Green smoothies
Found this interesting - I don't have a proper source, but it sounds reasonable:
Some greens are high in oxalic acid which binds with calcium to form calcium oxalate, an insoluble salt. Add these to a Green Smoothie for variety, but do NOT make an entire Smoothie out of them. They are lambsquarters, beet leaves, purslane, spinach, Swiss chard, parsley, amaranth leaves and sorrel. Leave rhubarb leaves alone, they can poison you when eaten in large quantities.
Sweet baby-leaf spinach is fine to eat in quantity. It tastes like the acid has been bred out of it.
via Green Smoothie Recipes + In-depth Info on Green Smoothies.
I'm now staying in Jakarta, Indonesia. I'm new to making green smoothies and haven't figured out how to make them successfully yet. Vegetables here are often different (e.g. amaranth greens are available, but for spinach you'd have to go to a rich-people's supermarket). Even when the vegetables are familiar, the taste can be different - e.g. the lettuce (the light green floppy kind) tends to be bitter, with plenty of white sap. Will try again tomorrow without the lettuce, and with different fruit (pineapple).