Oxalates in Kale vs. Spinach
Some opponents of Green Smoothies argue that oxalate-rich foods commonly used in Green Smoothies may cause kidney stones in susceptible individuals. In this context, kale and spinach are frequently mentioned. However, while raw spinach is indeed a rich source of oxalates, kale – contrary to popular belief – has in fact been reported to be low in oxalates. But exactly how much (or rather, little) oxalic acid does kale contain? And what about spinach?
Before delving into the exact values, it is important to note that the oxalic acid content of a specific vegetable can vary significantly depending on several factors. For example, the type of soil in which the vegetable was grown as well as the age of the vegetable when it was harvested can have a great impact on the oxalate content of that food. Furthermore, oxalate concentrations of vegetables such as kale and spinach have been shown to vary significantly depending on the season.
Oxalates in Kale
According to nutrient data provided by the U.S. Department of Agriculture (USDA), 100-gram serving of kale (Brassica oleracea L. var. acephala) contains only 20 milligrams of oxalates. This translates into about 13 milligrams per one cup of chopped kale. And this may even be an overstatement: According to data provided by the Department of Nutrition at the Harvard School of Public Health, a cup of chopped kale contains merely 2 milligrams of oxalates! The websites that claim that kale is rich in oxalates do not usually provide actual values for oxalate-containing foods, nor do they state the source of their information. In the light of all this evidence, the claim that raw kale is rich in oxalates appears to be nothing but a myth.
Oxalic Acid in Spinach
While kale is low in oxalates, spinach (Spinacia oleracea) is indeed a significant source of oxalates. On an ounce-to-ounce basis, only a few foods contain more oxalates than raw spinach (these foods include parsley, chives, purslane, cassava and amaranth, all of which have been reported to contain over 1 gram of oxalic acid per a 100-gram serving). According to USDA data, a 100-gram portion of spinach provides a whopping 970 milligrams of oxalic acid! And according to the Harvard data, the oxalate content of spinach may be even higher.
If you're looking for ways to reduce oxalic acid in your diet but love the health benefits (not to mention the culinary value!) of fresh spinach, you may want to focus on cooking with spinach in fall rather than spring or summer. According to a 2006 study published in HortScience, a bi-monthly publication of the American Society for Horticultural Science, the concentration of oxalic acid in spinach was lowest in fall.