In the bid to stay healthy, we sometimes do things that harm us unknowingly.
Sometime last year, someone sent me a recipe that included the use of raw spinach in juicing.
Spinach is one vegetable that should not be eaten raw because it is high in oxalate.
This week, we shall be talking about oxalate and everything you should know about it.
I will not be doing justice to this discussion if I do not refresh our memories about plant defence mechanisms.
Animals employ various strategies/defence mechanisms to avoid their predators.
Behavioural responses, like running away, are very important. In contrast, plants are sedentary, meaning they cannot move. Roots anchor them to the soil, so plants cannot run.
Therefore, they have developed physical and chemical defences to protect themselves against herbivores, which are animals that live by eating plant tissues.
A form of defence can be physical. For example, a rose’s thorns, technically called prickles, are extremely important for its survival. They do everything from helping rose bushes grow to keeping them safe from predators and casual flower pickers.
There is also a chemical form of defence, also known as anti-nutrients. The name comes from how they function in your body once you eat them.
They block or interfere with how your body absorbs other nutrients. Thus, anti-nutrients may decrease the amount of nutrients you actually get from your food. They most commonly interfere with the absorption of calcium, iron, potassium, magnesium and zinc.
Plants evolve these compounds as a defensive mechanism against insects, parasites, bacteria and fungi infestations. For example, some anti-nutrients can cause food to taste bitter; animals won’t want to eat it, leaving the seed, for instance, to provide nourishment for future seedlings. However, if birds and animals were to eat the fruits containing the seeds and digest them to obtain additional nutrients and calories, then there would be no seeds left for the continuation of the plant species.
Some anti-nutrients block the digestion of seeds that are eaten. The seeds disperse when they come out of the other end of the animal’s faecal matter and can go on to grow new plants.
Some of these compounds may also prevent the activity of digestive enzymes so that even if the seeds are consumed, they are excreted without altering their physiology.
Both of these survival tactics help the plant species grow and spread. In terms of foods that people eat, you will most commonly find anti-nutrients naturally occurring in whole grains and legumes.
Thus, anti-nutrients are part of the plant’s chemical defence system against invaders in order to ensure its survival.
Now that we know what anti-nutrients are, let us get back to our discussion on oxalate.
Oxalic acid is an organic compound found in many plants, including leafy greens, vegetables, fruits, cocoa, nuts and seeds.
In plants, it is usually bound to minerals, forming oxalate. The terms “oxalic acid” and “oxalate” are used interchangeably in nutrition science.
Your body can produce oxalate on its own or obtain it from food. Vitamin C can also be converted into oxalate when it is metabolised.
Once consumed, oxalate can bind to minerals to form compounds, including calcium oxalate and iron oxalate. This mostly occurs in the colon but can also take place in the kidneys and other parts of the urinary tract.
In most people, these compounds are then eliminated in the stool or urine.
However, in sensitive individuals, high-oxalate diets have been linked to an increased risk of kidney stones and other health problems.
The itchy sensation caused by raw yam is primarily due to microscopic, needle-like crystals of calcium oxalate (often called raphides) found in the tuber’s skin and slimy outer layer. These tiny needles penetrate the skin, causing intense itching and burning.
If you have ever broken out with massive skin eruptions after drinking bitter leaf water, this is due to the presence of raphides, which are bundles of needle-like crystals of calcium oxalate in the bitter leaf.
Let us see some of the effects of oxalate:
- Oxalate can reduce mineral absorption
One of the main health concerns about oxalate is that it can bind to minerals in the gut and prevent the body from absorbing them.
For example, spinach is high in calcium and oxalate, which prevents a lot of the calcium from being absorbed into the body.
Eating fibre and oxalate together may further hinder nutrient absorption.
Nevertheless, it is important to remember that only some of the minerals in food will bind to oxalate.
Even though calcium absorption from spinach is reduced, calcium absorption from milk is not affected when milk and spinach are consumed together.
- Oxalate may contribute to kidney stones
Normally, calcium and small amounts of oxalate are present in the urinary tract at the same time, but they remain dissolved and cause no problems.
However, sometimes they bind to form crystals. In some people, these crystals can lead to the formation of stones, especially when oxalate is high and urine volume is low.
Small stones often don’t cause any problems, but large stones can cause severe pain, nausea and blood in the urine as they move through the urinary tract.
Although there are other types of kidney stones, about 80 per cent are made up of calcium oxalate.
For this reason, people who have had one episode of kidney stones may be advised to minimise their consumption of foods high in oxalate.
However, across-the-board oxalate restriction is no longer recommended for every person with kidney stones. This is because half of the oxalate found in urine is produced by the body rather than absorbed from food.
Most urologists now prescribe a strict low-oxalate diet (less than 100 milligrams per day) only for patients who have high levels of oxalate in their urine.
Therefore, it is important to be tested from time to time to determine how much restriction is necessary.
A study titled Dietary oxalate and kidney stone formation by Tanecia Mitchell et al concludes that dietary oxalate may have an important influence on the risk of formation of calcium oxalate kidney stones. Reduced intake of high-oxalate-containing foods and normal intake of dietary calcium may be a practical method for attenuating calcium oxalate supersaturation and, thus, may limit stone risk in kidney stone-forming individuals.
In another study titled Oxalate content of cereals and cereal products by Roswitha Siener et al, the conclusion is that cereals and cereal products contribute considerably to daily oxalate intake. Vegetarian diets may contain high amounts of oxalate when whole grain wheat and wheat products are ingested.
Recommendations for the prevention of recurrence of calcium oxalate stone disease have to take into account the oxalate content of these foodstuffs.
Some proponents of low-oxalate diets say people are better off not consuming foods rich in oxalates, since they may have negative health effects.
However, it is not that simple. Many of these foods are healthy, containing important antioxidants, fibre and other nutrients.
Therefore, it is not a good idea for most people to completely stop eating high-oxalate foods.
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