Oxalate after gastric bypass or with Crohn's
By Oliver Silen · Updated November 2, 2026 · Not medical advice
Gastric bypass, Crohn's disease, short bowel and other conditions that leave fat unabsorbed can make the gut take up much more oxalate. Doctors call it enteric hyperoxaluria, and it's a group where clinics often use the strict 50 mg target.
Why it happens
Normally calcium in the gut binds oxalate so it leaves in stool. When fat isn't absorbed, the leftover fatty acids grab that calcium first, leaving oxalate free to be absorbed in the colon. Bile acids reaching the colon also make its lining more permeable [1]. The result is more oxalate in urine from the same food.
What usually changes
- Calcium with every meal, often as a supplement taken with food, to bind oxalate in the gut [1].
- A low-oxalate diet, closer to 50 mg a day. See 50 or 100 mg.
- Less fat, which leaves more calcium free to bind oxalate [1].
- More fluid and citrate, since diarrhea lowers urine volume and citrate. See lemon water and citrate.
These are treated together with your gastroenterologist or bariatric team, since doses depend on how much fat you absorb. A 24-hour urine test shows whether it's working.