Some vegetables are noted for their high content of oxalic acid that binds to calcium in your digestive tract and keeps the calcium from being absorbed. Oxalic acid is also the primary component of kidney stones. These are good reasons to reduce oxalic acid in your diet.
A 1988 study in the American Journal of Clinical Nutrition made the point clearly: absorption of calcium from milk was over five times greater than from spinach. The oxalates in spinach inhibited the absorption of calcium from the spinach.
Improving Calcium Absorption by Choosing Lower Oxalate Vegetables
In an interesting study reported in the British Journal of Nutrition in 2004, researchers compared magnesium absorption from high-oxalate and low-oxalate food: spinach and kale. Participants were given a meal of phytate-free white bread and either spinach or kale, cooked and puréed. In the high oxalate spinach meal, study participants absorbed about 27 percent of the magnesium in the meal. In the low oxalate kale meal, participants absorbed about 37 percent of the magnesium. This study shows that you can increase your mineral absorption by eating foods lower in oxalic acid. We have no reason to think these findings would not apply to calcium as well.
This study suggests an important strategy: Simply, choose lower-oxalate vegetables over vegetables high in oxalic acid.
Vegetables High in Oxalic Acid
The oxalic acid big list below—the foods highest in oxalate—are the foods you would be better off avoiding if you have a choice. The big list includes foods you should avoid eating raw in large quantities. Beets are a popular choice in homemade raw vegetable juice, yet are high in oxalic acid. Carrots, parsley, and spinach tend to be eaten raw as well. Do not eat them in their raw form in great quantities; consider boiling them and tossing out the boiling water as an alternative to sautée. Boiled vegetables can then be browned in oil if you do not like the taste of boiled produce.
The amount of oxalic acid in food samples is highly variable and, thus, so are oxalic acid food lists. Oxalate varies across foods, plant varieties, and picking times. To create this list, I used foods that appeared multiple times in these five sources: Brzezinski et al. 1998; Duke 1992; Hodgkinson 1977; Chai and Liebman 2005; USDA 1984.
- Brussels sprouts
- Collard greens
- Sweet potato
- Swiss chard
Reducing Oxalic Acid: Boil, Steam, or Ferment
A 2005 study found that boiling reduced the level of oxalic acid in food. In a test of foods high in oxalic acid, researchers found that boiling spinach reduces oxalic acid by 87 percent whereas steaming reduces it by 42 percent. In every vegetable studied, boiling is more effective than steaming. And there is a good reason: oxalic acid simply falls off of the food and into the water. You can then remove the oxalic acid by pitching the cooking water.
Based on this research, the best cooking strategy is to boil (or at least steam) the food and discard the cooking water. I know that all our mothers saved this liquid for soup or gravy, but you’ll want to toss the water of heavy offenders.
A more effective strategy is to ferment foods high in oxalates. This is my favorite strategy of course because you maintain the enzymes in the raw food, add beneficial bacteria to your diet, and increase the B vitamin content as I describe above. Boiling or steaming will also cause some mineral loss in the food.
In a 2005 study in Food Microbiology, researchers found that the soluble iron in the homemade vegetable juice in the study increased sixteen times with fermentation. What this means is that if you juice your own vegetable juice with a high iron vegetable like spinach and you ferment it, your body may absorb sixteen times more iron than it would have absorbed had you consumed the juice right out of the juicer.
The same study found that fermenting commercial juice increased the solubility of iron by seven times. So you can also buy a ready-to-drink juice and ferment it and digest about seven times the iron in the original juice.
Rules of Thumb
But do not let oxalic acid drive you crazy. Spinach, for instance, is high in oxalates which bind to minerals but it is still a very good source of folate. Here are some good rules of thumb:
- If you eat a lot of a high-oxalate food, try to find a reasonable alternative for some of it. Not all raw vegetable juices need beets and carrots, for instance.
- Try some wilted salads. To reduce oxalates (at the expense of some folate loss), steam spinach slightly and use as a base of a “wilted salad.”
- Learn fermentation techniques.