Many are advised to follow a low oxalate diet to maintain or improve their kidney health. This often means lowering oxalate intake to around 50 mg per day. The first step for many towards being successful on this diet is research on what foods to avoid and what foods to eat. There are several resources available online that report the relative oxalate concentration of foods.
What tends to be discouraging is the inconsistency across these lists. For example, according to the food list released by Harvard’s T.H. Chan School of Public Healthy Nutrition Department, tomatoes have a moderate level of oxalate. However, The Low-Oxalate Cookbook Two by The VP Foundation, Annie Gottlieb and Joanne Yount reports them to have a high level of oxalate.
Oxalate levels can differ even within the same plant type based on environmental factors including geography, weather conditions, and time of year the food is grown and harvested. Additionally, there may be differences based on the cultivar or plant species, and methodology used to determine oxalate content.
Factors Affecting Oxalate Content
Research shows that oxalate levels are strongly affected by growth season. In plants, this is thought to be partly due to temperature. For example, one study showed that plants picked in early summer had higher oxalate content when compared to similar plants picked later in the season. Also, in dry conditions, oxalic acid tends to accumulate in plants. In addition, the nutrients in the soil can affect the plant’s development and oxalate levels as well as the length of time it’s grown; certain plants may accumulate oxalate as they ripen.
In some plants, there is a rapid increase in oxalate during the beginning stages of growth followed by a decrease as the plant ripens and is harvested. However, in some plant species such as rhubarb, oxalate tends to increase as the plant matures.
Oxalate may vary by cultivar. Some varieties of spinach are reported to contain 400 to 600 mg/100 g, while others range from 700 to 900 mg/100 g. This is 8-12 times and 14-18 times the recommended daily intake of 50mg per day for those on a low oxalate diet, respectively. Wilson and colleagues found that 15 cultivars of carambola (star fruit) had oxalate levels that varied 10-fold.
Soluble and Insoluble Oxalate
When discussing oxalate content it is also important to recognize that it comes in two different forms, soluble and insoluble. Which form it is becomes especially important when testing for oxalate since soluble oxalate can often be measured directly, but insoluble oxalate sometimes requires a pre-treatment of the sample, such as mixing with acid or heating up the sample.
As you have read in our prior article on “How to Reduce Oxalate by Cooking,”oxalate in food can be reduced by cooking or blanching. This is partly due to the fact that some oxalate is referred to as “free” or “soluble” oxalate. This means it easily moves from the food into water (for example, in your pot). Insoluble oxalate, on the other hand, is bound to something in the food, therefore rendering it less accessible. The insoluble form is usually calcium oxalate.
Another thing that can cause differences in the reported oxalate values is that different test methods are used for determination of oxalate, and some are more sensitive than others.
Oxalate Detection Methods
Colorimetric methods are based on an enzyme reaction that results in a colored product. The extent of the color change reflects the oxalate concentration. This method is mostly used for measuring urinary oxalate because vitamin C in foods can interfere with the colorimetric estimation of oxalate.
Chromatography is a method that separates molecules based on their chemical properties and then detects individual molecules using various techniques such as light absorption, conductivity, or through ion detection (ions created from burning the organic compound).
High-pressure liquid chromatography (HPLC)
High-pressure liquid chromatography is a form of column chromatography that pumps a sample, in this case, an oxalate extracted food mixture into a solvent, known as the mobile phase, at high pressure through a column. The detection is based on the absorption of UV light. Limits of detection are typically in the millimolar (mM) oxalate range.
Ion Chromatography (IC)
Ion chromatography is another form of column chromatography that allows for the measurement of oxalate based on its conductivity in water (i.e. charge). This method requires a pre-sample treatment extracting the oxalate but it can measure to the micromolar (uM) concentration range. IC is ideally used for measuring oxalate content in foods with less oxalate as it will be able to detect low concentrations.
Capillary electrophoresis (CE) is an analytical technique that separates ions based on their migration rate with the use of an applied voltage. Molecules can be either positively charged (cations), negatively charged (anions), or neutral. Charged particles will seek molecules having an opposing charge; in other words, opposites attract. Different anions are then detected by indirect absorption from UV light.
In one’s efforts to find the most accurate and reliable list, it’s important to find a credible source. Testing methodologies, food sources, growth season and time of harvest all have an effect on the measured oxalate concentration. Knowing this, it comes down to personal evaluation in choosing which list to go by.