How Oxalic Acid and Oxalates Affect Human Health and Nutrition
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. Toxic levels of oxalic acid 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, but some contain enough oxalic acid to cause health problems in certain people. The best known of these problems is the formation of calcium oxalate kidney stones.
What are Oxalates?
Oxalic acid often exists in the form of oxalates 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 oxalate 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 calcium oxalate kidney stones are generally advised to follow a low oxalate diet.
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. Concentrated oxalic 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 should form a large part of our diet. However, a lot of plants do contain a significant amount of oxalic acid or oxalates. This may not be a problem for some people, but for others the oxalates in food can 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 acid.
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 sorrel, beets, leeks, spinach, Swiss chard, okra, sweet potatoes, tea, cocoa, certain legumes or pulses - including soybeans and peanuts - and some nuts, seeds and berries. A more detailed list is given in the link at the end of this article.
Since oxalates tend to be most common in some very healthy foods, they shouldn't be avoided unless this is recommended by a doctor.
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 of oxalates, 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.
How to Prevent Kidney Stones
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. Oxalate binds to both iron and calcium, preventing it from being absorbed from our small intestine. However, some researchers say that provided we eat a wide variety of food and don't eat a huge amount of high-oxalate food, we don't need to worry about a lower intake of nutrients from plants that contain oxalate.
Many plant foods containing a high oxalate level have important health benefits. It would be a shame to eliminate them from a diet, or even severely limit them, unless there is a medical reason to do so.
The Urinary System
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. However, a larger stone trying to pass through a ureter can cause excruciating pain. The stone can be removed by a variety of surgical treatments, but once a person has experienced one kidney stone the chance of developing another one is greatly increased.
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 can increase the risk of kidney stones, since it increases the probability that oxalates 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 oxalate.
- Eating a lot of animal protein reduces the amount of citrate in the urine. Citrates, or 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 or with symptoms suggesting the presence of a kidney stone should seek a doctor's advice.
Genetics and Kidney Stones
Calcium in the Diet
Interestingly, patients who are susceptible to calcium oxalate kidney stones don't need to follow a low-calcium diet. Calcium in the body isn't a problem, as long as it doesn't collect in the urine. In fact, calcium in food seems to be helpful to people who tend to develop kidney stones. Some researchers recommend that calcium supplements be taken with a meal, however. Some of the calcium binds with oxalate in food, forming insoluble calcium oxalate and preventing the oxalate from being absorbed from the digestive tract.
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 lots 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?
© 2012 Linda Crampton