburgh Medical Center
- "How to Eat a Low Oxalate Diet" from the University of Chicago Kidney Stone Evaluation and Treatment Program
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
© 2012 Linda Crampton