Antinutrients: are they a health risk?
In the previous steps of this course we mostly discussed the good properties of foods and superfoods.
However, in recent years a term that was previously confined to technical literature started to appear also in health magazines and websites. This term is antinutrient.
The definition of antinutrients includes a broad range of molecules, such as tannins, oxalic acid, phytic acid, and lectins.
In general, they are defined as molecules that are able to reduce the absorption of nutrients, and they can have a significant economic impact when, for instance, an antinutrient-rich plant is used as animal feedstock.
This is one of the reasons why in many cases their chemical properties are well known, and substantial effort has gone into finding ways to reduce or eliminate them. These efforts include, for instance, the breeding of new varieties of crops, or processing techniques that remove or destroy these “bad” molecules.
Of course, antinutrients can also be present in fruits and vegetables usually grown for human consumption, including superfoods and other healthy foods like whole grains and beans.
Should we be aware of their presence?
Can they pose a health risk, or would any concern be unjustified?
Surprisingly, the answer is: both.
Antinutrients can cause nutritional deficiencies when consumed in high quantities, but their concentration in the average diet is not likely to be excessively high.
Moreover, it seems that molecules with antinutritional properties can have positive effects on our health when consumed in lower concentrations.
In the rest of this article we will discuss, as an example, two classes of antinutrients: tannins and phytic acid, and we will discuss their positive and negative properties, as well as the foods where they can be found.
Tannins belong to polyphenol family of compounds. Polyphenols constitute a large class of molecules, and they include the subject of the last week, flavonoids. The difference is that tannins are composed of 2 or more “flavonoid” units.
Due to that fact, they are big, heavy molecules, that are weakly digested and absorbed. Those features – big molecular size and weakly absorption have an impact on their properties and behaviour during their passage through the gastrointestinal tract.
As other polyphenols, tannins are characterized by antioxidant activity and they are one of few antioxidants that can reach the large intestine in an almost intact form. There they might still act as antioxidants and are also utilized and metabolized by our intestinal flora, or microbiome.
For these reasons, tannins could have a positive effect on our health. But, as we said before, tannins are also considered as antinutrients. Their antioxidant properties are due to the chemical groups responsible for most of the antioxidant potential of all flavonoids, but in case of tannins those special chemical groups exhibit two additional properties.
First is the fact that tannins are very good at binding to free metal ions, like iron, copper or zinc. This means that the overall absorption of certain microelements can be disrupted, and in some cases, this can lead to nutritional deficiencies.
As the iron contained in meat products (heme iron) is not affected by tannins, people on a vegetarian or vegan diet are more likely to incur in this risk. On the other side, tannins can also bind to toxic metal ions (heavy metals), reducing their absorption and therefore exerting a positive effect on our health.
Secondly, tannins can bind to proteins. Interestingly, this phenomenon is responsible for the bitter or astringent taste of foods rich in tannins, such as some red wines or black tea. From a nutritional point of view, this is relevant because this phenomenon can lead to lower absorption of proteins.
Where can tannins be found? It’s very hard to avoid them because they are one of the most widespread plant metabolites. Those molecules can be found, for instance, in many foods sold as superfoods, such as cocoa, berries or nuts, as well as in other vegetables such as plums, apples or sorghum.
Pulses (beans, lentils, chickpeas….) are sometimes described as superfoods, but they are part of traditional diets in almost all regions of the world. They contain both tannins and phytic acid, the second antinutrient we discuss in this article.
Phytic acid is also found in the bran of cereals and in oilseeds.
Its antinutritional properties are due to its ability to bind to positively charged ions, such as calcium and zinc, therefore reducing their absorption. Phytic acid also can interact with proteins, and could also decrease the absorption of starches.
While these characteristics all contribute to the antinutritional properties of phytic acid, they could also be responsible for some positive effects that are currently under scientific investigation. In some studies phytic acid has been shown to reduce levels of triglycerides in the blood, as well as reducing the level of glucose after the consumption of starchy foods.
So, in conclusion, while the antinutritional properties of certain chemical compounds are certain, but their concentration in a healthy and varied diet is not likely to cause health concerns. Furthermore, food preparation techniques such as soaking, washing and boiling can significantly reduce their concentration. Finally, some antinutrients (but not all of them!) could have also a beneficial effect on our health, though they are still under investigation.