Kale is one of the lowest-oxalate leafy greens you can eat. A 150-gram serving of kale contains roughly 0.01 grams of oxalic acid, which puts it at the very bottom of the oxalate spectrum among vegetables. For comparison, the same weight of spinach packs more than a hundred times that amount. If you have been told to limit oxalate or just want nutrient-dense greens without the antinutrient baggage, kale is about as safe a choice as you will find.
How Kale Stacks Up Against Other Leafy Greens
The gap between kale and high-oxalate greens is not a small difference. In a study that compared spinach and kale meals head to head, 150 grams of spinach delivered 1.27 grams of oxalic acid while the same weight of kale delivered just 0.01 grams.1PubMed. Oxalic acid does not influence nonhaem iron absorption in humans: a comparison of kale and spinach meals A separate trial looking at magnesium absorption used 300-gram portions and found 6.6 millimoles of oxalate in the spinach serving versus 0.1 millimoles in the kale.2PubMed. Fractional magnesium absorption is significantly lower in human subjects from a meal served with an oxalate-rich vegetable, spinach, as compared with a meal served with kale, a vegetable with a low oxalate content That is roughly a 66-fold difference in the same serving size. Spinach, rhubarb, beets, and Swiss chard regularly top the charts for oxalate content. Kale lives in a completely different category.
Researchers who classify vegetables by oxalate level typically define “low oxalate” as containing less than 10 milligrams of oxalic acid per 100 grams of fresh weight.3Journal of Food Research. Oxalate Content of Egyptian Grown Fruits and Vegetables and Daily Common Herbs Some analytical work sets the bar for “very low” even tighter, at under about 2 milligrams per 100 grams.4PubMed. Estimation of the oxalate content of foods and daily oxalate intake Kale lands comfortably in either range, depending on the variety you pick. That makes it a genuinely low-oxalate food by any standard classification in the nutrition literature.
Not All Kale Varieties Are Identical
If you shop at a farmers’ market or grow your own greens, you may encounter several types of kale, and their oxalate levels are not perfectly uniform. A laboratory analysis of two common varieties found that curly kale (the ruffled kind you see in most grocery stores) contained about 0.69 milligrams of oxalate per 100 grams, while Tuscan kale, sometimes called kale nero or lacinato kale, came in at roughly 1.70 milligrams per 100 grams.5Jurnal Ilmiah Ibnu Sina (JIIS): Ilmu Farmasi dan Kesehatan. ANALISIS KADAR OKSALAT PADA DUA JENIS TANAMAN KALE (Brassica oleracea var. acephala dan Brassica oleracea var. palmifolia) DENGAN METODE SPEKTROFOTOMETRI UV
That means Tuscan kale has about two and a half times the oxalate of curly kale. Before that sounds alarming, remember that both figures are still well under the 10 mg/100g threshold for “low oxalate.” Even the higher-oxalate Tuscan variety would need to be eaten in extremely large quantities before its oxalate load came anywhere close to a single serving of spinach. So if you prefer the flatter, darker leaves of lacinato kale for soups or sautés, you are still firmly in low-oxalate territory.
What Low Oxalate Means for Calcium Absorption
This is where kale’s low-oxalate status has a real payoff that goes beyond kidney stone concerns. Oxalate binds to calcium in food, forming calcium oxalate crystals that your body cannot absorb. A food can be loaded with calcium on paper but deliver very little of it to your bloodstream if it also contains a lot of oxalate. Spinach is the classic cautionary tale: it has a high calcium content but spectacularly poor calcium bioaccessibility, more than 300 times lower than skimmed milk by one estimate.6Food Research International. A comparison of the bioaccessible calcium supplies of various plant-based products relative to bovine milk
Kale plays the opposite game. It has extremely high total calcium, measured at about 959 milligrams per 100 grams of fresh weight in one analysis, and because there is almost no oxalate present to trap it, the calcium bioaccessibility is comparable to that of skimmed milk.6Food Research International. A comparison of the bioaccessible calcium supplies of various plant-based products relative to bovine milk Earlier research described kale as exhibiting “excellent absorbability for its calcium,” in direct contrast to the poor absorption seen with spinach.7PubMed. Calcium absorption from kale A single 80-gram serving of kale can supply around 328 milligrams of bioaccessible calcium, which is roughly half the recommended daily intake for an adult and far more than an equivalent serving of milk delivers.6Food Research International. A comparison of the bioaccessible calcium supplies of various plant-based products relative to bovine milk
The magnesium story follows the same pattern. When people ate a meal made with spinach, their fractional magnesium absorption was significantly lower than when they ate the same meal made with kale, and the researchers attributed the difference to the oxalate content of spinach interfering with absorption.2PubMed. Fractional magnesium absorption is significantly lower in human subjects from a meal served with an oxalate-rich vegetable, spinach, as compared with a meal served with kale, a vegetable with a low oxalate content If you are eating greens specifically for their mineral content, the oxalate level of the green you choose matters more than its raw mineral numbers.
