Tofu and Soy on a Kidney Stone Plan: The USDA Data Most People Haven't Seen

2026-08-24 · By Sarah Chen, Data Editor · Dietary reference, not medical advice

Soy is the protein backbone of plant-based eating, and for stone formers the question has always been whether tofu belongs on the plate. The most useful answer comes from a lab study most diet lists never cite: a USDA-supported analysis that measured oxalate across 30 commercial soy foods and found the tofu family largely in the clear.

The finding, plainly: A 2005 study from Washington State University and the USDA (Al-Wahsh et al., Journal of Agricultural and Food Chemistry) measured oxalate in 30 commercial soy foods. Eighteen of 19 tofu brands and two soymilk brands contained less than 10 mg of oxalate per serving — the standard cutoff for a low-oxalate food. Soy flour, textured vegetable protein, edamame, soy nuts, tempeh, and soy nut butter exceeded 10 mg per serving, with total soy-food oxalate ranging from 2 to 58 mg per serving across the category. The same study found soy foods also carry phytate, which is itself an inhibitor of calcium stone formation.

Why Tofu Is Different From Soy Flour

The processing matters. Tofu is made by coagulating soymilk into curds and pressing out the liquid — and a large share of the oxalate leaves with that liquid. The 2005 data show most tofu brands end up below the 10 mg low-oxalate line, which makes firm and silken tofu reasonable protein options for stone formers. The concentrated soy products are the ones to watch: soy flour and textured vegetable protein keep the oxalate that processing removed from tofu, and soy nuts and tempeh land above the low-oxalate cutoff.

The second useful finding is the phytate connection: soy foods with more oxalate also tend to carry more phytate, and phytate binds minerals and inhibits calcium crystallization. That does not cancel the oxalate — but it is a reminder that soy foods are not a one-note oxalate source, and that the category spans a wide range.

The boundary, stated clearly: these are measured values from one USDA-supported lab study of specific brands; values vary by product, coagulant, and brand. The practical takeaway is ordering within the soy category — tofu and soymilk low, concentrated soy products higher — not a blanket ban or a blanket pass.

The Protein Shelf, Measured

For context, here is where the low-oxalate protein sources sit:

Food	Oxalate (mg)	Serving	Rating
Chicken breast, roasted	0 mg	4 oz	Safe
Eggs, large	0 mg	1 egg	Safe
Salmon, cooked	0 mg	4 oz	Safe
Yogurt, plain whole-milk	5 mg	6 oz	Safe
Cheese, hard	1 mg	1 oz	Safe

The oxalate-free protein anchors — chicken, eggs, salmon — and the dairy staples make the low-oxalate base of any plate. Tofu belongs near this end for most brands (under 10 mg per serving in the USDA data), which means plant-based eaters are not forced off soy entirely; they are pointed toward tofu and soymilk and away from soy flour, tempeh, and soy nuts.

What to Do With This

Three habits cover most of the upside. If you eat plant-based protein, keep tofu and soymilk in the rotation — the measured data put them at the low-oxalate end. Treat tempeh, soy nuts, edamame, and soy flour as portioned or occasional, since they sit above the low-oxalate cutoff. And pair soy meals with a calcium source, because calcium binding oxalate in the gut is the same lever that makes any protein choice safer.

Frequently Asked Questions

Is tofu high in oxalate?

In the USDA-supported 2005 study, 18 of 19 tofu brands measured under 10 mg of oxalate per serving — below the low-oxalate cutoff. Tofu is generally a reasonable protein for stone formers, unlike soy flour and soy nuts, which run higher.

What about soymilk and tempeh?

Soymilk also measured under 10 mg per serving in the same study. Tempeh and soy nuts exceeded 10 mg per serving, so they are the soy foods to portion rather than use daily.

Sources:
Harvard T.H. Chan School of Public Health Oxalate Database (2024); USDA FoodData Central; Al-Wahsh IA, et al. Oxalate and phytate of soy foods. J Agric Food Chem. 2005 (USDA/WSU; 30 soy foods measured); Harvard oxalate database values for rating thresholds. All oxalate values are lab-measured via ion chromatography. This page is a dietary reference tool, not medical advice.

See how oxalate values are measured and verified on our data methodology page.

Medical Disclaimer: This site is a dietary reference tool based on published food composition data. It does not provide medical advice, diagnose, or treat any condition. Talk to your doctor or a registered dietitian before changing your diet, especially if you have kidney stones, kidney disease, or other medical conditions.