Onion — Low or High Oxalate?

Based on Harvard's 2024 lab measurements, Onion falls in the **Low Oxalate** range (under 25 mg/serving). That's the safest category for kidney stone patients — you can eat this food regularly without oxalate concerns. At 4 mg per serving, Onion is a dependable low-oxalate choice.

✓ Medically Reviewed Based on Harvard T.H. Chan SPH oxalate data (2024) and NKF/AUA dietary guidelines.

4
mg oxalate
per serving

Food: Onion, Variety, boiled or sautéed
Serving: 1/2 cup (105 g)
Verdict: 🟢 Safe
Per 100g: 4 mg
Data: Harvard T.H. Chan SPH (2024)

💡 What This Means for Your Diet

What this means: Onion is rated Safe (🟢). At 4 mg oxalate per serving, it's well under the 25 mg safety threshold. You can include it regularly in a kidney stone diet. Pair it with calcium-rich foods (dairy, fortified plant milks) for additional protection, and drink plenty of water throughout the day.

📊 How Onion Compares

Among vegetables, Onion sits well below the category average of 27 mg. For context, 219 out of 271 vegetables (80%) are rated 🟢 Safe (under 25 mg oxalate/serving). The average vegetables in our database has 27 mg oxalate per serving — which means Onion is 86% below the category norm. Within its specific category, Onion ranks among the lower-oxalate options. This context matters because two 'Safe' foods can have very different oxalate levels — one might have 5 mg while another has 24 mg. Both are Safe, but the first gives you more dietary flexibility. Browse all vegetables rated Safe to compare within the same verdict level.

❓ Related Questions

What's the difference between 'Low' and 'Safe'?

Low Oxalate means under 25 mg per serving — the same threshold as Safe. They're equivalent in practice. The verdict system uses Low/Moderate/High to describe oxalate categories and Safe/Caution/Avoid to recommend dietary action.

Can oxalate levels change with cooking?

Yes. Boiling and discarding water can reduce soluble oxalate by 30-50% in many vegetables. Check the food's main page or cooking-specific angle page for details on preparation effects.

Is 'Low Oxalate' the same as 'No Oxalate'?

No — almost all plant foods contain some oxalate. 'Low Oxalate' means under 25 mg per serving, which is considered safe for kidney stone patients. Only purified products like sugar, oil, and salt have near-zero oxalate.

This is dietary reference information, NOT medical advice. Talk to your doctor or a registered dietitian before changing your diet. Oxalate tolerance varies by person, stone type, and medication.