The short version
Oxalate is a plant compound that grabs onto calcium, iron and other minerals in the gut, and it is the specific reason spinach and a handful of other plant foods get singled out as a health worry.
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Oxalic acid, or "oxalate," is a naturally occurring plant chemical which, in the human diet, can react with calcium, iron, and other minerals to inhibit mineral absorption. Foods such as tea, beets, rhubarb, strawberries, spinach, and chocolate are known to increase oxalate in the urine and may contribute to kidney stone formation.
USDA's own summary does not stop at the warning. It adds the other half of the picture in the very next line.
Primary source 02 / 10
Spinach in particular contains higher concentrations of oxalate than most crops, but it is also considered healthful because it is rich in a number of key nutrients.
What it binds, and how much actually gets absorbed
Spinach is the extreme case among common foods: a 2023 review put its published oxalate content across a huge range depending on variety and growing conditions.
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published oxalate values are 329.6–2350 mg total oxalates/100 g fresh weight (FW) for spinach
That large a number in the food is not the same as that much reaching the bloodstream. Most oxalate passes straight through.
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soluble oxalates are absorbed through the intestines and colon (5–10% of ingested oxalate, under normal conditions), going into the bloodstream.
Eating oxalate alongside calcium or magnesium cuts that further, by the same binding mechanism that makes oxalate a mineral thief in the first place.
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a simultaneous consumption of oxalate with calcium or magnesium can reduce its bioavailability and absorption due to the formation and fecal excretion of insoluble salts, lowering the health risk
Absorption also is not fixed across people, and this is where the kidney-stone risk actually concentrates: in a subset of absorbers, not the general population.
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intestinal absorption of oxalates in individuals with a history of stone formation was expressly higher than in healthy individuals (9.2% and 6.7%, respectively)
Preparation changes the numbers
The same review credits ordinary cooking with cutting a large share of the load before it ever reaches the gut.
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boiling has been associated with decreased oxalate content, especially soluble oxalates, due to its leaching and thermal degradation
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boiling, steaming, soaking, and processing with calcium sources are some procedures to reduce the content of soluble oxalates, the most harmful oxalates
The kidney-stone question, and why cutting calcium backfires
For anyone who has actually had a calcium oxalate stone, NIDDK's own patient guidance runs against the intuitive fix.
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Even though calcium sounds like it would be the cause of calcium stones, it’s not. In the right amounts, calcium can block other substances in the digestive tract that may cause stones.
Its practical advice leans on swapping meat protein for legumes rather than avoiding oxalate-containing plants outright.
Primary source 10 / 10
Consider replacing some of the meat and animal protein you would typically eat with beans, dried peas, and lentils, which are plant-based foods that are high in protein and low in oxalate.
None of this is medical advice, and NIDDK repeats that point in its own guidance: anyone with a history of stones should work out sodium, calcium, protein and oxalate targets with a health care professional or dietitian, since the right diet differs by stone type. For most people who have never had a stone, oxalate is one input into a mineral supply chain that a varied diet, some cooking, and ordinary calcium intake already manage well.