Oxalates in Tea: Comparison of Green, Oolong and Black Tea
Although all types of tea – green, black, white and oolong – contain oxalates, there are significant differences between different types of tea. The purpose of this article is to provide a comparison of the oxalate content of different types of tea, including green tea varieties (such as tencha, sencha and gyokuro), black tea, and oolong tea. But before we delve into the details, let's back up a step and look at the role of oxalates in human health.
Oxalates and Health
Tea, especially green tea, has been praised for its health benefits. Most of these benefits have been attributed to polyphenols, such as catechins, which are abundant in the leaves the tea plant (Camellia sinensis). However, tea – whether black, oolong or green tea – has also been shown to contain oxalates, controversial substances that have been linked to some health problems. Oxalates, which are abundant in a number of foods and drinks, may increase the risk of kidney stones (also known as renal calculi) in people susceptiple to developing calcium-oxalate kidney stones. In addition, people with certain medical conditions such as Primary Hyperoxaluria and Enteric Hyperoxaluria may be advised to restrict their dietary intake of oxalate-containing foods and drinks.
Oxalate Content of Tea
According to a study published in the Asia Pacific Journal of Clinical Nutrition in 2002, loose black tea leaves contain 5.11 milligrams of soluble oxalates per one gram of tea leaves. Black tea in tea bags contains slightly less oxalates: 4.68 milligrams per 1 gram of tea leaves. When tea is steeped, some of the soluble oxalates leach into the hot water. The amount of oxalates provided by one cup of black tea made by steeping loose tea leaves is estimated to range from 4.41 milligrams to 16.43 milligrams. The oxalate content of black tea made by steeping tea bags is estimated to vary between 2.98 milligrams and 15.61 milligrams per one cup of steeped tea. For the sake of comparison, spinach – which is considered one of the most oxalate-rich food sources – contains 970 milligrams of oxalic acid per 100 grams (3.5 oz).
Green tea and oolong tea contain less oxalic acid than black tea. According to the same study that compared the oxalate content of loose leaf vs bagged black tea, the soluble oxalate content of green tea and oolong tea ranged from 0.23 to 1.15 mg per one gram of tea leaves. A more recent study, published by the Japanese Society of Soil Science and Plant Nutrition, focused on comparing the oxalate content of different types of green tea. This comparison revealed that the tested green tea varieties that were cultivated with shading (Gyokuro and Tencha) contained more oxalates than Sencha green tea which is cultivated without shading. In Japan, shading tea plants is used to produce sweeter tasting tea leaves.