What Exactly Are Oxalates?
Oxalates, or oxalic acid, are natural compounds found in many plants. When consumed, they can bind to minerals like calcium and magnesium, sometimes forming calcium oxalate crystals. For most healthy individuals, oxalates are simply filtered by the kidneys and excreted from the body without issue. However, for those with a history of kidney stones, certain gut conditions, or other health concerns, high oxalate intake can be problematic.
The High-Oxalate Contenders
Several leafy greens stand out for their particularly high oxalate levels. While these greens are often nutrient-dense, moderation and preparation techniques are important, especially for susceptible individuals.
Very High-Oxalate Greens (>100 mg per serving)
- Spinach: This is arguably the most notorious high-oxalate green. Raw spinach can contain exceptionally high amounts, with studies showing a range of 329.6–2350 mg per 100g fresh weight depending on growing conditions and season.
- Swiss Chard: A nutritional powerhouse, Swiss chard is also very high in oxalates, with reported values from 874 to 1,458 mg per 100g fresh weight.
- Rhubarb: Though often used in desserts, rhubarb is a vegetable with very high oxalate content, concentrated primarily in its leaves (which should never be consumed) but also significant in its stalks.
- Beet Greens: These leafy tops of beets are high in oxalates, with one study documenting high levels in various cultivars.
- Purslane: This succulent green, popular in some cuisines, contains high levels of oxalates.
Moderate-Oxalate Greens (10–100 mg per serving)
Some greens have moderate levels of oxalates, and their consumption is often acceptable for many on a low-oxalate diet, especially when prepared properly.
- Kale: While sometimes mistakenly grouped with high-oxalate greens, kale's oxalate content is significantly lower than spinach or chard. Some estimates place it as low as 2mg per cup, though levels can vary.
- Collard Greens: These greens contain moderate levels of oxalates and can be a good alternative to higher-oxalate varieties.
- Mustard Greens: Providing a spicy kick, these greens also fall into the moderate oxalate category.
- Dandelion Greens: Often foraged, these bitter leaves contain appreciable amounts of oxalates.
Impact on Your Health
For those sensitive to oxalates, excessive intake can lead to several health issues, primarily linked to the formation of calcium oxalate crystals. The main concerns include:
- Kidney Stone Formation: The most common type of kidney stone is made of calcium oxalate. High oxalate levels in urine, known as hyperoxaluria, increase the risk of stones forming.
- Reduced Mineral Absorption: Oxalates bind to minerals like calcium and magnesium in the digestive tract, potentially hindering their absorption.
- Systemic Issues: In rare, severe cases of oxalosis, calcium oxalate crystals can deposit in various organs and tissues, causing broader health problems.
How to Reduce Oxalates Through Preparation
Fortunately, simple cooking methods can significantly reduce the soluble oxalate content of leafy greens. The key is to discard the cooking water, which leaches the oxalates away from the food.
Best Practices for Preparation
- Boiling: This is the most effective method for reducing soluble oxalates. Studies show that boiling can reduce oxalate levels by 30% to over 60%, depending on the vegetable.
- Steaming: While less effective than boiling, steaming also helps lower oxalate content, especially if the greens are not cooked in the same container as other foods.
- Pair with Calcium: Consuming high-oxalate greens alongside calcium-rich foods (like dairy products or calcium-set tofu) allows the calcium to bind with the oxalates in the gut, preventing their absorption into the bloodstream.
Comparison of High vs. Low Oxalate Greens
This table provides a quick overview of the oxalate content in various greens, helping you make informed choices. Please note that oxalate content can vary based on growing conditions.
| Leafy Green | Typical Oxalate Content (per 100g raw) | Primary Recommendation |
|---|---|---|
| Spinach | Very High (970 mg+) | Consume in moderation; boil and discard water |
| Swiss Chard | Very High (874-1458 mg) | Consume in moderation; boil and discard water |
| Beet Greens | Very High (High levels) | Consume in moderation; boil and discard water |
| Rhubarb (Stalks) | Very High (800 mg) | Consume sparingly; boil and discard water |
| Kale | Low (approx. 2 mg/cup) | Excellent for regular consumption |
| Bok Choy | Low (approx. 1 mg/cup) | Excellent for regular consumption |
| Arugula | Low-Moderate | Excellent for regular consumption |
| Collard Greens | Moderate (approx. 10-40 mg) | Good for regular consumption; cook to reduce further |
| Lettuce (Romaine) | Low | Excellent for regular consumption |
Conclusion
While the nutritional benefits of leafy greens are undeniable, understanding their oxalate content is a vital part of a balanced diet, especially for individuals at risk of kidney stones. Greens like spinach, Swiss chard, and beet greens are very high in oxalates and should be consumed in moderation, or prepared using methods like boiling that significantly reduce their oxalate load. For most people, a varied diet with both high and low-oxalate greens is perfectly healthy. However, for those with a history of kidney stones or other related conditions, choosing lower-oxalate options like kale, bok choy, and romaine lettuce is a safer strategy, along with staying well-hydrated and pairing high-oxalate foods with calcium. For personalized advice, it's always best to consult with a healthcare provider or a registered dietitian. For further reading, an extensive review on oxalates in foods can be found in this MDPI review on oxalate in foods.
