“In humans, oxalic acid is produced as a waste product and excreted in the urine. It occurs as the result of the breakdown of the amino acids glycine and hydroxyproline and of a molecule called glyoxalate in a process termed the glyoxalate pathway. In a small number of individuals with a rare genetic disorder known as primary hyperoxaluria, or PH, mutations in this pathway lead to much larger endogenous formation and excretion of oxalates (100mg to 600mg per day). People with primary hyperoxaluria suffer from the formation of frequent and severe calcium oxalate kidney stones, often leading to permanent kidney damage or renal failure as well as oxalate deposition in bones, joints, bone marrow, and other tissues outside of the kidneys. This is a condition known as systemic oxalosis.

“Plants also use oxalic acid as a defense weapon against predators.

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Oxalic acid crystalizes into multiple shapes in both plants and humans. One of these is the raphide form depicted in the graphic below that shows the foods with the highest concentrations of oxalates. Raphides are basically microscopic needles, not exactly the type of thing insects, animals, or humans want to be biting into.”

Excerpt From The Carnivore Code: Unlocking the Secrets to Optimal Health by Returning to Our Ancestral Diet Paul Saladino, MD

There's the book Toxic Superfoods, great info. You can also listen to some interviews with the author, she's great.

There's a condition called oxalate dumping, which can hit people who go full carnivore (which has zero oxalates). Your body is getting rid of the toxin - oxalates - which is causing damage in many places. But the process itself can cause issues.