Why much of the calcium in many leafy green vegetables notably spinach is not absorbed because of the presence of oxalates

November 3, 2025 •

2 min read

In a study involving healthy adults, researchers found that the body absorbs only about 5.1% of the calcium from spinach, compared to 27.6% from milk. This stark difference highlights a common nutritional paradox, explaining why much of the calcium in many leafy green vegetables notably spinach is not absorbed because of the presence of a specific compound.

Quick Summary

This article explains how oxalates in leafy greens bind to calcium, creating an unabsorbable compound called calcium oxalate. Learn to identify high-oxalate greens and apply preparation methods that can help increase mineral absorption from other food sources.

Key Points

Insoluble Calcium: The calcium in high-oxalate greens like spinach is largely not absorbed because it binds with oxalic acid, forming insoluble calcium oxalate.
Low Bioavailability: Studies show calcium absorption from spinach is very low compared to sources like dairy, despite its high calcium content.
Boiling Reduces Oxalates: Cooking methods like boiling can reduce the oxalate content of leafy greens, making more of their minerals accessible.
Choose Low-Oxalate Greens: Opt for greens such as kale, bok choy, and collard greens, which have lower oxalate levels and higher calcium bioavailability.
Eat a Balanced Diet: For most healthy people, eating a variety of foods prevents oxalate buildup from becoming a problem and still provides numerous health benefits.
Kidney Stone Risk: Individuals prone to calcium oxalate kidney stones may need to monitor their intake of high-oxalate foods more closely.

Understanding Oxalates and Calcium Absorption

While spinach contains calcium, its effectiveness as a calcium source is limited due to oxalates, natural plant compounds. Oxalates bind to calcium and other minerals in the digestive system, forming crystals the body can't absorb, reducing the bioavailability of calcium. Although generally not harmful in moderation for healthy individuals, understanding oxalates is key for optimizing mineral intake.

The Chemistry Behind Calcium Oxalate

Oxalic acid in foods like spinach binds with calcium during digestion to create insoluble calcium oxalate. Unlike soluble calcium found in dairy, calcium oxalate isn't absorbed and is excreted. This is why much of the calcium in spinach isn't utilized by the body. The amount of oxalate varies significantly between different leafy greens, impacting their calcium bioavailability.

Comparing High-Oxalate and Low-Oxalate Greens

The impact of oxalates on calcium absorption is evident when comparing different greens. Spinach and rhubarb are high in oxalates, while others offer more absorbable calcium. A comparison can illustrate the difference:


Feature	High-Oxalate Greens (e.g., Spinach)	Low-Oxalate Greens (e.g., Kale)
Oxalate Content	Very high	Low
Calcium Content	High	Moderate to High
Calcium Absorption	Very low (approx. 5%)	High (comparable to dairy)
Best for Mineral Source	Focus on other nutrients (e.g., vitamins, fiber)	Excellent source of absorbable calcium
Health Consideration	May contribute to kidney stones in sensitive individuals	Generally safe for those prone to kidney stones

Strategies to Maximize Calcium Intake from Leafy Greens

Certain methods can help reduce oxalate impact and increase overall calcium intake from high-oxalate vegetables. Boiling can significantly lower oxalate content if the cooking water is drained. Pairing high-oxalate foods with a good calcium source can help oxalates bind to the external calcium. Diversifying with low-oxalate options like kale, bok choy, and collard greens offers more absorbable calcium. Staying hydrated can also help.

The Bigger Picture: A Balanced Approach to Nutrition

High-oxalate vegetables like spinach are nutritious, rich in vitamins, antioxidants, and fiber. For most without a history of kidney stones, oxalates in typical amounts aren't a major concern. A low-oxalate diet is usually only advised for those with a history of calcium oxalate kidney stones. Spinach is healthy as part of a varied diet but not a primary source of bioavailable calcium. Dairy, fortified foods, and low-oxalate greens are better reliable sources.

For more detailed information on dietary oxalates and kidney health, you can consult the {Link: National Kidney Foundation https://www.kidney.org/kidney-topics/calcium-oxalate-stones}.

Conclusion

Much of the calcium in leafy greens like spinach is poorly absorbed due to oxalic acid, which forms insoluble compounds. This makes its calcium less available than that from low-oxalate greens. However, this doesn't negate spinach's overall nutritional benefits. A balanced diet with various greens is recommended, and methods like boiling can help. Reliable calcium needs should be met through other sources.

Frequently Asked Questions

Spinach, rhubarb, and beet greens are among the leafy green vegetables with the highest concentrations of oxalates.

Cooking methods like boiling can significantly reduce the soluble oxalate content of vegetables, with boiling being more effective than steaming. It's important to drain and discard the cooking water.

Yes, oxalates can bind with calcium in the kidneys to form calcium oxalate crystals, which are the most common type of kidney stone. This is primarily a concern for individuals with a history of kidney stones or metabolic issues.

Good sources of highly absorbable calcium include low-oxalate greens like kale, bok choy, and turnip greens, as well as dairy products, fortified plant-based milks, and fortified tofu.

Absolutely. High-oxalate greens like spinach are still very nutritious, offering a wealth of vitamins, minerals, and antioxidants. The key is to not rely on them as your primary source of calcium.

Consuming a high-calcium food with a high-oxalate one causes the oxalates to bind to the calcium in the gut, helping to excrete them and ensuring that calcium from the separate food source is more readily available for absorption.

Vitamin D is essential for the body to absorb calcium from the intestines. Without adequate vitamin D, calcium absorption is compromised, regardless of the dietary source.