Oxalates are naturally occurring compounds found in various plants, where they function as a defense mechanism against potential predators. The presence of these compounds in many nutrient-dense foods, particularly leafy greens, often leads consumers to question the content of specific items like kale. Understanding a food’s oxalate level is relevant for individuals managing certain health conditions. Kale is generally classified as a low-oxalate food, which often surprises people who associate all dark leafy greens with high oxalate levels.
Understanding Dietary Oxalates
Oxalates are organic acids that the body produces as a metabolic byproduct, but they are also readily absorbed from dietary sources. In plants, oxalates are often bound to minerals, and when consumed, they can act as an “anti-nutrient” by interfering with the absorption of those same minerals in the digestive tract. This binding action primarily targets calcium and iron, reducing their bioavailability from the food source itself.
The primary health concern related to high oxalate intake centers on the formation of kidney stones. Approximately 75% of all kidney stones are composed of calcium oxalate crystals, which can form when there are high concentrations of oxalate in the urine. Individuals with conditions that lead to excessive oxalate excretion, known as hyperoxaluria, are often advised to follow a diet that limits these compounds. Managing the intake of high-oxalate foods becomes a consideration for long-term health management.
The Specific Oxalate Content of Kale
Kale is considered a low-oxalate green. Research shows that kale’s oxalate content is remarkably low, often measured at around 2 milligrams per cup of raw, chopped leaves. This low concentration places it firmly in the category recommended for individuals on restricted diets. The specific oxalate content can vary depending on the variety of kale and the testing method used.
For instance, raw curly kale has been reported with a slightly higher, but still moderate, level of approximately 9.3 milligrams per half cup. Varieties like Lacinato kale, also known as dinosaur kale, show even lower concentrations, especially after cooking. This variability highlights that while all kale is relatively low, the exact milligram count can fluctuate based on growth conditions, plant variety, and preparation.
Kale Compared to Other Leafy Greens
Kale’s low oxalate level makes its nutritional profile stand out when compared to its leafy green counterparts. Certain popular greens are known to contain extremely high concentrations. Spinach, for example, is an outlier, containing hundreds of milligrams per serving, with raw spinach measured at approximately 656 milligrams per cup.
Other greens high in oxalates include Swiss chard and beet greens, which also contain hundreds of milligrams per cup. Kale’s typical content of 2 milligrams per cup positions it as a suitable substitute for these high-oxalate greens in salads, smoothies, and other recipes. Other low-oxalate greens include bok choy (about 1 milligram per cup) and arugula (about 2 milligrams per cup).
Even collard greens, which are also considered low-oxalate, contain a slightly higher amount than kale, measuring around 10 milligrams per cup. Choosing kale over greens like spinach allows dieters to maintain a high level of nutrient intake without the concern of excessive oxalate consumption.
Reducing Oxalates Through Preparation
For any food containing oxalates, certain preparation methods can help reduce the final concentration. Oxalates are water-soluble, meaning they can leach out of the food and into the surrounding liquid during cooking. Boiling is the most effective technique for reducing soluble oxalate content, with studies showing a reduction of 30% to 87% in various vegetables.
To maximize this reduction, the cooking water must be discarded rather than consumed, as this water holds the leached oxalate compounds. Steaming is a less effective method because it involves less direct contact with water. Another strategy to manage oxalate absorption is to consume foods containing calcium alongside the meal, as the calcium can bind to the oxalate in the digestive tract, preventing its absorption.