Green smoothies are a common way for people to sneak in their greens every day. As you know, we recommend you always chew your food, because chewing is the first step in good digestion. But there is another risk to green smoothies that most people don’t realize – the high level of oxalates found in spinach, beet greens and swiss chard.

Oxalates are natural substances occurring in many foods. Usually they bind to calcium and leave the body with the trash in the large intestine. However, if it doesn’t bind to calcium it has to be removed from the blood by the kidneys and sent out with urine. That is where it can cause a problem. Too many oxalates and too little liquid can lead to kidney stones.

One in ten people in the US will experience kidney stones in their lifetime (Russ and I both have). The most common type is made up of oxalates (Russ’ was. Mine were protein). But people who get stones don’t eat anymore oxalates than those who don’t. Instead, it’s a matter of how much of it you absorb.

Since the human body creates oxalates as a waste product, we do have the ability to eliminate it from our system. But it requires the alkalizing affect of fruits and vegetables. So even though some greens are high in oxalates, if you remove fruits and vegetables from your diet your risk of getting kidney stones goes up.

Additionally, the acid created by eating animal products also increases the risk of kidney stones. The amount of animal protein in a single can of tuna daily increased your risk of kidney stones by 250%. And having gastric bypass surgery increases oxalate absorption and thus, kidney stone risk.

Spinach has 656mg of oxalate per cup (as compared to kale with 2mg per cup) and accounts for more than 40% of oxalate intake in the US. Oxalates are absorbed more rapidly when food is blended rather than chewed. So, the two cups of spinach you are putting in your green smoothie on a daily basis may be putting you at risk for kidney stones.

Blanching spinach will reduce oxalates by about a third and boiling by about half. But it would be wise to not eat large doses of high oxalate foods on a daily basis. Instead, switch out lower oxalate options at least a few days a week.

How much is too much?

  • More than four cups a day of rhubarb (I have no idea how anyone eats rhubarb at all. I personally can’t stand it)
  • More than one cup a day of almonds and/or cashews
  • More than 1 ¼ cup star fruit juice or 4-6 whole star fruits a day
  • Instant tea (based I what I read, any amount every day could be a problem)
  • More than a gallon of tea a day
  • More than a teaspoon of turmeric a day (we do ½ a tsp in our oatmeal)
  • More than two cups of spinach a day (apparently seven cups of spinach a day will kill you. Although it didn’t say in what timeframe)

None of that is to say you shouldn’t eat those foods. That is not the case. They are all loaded with great nutrients and are part of a healthy, nutrient dense diet. But the healthiest diets include variety. If you want to have a green smoothie every day (again, we recommend chewing your food not drinking it), vary what makes it green. And of course, always drink plenty of water.

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Dr Robyn is a former competitive volleyball player turned psychologist with continuing education in nutrition. Russ is a former competitive bodybuilder and trainer on the Mr. Olympia Tour. They are the co-founders of Whole Food Muscle and the authors of How to Feed a Human The Whole Food Muscle Way. To work with them one on one to improve your health and fitness or to have them speak at your event or organization email them at