Oxalates: Why Does the Same Food Have Many Oxalate Values?

by Arpi Gasparyan | Last updated on February 19, 2024

Medically reviewed by Elen Khachatrian

Introduction

Sometimes, adhering to a low oxalate diet for people with oxalate kidney stones is challenging. Some foods are well-known to be naturally high in oxalates, such as spinach and other leafy green vegetables, almonds, and other nuts, beets, and soy products, and are easily excluded from the diet. However, some foods with significant reported variations in oxalate content could mislead and confuse the consumer.

Different plant-based foods contain different amounts of oxalates; the same food also contains different amounts of oxalates, and this article aims to explain why.

For a bit of background, oxalates (oxalic acids) are one of the end products of plant carbohydrate metabolism, accumulating in the cell’s vacuole. Many factors influence a food’s oxalate value: extraction conditions and analytical methods, growing conditions, food cultivar or variety, ripeness and part of the plant, seasonal changes and climate, soil conditions, time of harvest, and preparation methods (1, 2).

Extraction Conditions

In certain instances, oxalate extraction conditions and analytical methods can result in calculation errors.

Extraction may be difficult, and there are several factors to consider, such as extraction temperature, pH, and time. For example, incorrect extraction temperatures can lead to oxalate generation from precursors or incomplete extraction. Regarding time, 15 minutes is considered the minimum for oxalate extraction; increasing it has not been linked to any benefits, and the pH value should be at least 1 (2).

Analytical Methods

Several analytical methods include electrochemical detection, liquid or gas chromatography coupled with mass spectrometry, enzymatic oxalate degradation, and indicator-based methods employing fluorescent or UV light-absorbing compounds.

There is no analytical method that is ideal for every situation, as they all have different strengths, limitations, sensitivity, and specificity.

For example, acid extractions, used for colorimetric and gas chromatography methods, may increase the measured oxalate values depending on the acid preparation used. Atomic absorption used for oxalate quantification from precipitated calcium may also cause unknown losses (4).

Seasonal Changes and Time of Harvest

The average oxalate concentration varies depending on the cultivation season. Oxalate concentrations are studied to be the highest in winter and the lowest in fall.

One hundred eighty-two spinach cultivars were used in a study, and the results have shown that the highest oxalate concentrations were observed in winter (1092 mg/100g) and lowest in fall (614 mg/100g) (2,5).

The seasonal differences are explained by the length of the growing period, which is relatively higher in winter. The longer the growing period, the higher the oxalate accumulation in the cell’s vacuole.

Depending on the climate, some seasonal changes may appear. For example, the highest oxalate content of foods in the tropics occurs before the rainy season begins due to the accumulation of oxalates during the dry season and conditions (6, 7). The greater oxalate value may be due to the longer duration of solar irradiance, temperature, day length, and hours of sunlight (6).

A food’s oxalate concentration appears higher in the tropics than in the subtropics (6).

Food Cultivar and Ripeness

The developmental stage of plants, genetic variations, soil chemistry and moisture, fertilization, and plant parts are all significant factors impacting the oxalate content (4).

Fast-growing cultivars and immature foods contain fewer oxalates, whereas slow-growing and ripe ones contain higher oxalate levels (5, 8). The mechanisms are similar to the ones influencing seasonal variations: longer growing periods and slow-growing varieties result in greater oxalate accumulations.

One may also affect the other, as, for example, the growth rate of spinach depends mainly on the temperature. In cultivars with different growing rates within the same growing seasons, oxalate concentration may significantly differ among the four seasons (5).

The table below demonstrates how the season and growing rate affect spinach’s oxalate content (mg/100g fresh weight).

Growth rate	Winter	Spring	Summer	Fall
Fast	994.2	856.7	737.9	526.1
Moderate	1094.9	906.5	762.7	600.8
Slow	1175.5	894.8	734.8	714.0

Genetic differences other than the ones responsible for growth rate also impact food’s oxalate value. For example, researchers found that 15 cultivars of star fruit had 10-fold, and 116 soy cultivars had over 2-fold varying oxalate levels. Others studied nearly 80 rhubarb cultivars and noticed a 3-fold difference in oxalate values. The rhubarbs were grown in the same field in the same year and harvested on the same day to minimize the differences caused by growing practices (9).

Oxalate Concentrations on Different Parts of a Food

Oxalate distribution within the same plant’s different parts is uneven. In general, oxalates are the highest in the leaves, followed by the seeds, leaving the stems and fruits in the last place (7, 9).

Reports demonstrate that the stems or stalks of plants like spinach, amaranth, rhubarb, and beet have notably lower oxalate values than the leaves. In the buckwheat (Polygonaceae) family, the oxalates are 2 times higher in leaves than the stalk (7). Bamboo sprouts have 3 times more oxalates in younger parts of the food than in older parts (9).

As for whole grains, oxalates are concentrated in the bran; thus, about one-third of oxalates are lost in processing wheat to refined flour. The New Zealand yams have the greatest concentration of oxalates in the skin. In contrast, oxalate concentrations in potato flesh and peel are similar (9).

Preparation Methods

Cooking methods such as boiling or simply soaking the food in water for several minutes reduces oxalate concentrations.

In a study, boiling spinach for only 1 minute removed about 10% of insoluble and 47% of soluble oxalate salts. Boiling Japanese taro cormels reduced the soluble oxalate level in the cooked tissues below the detectable levels (10).

The greater reduction of soluble oxalates than the insoluble ones is more important, as the more bioavailable oxalates likely to affect health are the soluble ones (10).

In another study, 9 types of raw and cooked vegetables were analyzed, in which boiling markedly reduced soluble oxalate content by 30-87% and was more effective than steaming, where oxalates were reduced by 5-53% and baking (11).

Food’s cutting size before boiling may also determine the extent of oxalate loss. A study suggested boiling spinach in water (100 °C for 2 min) and cutting it to 1cm can maximize the removal of oxalates by 72% (12).

You can visit our “Food Oxalates: How to Reduce Oxalate Levels for Better Health” for a detailed article about the influence of food preparation methods on oxalate levels.

Summary

Many factors influence a food’s oxalate value: extraction conditions and analytical methods, growing conditions, food cultivar or variety, ripeness and part of the plant, seasonal changes, soil conditions, time of harvest, and preparation methods.

Extraction conditions and analytical methods are yet to be perfected and may lead to incorrect oxalate numbers, leading to incomplete oxalate extraction, generation, or loss.

Longer growing periods during winter, ripe and mature plants, and slow-growing varieties result in greater oxalate accumulation in the plant. In contrast, shorter growing periods during fall, immature plants, and fast-growing food varieties result in lower oxalate levels.

Cooking methods such as boiling or simply soaking the food in water for several minutes reduces oxalate concentrations.

References

Author name: Arpi Gasparyan

Education: General Medicine at YSMU

Last updated: February 19, 2024

Medically reviewed by Elen Khachatrian