In general, oxalic acid is found in all kinds of foods, it is naturally formed. The oxalic acid concentration in most foods is very low. The oxalic acid content of spinach and chard, is slightly higher compared to other leafy vegetables. Oxalic acid can cause the formation of kidney stones and kidney damage, therefore it is important not to consume too much oxalic acid. It is true that you would have to eat a kilo of spinach a day to exceed the specified limits. Nevertheless, we don't recommend using leafy greens containing oxalic acid several days in a row. Instead, we recommend using all the variety nature has to offer for your green smoothie. For example, you can replace spinach or chard with lamb's lettuce, baby lamb's lettuce, or even radish or kohlrabi leaves. Another tip: Oxalic acid can mainly be found in the large, fully grown leaves. So if you use spinach or chard, use the small, young leaves. More information about oxalic acid in green smoothies can be found here.