Kidney Stones and Dietary Oxalate
Most people who search for oxalate content in foods are either managing kidney stones or trying to prevent them. Calcium oxalate stones are the most common type, and dietary oxalate does contribute to the risk, though the relationship is more complicated than “high-oxalate food equals stones.”
Your body makes most of its own oxalate internally. Dietary oxalate adds to the total, but not in a simple, proportional way. A review of the evidence found that only eight foods consistently caused a meaningful increase in urinary oxalate excretion in study volunteers: spinach, rhubarb, beets, nuts, chocolate, tea, wheat bran, and strawberries.8PubMed. Effect of dietary oxalate and calcium on urinary oxalate and risk of formation of calcium oxalate kidney stones Kale was not on that list, and given its oxalate content is orders of magnitude below the foods that did make the cut, eating kale is unlikely to meaningfully affect your urinary oxalate levels.
Large cohort studies have shown that urinary oxalate excretion above roughly 25 milligrams per day is associated with increased stone risk, and that it behaves as a continuous variable, meaning even modest reductions can help.9PubMed Central. Dietary oxalate and kidney stone formation For someone who is actively trying to keep urinary oxalate low, replacing high-oxalate greens with kale is a straightforward win. You keep the nutritional benefits of a leafy green while dropping almost all of the oxalate load.
One point that often gets lost in oxalate-avoidance advice: calcium intake matters too. Adequate calcium in the diet, generally in the range of 800 to 1,200 milligrams per day, actually reduces intestinal oxalate absorption because calcium binds oxalate in the gut before it reaches the kidneys.10PubMed. Efficacy of dietary interventions targeting calcium and oxalate intake in the prevention of calcium oxalate stones: An integrative review Restricting calcium can backfire by freeing up more oxalate for absorption.8PubMed. Effect of dietary oxalate and calcium on urinary oxalate and risk of formation of calcium oxalate kidney stones Since kale delivers both very little oxalate and a lot of bioavailable calcium, it essentially works both sides of that equation simultaneously.
Does Cooking Change Kale’s Oxalate Level?
For high-oxalate vegetables, cooking method matters a lot. Boiling can strip out 30 to 87 percent of soluble oxalate, depending on the vegetable, because the oxalate leaches into the cooking water. Steaming is less effective, removing only about 5 to 53 percent.11PubMed. Effect of different cooking methods on vegetable oxalate content That research primarily looked at vegetables like spinach, beets, and carrots where oxalate reduction through cooking is a meaningful strategy.
For kale, the practical takeaway is simpler: it does not matter much how you cook it, because there is barely any oxalate to remove in the first place. Whether you eat kale raw in a salad, sautéed, steamed, or boiled into a soup, the amount of oxalate reaching your gut remains negligible. This is a genuine advantage over vegetables like spinach, where you need to think about preparation method if oxalate is a concern. With kale, you can cook it however you like without worrying that your chosen method left too much oxalate behind.
That said, if you are preparing a mixed dish that includes higher-oxalate ingredients alongside kale, boiling and discarding the cooking water remains your best bet for reducing total oxalate from the other ingredients. The oxalate lost during boiling migrates almost entirely into the water, so as long as you are not drinking the broth, it is gone.11PubMed. Effect of different cooking methods on vegetable oxalate content
The Role of Gut Bacteria in Oxalate Handling
Your intestines are not entirely passive bystanders when it comes to oxalate. A specialized gut bacterium called Oxalobacter formigenes uses oxalate as its sole energy source, breaking it down in the gut before it can be absorbed into the bloodstream. In animal studies, this bacterium plays a critical role in preventing kidney damage from oxalate-rich diets.12PubMed Central. Forty Years of Oxalobacter formigenes, a Gutsy Oxalate-Degrading Specialist Early human research showed that a single oral dose of O. formigenes reduced urinary oxalate excretion after an oxalate challenge and reestablished oxalate-degrading activity in the feces.13PubMed Central. Oxalobacter formigenes and its potential role in human health
The evidence that this bacterium meaningfully reduces kidney stone risk in humans is less settled, partly because O. formigenes is notoriously difficult to culture and study, and because the human studies done so far have not carefully controlled dietary oxalate intake.12PubMed Central. Forty Years of Oxalobacter formigenes, a Gutsy Oxalate-Degrading Specialist What is clear is that people vary in how much oxalate they absorb from the same food, and gut microbiome composition is one reason. Antibiotic use can wipe out O. formigenes, potentially leaving you more vulnerable to oxalate absorption for a period afterward. None of this matters much for kale specifically, since the oxalate dose from kale is so small that even a gut completely devoid of oxalate-degrading bacteria would have little to process. But it is relevant context if you are thinking broadly about oxalate management across your entire diet.
