I get quite a few emails from people, expressing concern about the oxalate content of greens in green smoothies, in particular the action of oxalates in blocking calcium uptake in the body.
Oxalic acid/oxalates is certainly present in high amounts in greens – in fact it is present to some degree in almost all plant foods. Plants such as spinach, chard, silverbeet, beet greens, parsley, purslane, chives, cassava, amaranth, and rhubarb have significant oxalic acid content. Does this mean that eating greens is bad for your bone health?
Not at all.
While the oxalic acid present in plant foods will in fact inhibit absorption of some calcium, it does not block ALL of the calcium present in that food. Oxalates do bind to calcium and prevent the absorption, but this amount is very small, and the other abundant nutritional benefits of eating your leafy greens considerably outweigh the small decrease in calcium.
Basically, if you are drinking daily green smoothies then you are almost certainly getting far more than the RDA of calcium, and even with the oxalates causing a drop in levels, you will still come out on top! It is incredibly easy to meet the recommended daily amount of calcium (450mg) through your diet, and if you eat plenty of fresh fruit, vegetables, nuts, and seeds then you should be getting close to double that amount, since people who eat wholefoods diets tend to get closer to 700-900mg daily calcium from their food.
Anyone who is truly concerned about their calcium intake should consider adding sesame seeds and tahini (sesame paste) to their diet, since 
sesame seeds have an incredibly high calcium content – nearly ten times that found in full fat milk! These tiny seeds also contain a host of other crucial minerals and nutrients for bone health, plus the calcium in sesame is readily bioavailable.
Dietary choices that inhibit calcium absorption even more than the oxalates in leafy greens include meat, salt, alcohol, and processed foods – these should be excluded from your plate long before leafy greens get the boot, if you want to ensure that you are getting the full spectrum of vitamins and minerals.
In The Tea Garden 