Perhaps a doctor or food scientist can confirm, but I have read somewhere that you can apparently mitigate the effects of Oxalic acid (that gives Sorrel its characteristic zingy taste) by consuming food that contains calcium at the same time. The idea being that Oxalic acid in high enough (though non-fatal) doses can lead to kidney stones as it binds with calcium, forming a non-soluble salt which precipitates out in the urine. So, by consuming calcium rich foods you encourage that binding process before it's been absorbed through the gut.