Oxalates are Found in Many Different European and Asian Foods - Effects of Cooking and Processing

Geoffrey Savage,Warinporn Klunklin

doi:10.5539/jfr.v7n3p76

Abstract

Plant foods contain a surprising number of different toxins. A few well-known plants, including some grown in Thailand are known to contain high levels of oxalates however, some plants have not yet been fully investigated. A few plants have become fashionable to promote health because they contain antioxidants but some of these plants will contain oxalates as well. In many cases there is little published data to confirm the oxalates levels of these plants. If plant leaves are boiled before they are consumed this allows soluble oxalate to be leached out and discarded in the cooking water. This means that the cooked food contains considerably lower levels of soluble oxalates than the original raw plants. Cooking in a wok generally concentrates the oxalate contents as much of the cooking water is removed as steam. However, during cooking some of the soluble oxalates can combine with free calcium in the food and be converted to insoluble oxalates; these are not absorbed in the digestive tract. The preparation of juices using fruit or vegetables are being promoted as healthy alternatives, this poses further problems, as they may be prepared from raw vegetable leaves, such as spinach, which contain high levels of oxalates. These juices are not cooked so the oxalate concentration is not reduced during their preparation. Recent research has shown that the addition of calcium salts to these juices can considerably reduce the soluble oxalate content of the drink prepared without changing the taste.

Highlights

Oxalates can be found in relatively small amounts in many plants
Recent research has shown that the addition of calcium salts to these juices can considerably reduce the soluble oxalate content of the drink prepared without changing the taste
Soluble oxalates in food plants have two main effects: 1) they can bind in the gastrointestinal tract with soluble calcium in the food; or 2) they can bind with calcium in other foods eaten at the same time, but this makes the calcium unavailable

Summary

Introduction

Oxalates can be found in relatively small amounts in many plants. Oxalate-rich foods are usually minor components in human diets but are sometimes important in seasonal diets in certain areas of the world. Kidney stones have a very rough crystalline structure so can cause considerable pain as they pass down the ureter This pain usually motivates people to reduce the possibility of developing further kidney stones by decreasing their intake of high oxalate containing foods. There is a consensus by reputable authorities (National Kidney Foundation, 2014; Harvard Health Publications, 2017; Mayo Clinic (nd); National Institute of Diabetes and Digestive and Kidney Diseases, 2016) that several steps need to be followed to protect from, or lessen, the probability of kidney stones These are: 1) moderating the intake of oxalate-rich foods; 2) drinking enough water to prevent the urine from being too concentrated; 3) reducing the animal protein intake; 4) increasing the calcium intake; 5) reducing the intake of sodium; and 6) drinking juices that contain citric acid. The current trend of preparing fresh juices from raw vegetables and fruit (commonly called juicing) needs to be recognised as a potential way to consume very large amounts of soluble oxalates

Oxalate Content of European and Asian Plant Foods

New foods

Findings

Conclusions