Swiss chard is a leafy green vegetable prized for its flavor and nutritional density, but it contains high levels of naturally occurring compounds called oxalates. Oxalates, also known as oxalic acid, are organic molecules found in many plants, where they serve as a defense mechanism. The presence of oxalates means this otherwise healthy vegetable requires consideration, particularly for individuals with specific health concerns. Swiss chard is classified among the foods with the highest oxalate concentrations.
The Role of Oxalates in the Human Body
Oxalates enter the body through the diet, but they are also produced internally as a waste product of metabolism. Once consumed, oxalates have a strong tendency to bind with minerals, primarily calcium, in the digestive tract. This binding action forms a compound known as calcium oxalate, which is typically excreted from the body in the stool.
Oxalates are a focus of dietary concern because they interfere with mineral absorption. By binding to nutrients like calcium and iron, they limit the body’s ability to utilize them, sometimes classifying oxalates as anti-nutrients. For instance, despite chard being rich in calcium, its bioavailability is reduced due to the high oxalate content.
A more significant health concern involves the formation of calcium oxalate kidney stones, which account for roughly 75% of all kidney stones. When oxalates are absorbed from the gut into the bloodstream, they travel to the kidneys for filtration. If the concentration of oxalate in the urine becomes too high, it can combine with calcium to form crystals that aggregate into stones.
Dietary oxalates exist in two forms: soluble and insoluble. Soluble oxalates are not bound to minerals in the food matrix and are more readily absorbed from the gut into the bloodstream. Because this form is better absorbed, it poses the greater risk for increasing oxalate concentration in the urine, directly contributing to stone formation.
Quantifying Oxalate Levels in Swiss Chard
Swiss chard is consistently ranked as a very high-oxalate food, with a significant portion of its total oxalate content present in the readily absorbable, soluble form. Raw Swiss chard leaves can contain total oxalate levels ranging from approximately 874 milligrams to over 1,450 milligrams per 100-gram serving. To put this in perspective, some health professionals recommend that people prone to kidney stones limit their daily total oxalate intake to less than 50 milligrams.
The specific oxalate content can vary based on factors like the plant’s age, cultivar, and growing conditions. However, raw Swiss chard is firmly in the high-risk category for individuals sensitive to oxalates. A considerable amount of the oxalate in chard is in the soluble form, which is the fraction most affected by cooking.
The distinction between raw and cooked chard is substantial when considering oxalate intake. Once cooked, the oxalate content is drastically reduced due to the compound’s water-soluble nature. Studies show that boiling can reduce the soluble oxalate content in Swiss chard leaves by 84% to 85%. This demonstrates that the preparation method is a tool for managing the vegetable’s oxalate load.
Preparation Methods to Reduce Oxalate Content
Cooking is the most effective method for lowering the oxalate content of Swiss chard, primarily by reducing the amount of soluble oxalate. The water-soluble nature of the compound means that when the chard is exposed to boiling water, the oxalates leach out of the leaves and into the liquid. To maximize the reduction, it is necessary to discard the cooking water after boiling, as the oxalates remain suspended in this liquid.
Boiling is generally considered more effective than steaming for high-oxalate greens like chard because the greater volume of water allows for more efficient leaching. Research shows that while boiling can remove up to 85% of the soluble oxalates, steaming typically removes less. For the greatest benefit, the chard should be chopped before boiling. Chopping increases the surface area exposed to the water, facilitating the leaching process.
Another strategy involves combining Swiss chard with a source of calcium at the time of the meal. When calcium-rich foods like milk, cheese, or a calcium supplement are consumed alongside high-oxalate foods, the calcium binds to the oxalate in the stomach and intestines. This binding forms insoluble calcium oxalate before the compound can be absorbed, meaning it is passed out of the body in the stool. This pairing diverts the oxalate away from the urinary tract.
Comparing Swiss Chard to Other High-Oxalate Vegetables
Swiss chard is not alone in its high oxalate concentration; it is often grouped with spinach and beet greens as one of the three highest-oxalate leafy vegetables. Raw spinach can contain around 970 milligrams of oxalate per 100 grams, a level comparable to the lower end of the Swiss chard range. However, some studies place the oxalate content of raw chard at a level that can exceed raw spinach, sometimes reaching over 1,400 milligrams per 100 grams.
Other foods commonly recognized for their high oxalate content include rhubarb stalks, which contain approximately 800 milligrams per 100 grams, and beetroots. Dark chocolate, nuts like almonds and cashews, and wheat bran are also known sources of high dietary oxalate. Understanding where Swiss chard ranks helps to contextualize its consumption, as it is among the most potent sources of the compound.
The key difference between these high-oxalate greens and other healthy vegetables like kale, cabbage, or collard greens is the magnitude of the oxalate load. Kale, for example, contains hundreds of times less oxalate than spinach or chard, placing it in the low-oxalate category. Individuals with a history of kidney stones may need to limit the “big three” high-oxalate greens, including Swiss chard.