Other Vegetables in the Same Low-Oxalate Category
If you want variety alongside kale, several other vegetables fall into the low-oxalate bracket of less than 10 milligrams per 100 grams. Cabbage, courgette (zucchini), cucumbers, garlic, spring onions, and turnips all qualify.3Journal of Food Research. Oxalate Content of Egyptian Grown Fruits and Vegetables and Daily Common Herbs Broccoli and cabbage are worth particular mention because, like kale, they are brassicas with decent calcium content and good calcium bioaccessibility. Broccoli’s calcium bioaccessibility actually surpassed that of skimmed milk in laboratory testing, though its total calcium is much lower than kale’s, so it takes several servings of broccoli to match one serving of kale for total bioaccessible calcium delivered.6Food Research International. A comparison of the bioaccessible calcium supplies of various plant-based products relative to bovine milk
The vegetables that stone-formers are typically told to restrict or watch carefully are the same eight that show up in the urinary oxalate research: spinach, rhubarb, beets, nuts, chocolate, tea, wheat bran, and strawberries.8PubMed. Effect of dietary oxalate and calcium on urinary oxalate and risk of formation of calcium oxalate kidney stones If you are building a low-oxalate meal plan and wondering what leafy greens to rotate with kale, bok choy, lettuce, and watercress are also generally low, though published oxalate values vary by growing conditions and measurement methods.
Growing Conditions and Oxalate Variation
One reason oxalate tables sometimes disagree with each other is that the same vegetable can contain different amounts of oxalate depending on how and where it was grown. Nitrogen fertilization is one known factor. Research on apple trees demonstrated that higher nitrogen availability promotes oxalate production in leaves by switching on genes involved in oxalate synthesis while switching off genes responsible for breaking it down.14PubMed Central. Nitrogen reduces calcium availability by promoting oxalate biosynthesis in apple leaves Although that study looked at apple leaves rather than kale, the underlying biochemistry is shared broadly across plants. Soil composition, water stress, light exposure, and harvest timing can all shift oxalate content in any given crop.
For kale, the practical implication is that numbers in oxalate databases should be treated as approximate guides, not exact values. The good news is that kale’s baseline oxalate is so low that even meaningful percentage swings from growing conditions are unlikely to push it into the medium- or high-oxalate range. A doubling or tripling of a number that starts below 2 milligrams per 100 grams still leaves you well under the threshold where oxalate becomes a nutritional concern.
Kale, Thyroid Health, and Goitrogens
Oxalate is not the only antinutrient people wonder about in kale. Because kale belongs to the brassica family, it contains glucosinolates, some of which break down into compounds that can interfere with thyroid function. This comes up frequently enough that it is worth addressing directly.
Animal studies have shown that diets heavy in kale can affect thyroid hormone levels. Cattle fed kale showed nearly a fourfold decrease in triiodothyronine, and lambs on a low-iodine kale diet experienced thyroid enlargement and a dramatic drop in thyroxine.15PubMed Central. Do Brassica Vegetables Affect Thyroid Function?—A Comprehensive Systematic Review In those same studies, adding iodine supplementation neutralized the effect, which tells you iodine status is the key variable. Animals on an iodine-supplemented diet fed kale did not develop thyroid problems.
For humans eating normal amounts of kale, the risk appears minimal. An analysis of goitrogen levels across brassica vegetables found that kale belonging to the common species Brassica oleracea contains less than 10 micromoles of goitrin per 100-gram serving, a level considered too low to meaningfully decrease thyroid iodine uptake.16PubMed Central. Concentrations of thiocyanate and goitrin in human plasma, their precursor concentrations in brassica vegetables, and associated potential risk for hypothyroidism The threshold at which goitrin inhibits radioiodine uptake to the thyroid was estimated at about 194 micromoles, nearly 20 times what a serving of standard kale provides.16PubMed Central. Concentrations of thiocyanate and goitrin in human plasma, their precursor concentrations in brassica vegetables, and associated potential risk for hypothyroidism Collards, Brussels sprouts, and some varieties of Russian kale (a different species, Brassica napus) contain more goitrin and may warrant attention for people with existing thyroid conditions or very low iodine intake. Standard grocery-store kale does not.
The broader point is that kale has a remarkably clean antinutrient profile for a leafy green. Its oxalate is negligible, its goitrin content is well below the level thought to affect thyroid function in humans with adequate iodine intake, and the minerals it provides are unusually bioavailable. For people who have been told to avoid greens because of oxalate concerns, kale is one of the best substitutions available, and one that pulls its weight nutritionally rather than serving as a bland compromise.