Siyu-yu man,
Joseph : aren't amaranth & lambs quarters high in oxalic acid? i was juicing amaranth leaves & stems like crazy last month (we're growing it as a psuedograin crop this year) but stopped because i didn't want to overburden the kidneys.
If used raw, yes, they do have oxalic acid, so do most greens like spinach, silverbeet, beets, and other leafy greens, etc.
Simply blanching in or steaming over boiling water, and pouring off the water takes away most of the oxalic acid.
If just blanching, dip the drained, blanched leaves straight into very cold water to stop the cooking process and put in the fridge until completely cold if using as salad.
For cooked greens, steaming or boiling and pouring away the water is enough. Heat destroys the oxalates.
You are right about too much damaging kidneys as Oxalic acid binds with calcium and iron, making these minerals unavailable for absorption in the body
However, heat can break down oxalic acid which will allow the body to absorb higher amounts of vitamins, calcium, iron and fibre.
I find blanching or steaming best as many phytonutrients can get destroyed by a full cooking process.