Boiling is the most effective way to remove oxalates from spinach, reducing soluble oxalate by up to 87%. That drops the soluble oxalate content from roughly 803 mg per 100 grams of raw spinach down to about 107 mg. The key is that oxalates leach into the cooking water, so you need to discard the liquid afterward. Other methods like steaming and soaking also help, but less dramatically.
Why Spinach Is Especially High in Oxalates
Raw spinach contains about 1,145 mg of total oxalate per 100 grams. Of that, roughly 70% (803 mg) is soluble oxalate, the form your body can actually absorb and the type most likely to contribute to kidney stones. The remaining 30% is insoluble oxalate, which is already bound to minerals like calcium and magnesium and largely passes through your digestive tract without being absorbed.
This matters because the soluble fraction is what you’re really trying to reduce. Fortunately, it’s also the form that dissolves in water, which is why water-based cooking methods work so well.
Boiling: The Most Effective Method
Boiling spinach for about 12 minutes reduces soluble oxalate by 30 to 87%, with spinach specifically showing the highest losses among all vegetables tested. In one well-cited study published in the Journal of Agricultural and Food Chemistry, boiling cut spinach’s soluble oxalate from 803 mg to just 107 mg per 100 grams. That’s an 87% reduction.
The oxalates don’t disappear. They dissolve into the cooking water. Analysis of the cooking water confirmed it contained solely soluble oxalate, meaning the process is a straightforward transfer from spinach to liquid. This is why the single most important step is throwing out the cooking water. If you reuse it in a sauce or soup, you’re drinking the oxalates you just removed.
For the best results, use a generous amount of water relative to the amount of spinach. More water gives the oxalates more room to leach out. Bring the water to a rolling boil, add the spinach, cook for 10 to 12 minutes, then drain and squeeze out excess liquid.
Steaming Works, but Much Less
Steaming reduces soluble oxalate by only 5 to 53%, depending on the vegetable. For most vegetables, the reduction was just 5 to 19%. Spinach performed better than average with steaming, but the results still fall well short of boiling. The reason is simple: steaming involves far less direct contact between the spinach and water, so fewer oxalates have a chance to leach out.
Steaming does have one significant advantage, though. It preserves folate almost completely. Boiling spinach for a typical cooking period retains only about 49% of its folate content (dropping from roughly 192 to 94 micrograms per 100 grams). Steaming for up to 4.5 minutes caused no significant folate loss at all. If you’re eating spinach partly for its B vitamins, steaming gives you a moderate oxalate reduction without sacrificing that nutrition.
Soaking Before Cooking
Soaking spinach in water at room temperature before cooking can start the leaching process early. Since soluble oxalate dissolves in water, even cold water will pull some of it out over time. The amount removed depends on how long you soak, the water-to-spinach ratio, and whether you cut or chop the leaves first (smaller pieces expose more surface area).
Soaking alone won’t match boiling’s effectiveness, but combining the two gives you the best outcome. Soak chopped spinach for 15 to 30 minutes, discard the soaking water, then boil in fresh water and discard that too. You’re essentially getting two rounds of leaching.
Pair Spinach With Calcium-Rich Foods
Even after cooking, some soluble oxalate remains. You can further reduce how much your body absorbs by eating spinach alongside calcium-rich foods. When calcium meets soluble oxalate in your gut, the two bind together and form an insoluble compound that passes through without being absorbed into your bloodstream.
In a study with 10 volunteers, eating 100 grams of grilled spinach with sour cream and calcium-fortified milk significantly reduced the amount of oxalate available for absorption over both 6-hour and 24-hour periods. Cottage cheese, yogurt, or a glass of milk with your spinach all serve the same purpose. The calcium needs to be present in the same meal, not hours later, to bind the oxalates before they reach your intestines.
How Much Oxalate Is Too Much
For people managing calcium oxalate kidney stones, the University of Chicago Kidney Stone Program recommends keeping dietary oxalate below 100 mg per day, with 50 mg being ideal. To put that in perspective, a 100-gram serving of raw spinach contains over 1,100 mg of total oxalate. Even a small handful of raw spinach in a smoothie can blow past that daily budget.
Boiling changes the math considerably. If boiling drops the soluble oxalate to around 107 mg per 100 grams (with the insoluble portion remaining around 343 mg), a smaller cooked serving paired with calcium-rich food becomes much more manageable. You’re not eliminating all oxalate, but you’re reducing the absorbable amount enough that moderate portions can fit within a low-oxalate diet.
If your diet calcium intake is at least 800 to 1,000 mg per day, there’s more flexibility. Higher calcium intake means more opportunity for oxalate binding in the gut. In that context, keeping oxalate below 150 mg daily is generally considered reasonable for stone prevention.
What You Lose by Boiling
Boiling is the clear winner for oxalate removal, but it comes with nutritional trade-offs. Beyond the roughly 50% folate loss, boiling also reduces vitamin C and other water-soluble nutrients that leach out alongside the oxalates. Minerals like potassium follow the same path into the cooking water.
The practical solution is to compensate elsewhere in your diet. If you’re boiling spinach specifically to reduce oxalates, get your folate and vitamin C from foods you eat raw, like oranges, strawberries, or bell peppers. You don’t need spinach to carry the entire nutritional load of your diet. You just need to enjoy it without the oxalate burden.
Quick Summary of Methods by Effectiveness
- Boiling 10 to 12 minutes: Reduces soluble oxalate by up to 87%. Discard the water.
- Steaming up to 4.5 minutes: Reduces soluble oxalate by roughly 5 to 53%. Preserves folate.
- Soaking in water: Provides partial leaching. Best used as a pre-treatment before boiling.
- Pairing with dairy or calcium-rich foods: Reduces absorption of whatever soluble oxalate remains after cooking.
- Baking: No measurable oxalate loss. Not effective for this purpose.
For maximum reduction, combine strategies: soak, boil in fresh water, drain thoroughly, and serve with a calcium-rich side.