Low Oxalate Diet

If you’ve ever had kidney stones your doctor may have told you that you are at increased risk of developing them again. Research shows that the recurrence rate is 50% within 5 years and 70% or higher within 10 years. One of the ways to reduce your risk of recurrent kidney stones is to eat fewer high-oxalate foods.

What is oxalate?

Oxalate is a naturally occurring substance that is present in many plant foods, and is also produced in the liver. Additionally, the body converts vitamin C into oxalate. For some people, high levels of oxalates increase their chances of developing kidney stones.

What are kidney stones?

Kidney stones are crystal-like material formed from concentrated minerals and salts that stick together to create a stone that can become lodged in the kidney or in other parts of the urinary tract.

When the stone obstructs the flow of urine it can cause a sudden and severe pain that is often described as being worse than childbirth. Many patients also experience nausea and vomiting. Calcium oxalate stones are the most common type of kidney stones, accounting for up to 75% of stones formed. This is when oxalate from the foods you eat, as well as oxalate that is naturally made in your liver, binds with calcium in the body.

In fact, because of its binding capacity to calcium, plants use oxalate as a way to dispose of excess calcium.

Who should consume a low-oxalate diet?

You may need a low oxalate diet if you suffer from:

• Kidney stones that contain calcium oxalate crystals. The type of kidney stones you have determines your food choices
• A genetic defect that causes increased oxalate production in your body
• A gut problem that causes malabsorption such as fat malabsorption, chronic pancreatic disease, gastric bypass surgery, or inflammatory bowel disease
• A high intake of oxalate-rich foods and a diet rich in animal protein

However, for people who are not at special risk of calcium oxalate kidney stones or do not test high for urine oxalate, there are no benefits in this diet.

Which foods are high in oxalate?

It is often difficult to determine the oxalate content of foods as it depends on how the food is prepared, time of harvest, growing conditions and method of measurement. Thus, food tables listing the oxalate content of foods often show differences in oxalate values.

Nevertheless, consuming the following foods can raise the level of oxalate in your urine:

• Rhubarb
• Dark green leafy vegetables such as spinach, Swiss chard, and rocket
• Beetroot
• Wheat bran
• Nuts and seeds
• Soy products
• Chocolate
• Tea
• Strawberries

For a more comprehensive list: https://oxalate.org/

Can cooking remove oxalates from food?

Cooking does not seem to have a significant impact on the oxalate content of foods. Therefore, overcooking an oxalate-rich food will not reduce its oxalate content and at the same time will result in the loss of many vitamins and minerals. However, soaking soybeans, followed by cooking, have been found to reduce oxalate concentration.

Do vegetarians have higher levels of oxalates?

Studies have shown that despite consuming more plant foods, vegetarians have a lower risk of developing kidney stones compared with those who eat a high meat diet. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) recommends limiting consumption of animal protein to reduce your chances of developing kidney stones.
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What about calcium?

It may sound like dietary calcium may contribute to formation of calcium-oxalate stones, but studies have shown quite the opposite.

Reducing dietary calcium in patients with kidney stones may even worsen their stone disease. In the right amount, calcium can reduce absorption of oxalates in the body by binding to oxalates and excreting it from the body.

The current recommendations for people suffering from kidney stones specify that dietary calcium should not be restricted beyond normal, but sodium should be reduced. Higher salt intake has been associated with excessive calcium in the urine.
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How to follow a low-oxalate diet

It is difficult to avoid eating some oxalates as most plant foods have some. Eating a low-oxalate diet means that you should try and limit foods that contain moderate to high levels of oxalates. Here are some tips:

• Include fruit and vegetables that are low in oxalates, as they will provide many important nutrients.
• Limit portion size when consuming high-oxalate foods.
• Include high calcium foods in the same time as high oxalate food; the calcium will help reduce the amount of oxalate being absorbed by your body. Examples include low fat cheese with a spinach salad or yoghurt with berries.
• Drink plenty of fluid. Water increases your urine volume and dilutes the oxalates and other stone-forming substances in the urine. Aim for a minimum of eight glasses a day.
• Limit fat intake if you are suffering from malabsorption. Excess fat will bind with calcium in food, leaving oxalate to accumulate freely.
• A low oxalate diet for preventing recurrent kidney stone formation should be done along other dietary recommendations to reduce kidney stones, such as reducing salt and animal protein in your diet.
• Supplement of vitamin B6 with dosages up to 400-500 mg/d has been shown to reduce level of oxalates in the body - but should be taken under advice from your doctor.
• Avoid large doses of vitamin C, as it can convert to oxalate in the body. Some experts recommend limiting intake from supplements of no more than 500-1000 mg of Vitamin C daily.

References

Cleveland Clinic. Kidney Stones: Oxalate-Controlled Diet. Available at: https://my.clevelandclinic.org/health/articles/11066-kidney-stones-oxalate-controlled-diet

Dave, C., 2018. Nephrolithiasis. Available at: https://emedicine.medscape.com/article/437096-overview#showall

Dolan, L.C., Matulka, R.A. & Burdock, G.A., 2010. Naturally occurring food toxins. Toxins, 2(9), pp.2289–332. Available at: http://www.ncbi.nlm.nih.gov/pubmed/22069686  

Finkielstein, V.A. & Goldfarb, D.S., 2006. Strategies for preventing calcium oxalate stones. CMAJ : Canadian Medical Association journal = journal de l’Association medicale canadienne, 174(10), pp.1407–9. Available at: http://www.ncbi.nlm.nih.gov/pubmed/16682705

NIDDK 2017. Eating, Diet, & Nutrition for Kidney Stones. Available at: https://www.niddk.nih.gov/health-information/urologic-diseases/kidney-stones/eating-diet-nutrition

Shekarriz, B., 2015. Hyperoxaluria. Available at: https://emedicine.medscape.com/article/444683-overview#a3

Turney, B.W. et al., 2014. Diet and risk of kidney stones in the Oxford cohort of the European Prospective Investigation into Cancer and Nutrition (EPIC). European Journal of Epidemiology, 29(5), pp.363–369. Available at: http://www.ncbi.nlm.nih.gov/pubmed/24752465