Green leafy vegetables tend to be high in dietary oxalate - sometimes known as oxalic acid. This naturally-occurring compound is considered an "anti-nutrient" because it can stop your body from absorbing calcium properly. It may also increase how much oxalate is in your urine. This may lead to the development of some types of kidney stones.  People who are at risk from these conditions are usually advised to limit their oxalate intake by their medical provider. This means that green smoothies may not be ideal for them.  While it is always good to confirm your dietary choices with a medical professional, the vast majority of people can safely enjoy green smoothies without worrying about oxalates. Indeed, they are so rich in vitamins and minerals that it is likely the benefits will outweigh any risks!  That being said, there are ways in which you can minimize your risks from oxalates whilst still enjoying green smoothies. Try to use a variety of greens in rotation, rather than always sticking to the same one. This will give you access to a broader spectrum of nutrients and prevent you from consuming too much of any single anti-nutrient.  Consider steaming your greens before adding them to your smoothies. This will reduce the oxalates. Pick the leafy greens that are lowest in oxalates. These include kale, bok choy, broccoli, Romaine lettuce and zucchini.  Drink plenty of water - especially if you have kidney stones - and ensure that your diet contains lots of calcium to compensate for any reduction in absorption.