When we discussed purslane, one word needed a little more explanation: oxalates. They occur naturally in many foods, including some nutritious vegetables, nuts and cacao. Understanding them helps us make sensible choices without turning the vegetable basket into a list of suspects.
Oxalates have no established nutritional benefit for humans. However, many foods that contain them have considerable benefits. The important question is how much oxalate reaches your urine, and whether you are prone to forming kidney stones.
What are oxalates?
Oxalate is a chemical form of oxalic acid. Plants produce it naturally, and our bodies also make it during normal metabolism. It is not a vitamin or mineral we need to obtain from food.
Some dietary oxalate remains in the intestine and leaves in the stool. Some is absorbed into the bloodstream and is normally removed by the kidneys into the urine. Oxalate can bind to calcium, forming calcium oxalate. Where that binding happens matters. [1]
What happens in the kidneys?
When urine becomes sufficiently concentrated with calcium and oxalate, crystals can form. Under favourable conditions, these can grow into calcium oxalate kidney stones, the most common type of kidney stone.
A stone can cause severe pain, bleeding or a blockage. Excessive oxalate can also damage kidney tissue through crystal deposits, particularly in unusual circumstances such as rare inherited disorders, significant intestinal malabsorption or very large oxalate exposures. This is different from assuming that an ordinary portion of vegetables damages healthy kidneys. [1,2]
Risk depends on more than one food. Urine volume, calcium, oxalate, citrate, salt intake and individual medical conditions all matter. Not all kidney stones contain oxalate, so knowing your stone type is useful before changing your diet. [2,3]
Do oxalates have health benefits?
There is no established reason to deliberately increase oxalate intake for health. The benefits of spinach, nuts, purslane and cacao come from their other components—such as vitamins, minerals, fibre, useful fats and polyphenols.
Oxalate can bind some of a food’s calcium in the digestive tract, reducing its availability. This is one reason spinach should not be relied upon as your main calcium source. [1]
Nevertheless, a food containing oxalate is not automatically unhealthy. Plant-rich dietary patterns can help prevent kidney stones overall. Removing vegetables, beans, nuts and whole grains indiscriminately can make a diet less nutritious. [4]
Which foods contain oxalates?
These are useful examples, rather than a precise ranking. Amounts vary with variety, growing conditions, processing and serving size.
| Food group | Examples worth knowing | Practical point |
|---|---|---|
| Leafy vegetables | Spinach, Swiss chard, beet greens, purslane | These can be substantial sources. Rotate greens rather than using large amounts of the same one daily. |
| Other vegetables | Rhubarb stalks, beetroot; sweet potatoes also contain oxalate | Portion size matters. Rhubarb leaves should not be eaten. |
| Nuts and nut products | Almonds, cashews, peanuts and their butters | A spoonful and several handfuls provide different amounts. Nuts vary considerably. |
| Cacao and chocolate | Cocoa powder, cacao and dark chocolate | A small piece in a dish differs from a large daily serving or concentrated drink. |
| Grains | Wheat bran and some bran-rich products | “Whole grain” does not mean every product has the same oxalate content. |
| Drinks | Tea, particularly strong black tea | Brewing strength and the number of cups influence exposure. |
Sources: kidney-health food guidance and the purslane research discussed in our previous article. [3–6]
For lower-oxalate vegetable variety, options commonly include cabbage, broccoli, cauliflower, cucumber and lettuce. People who need a specific restriction should use a food list recommended by their kidney dietitian: online lists often disagree, and comparing milligrams per 100 grams with milligrams per serving can be misleading.
Why calcium can help
Calcium and oxalate binding inside the intestine can reduce how much oxalate is absorbed. The bound material can leave in the stool instead of reaching the kidneys.
That is why cutting dietary calcium to prevent calcium oxalate stones can backfire. Maintain an appropriate calcium intake, and include calcium-containing foods with meals. Plant-based options include suitable unsweetened calcium-fortified drinks and calcium-set tofu; check labels because calcium content varies. Supplements, if needed, should be discussed with your clinician, including their timing. [4,7]
Does cooking remove oxalates?
Boiling in water and discarding the water can reduce soluble oxalates. The reduction varies between foods and methods, and it does not remove all oxalate. A study comparing cooking methods found boiling more effective than steaming for the vegetables tested. [8]
For a high-oxalate green, you can boil it, drain it and then add it to your dish. If you use the same cooking water in soup, the oxalate that leached into it remains in the meal. Washing removes dirt but does not reliably remove oxalates within the leaves.
Fermentation should not be assumed to make a high-oxalate food low in oxalate. Its effects depend on the food and process.
A sensible approach to kidney health
Drink water regularly and replace fluid lost through heat and activity. People with recurrent stones are often advised to drink enough to produce around 2–2.5 litres of urine daily; that is a urine target, not a universal drinking prescription. Anyone with a prescribed fluid restriction needs an individual plan. [2]
Avoid excessive salt, maintain adequate calcium and eat a varied diet. Avoid high-dose vitamin C supplements without a reason: some vitamin C is converted to oxalate. This does not mean avoiding normal portions of fruit. [2,4]
If you repeatedly form stones, ask about stone analysis and a 24-hour urine collection. These can show whether oxalate is actually a problem for you. Current CARI guidance discourages routinely prescribing a strict low-oxalate diet to everyone with calcium oxalate stones, while recognising that people with particular absorption problems need tailored advice. [7]
Dr Michael Greger’s NutritionFacts.org also discusses oxalates and cautions against overdoing very high-oxalate greens. It is a useful addition to the kidney-health sources below. [9]
You can appreciate the nutrition in purslane, cacao and nuts while paying attention to portions and variety. For most readers, the goal is a balanced plate. For someone with recurrent stones or kidney disease, the goal is a balanced plate adapted to their test results.
Before making substantial dietary changes because of this article, consult your health practitioner, particularly if you have kidney disease, recurrent stones or a prescribed diet.
Sources and further reading
- Dietary Oxalate Intake and Kidney Outcomes — oxalate metabolism, absorption and kidney effects.
- Endotext: Nephrolithiasis — stone formation, hydration and dietary factors.
- NIDDK: Eating, Diet, and Nutrition for Kidney Stones — individual advice by stone type.
- National Kidney Foundation: Eat Smart to Prevent Kidney Stones — plant foods, calcium and testing.
- UCI Kidney Stone Center: Oxalate Content of Foods — illustrative food data; values are not universal.
- Poeydomenge and Savage: Oxalate Content of Raw and Cooked Purslane.
- CARI Guidelines: Nutrition Therapy for the Prevention of Kidney Stones — current recommendations and individual exceptions.
- Chai and Liebman: Effect of Different Cooking Methods on Vegetable Oxalate Content.
- Dr Michael Greger: Oxalates, NutritionFacts.org.