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Antinutrients affect the nutritional quality of food products.

Blog post
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28.11.2022
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Markus Nurmi

We get the nutrients we need from the food we eat every day. There are differences in the nutritional quality of different foods, and it does matter what food we choose on our plates. How well we can utilize the nutrients in our food affects the nutritional quality. Antinutrients reduce the absorption of nutrients, but knowledge about their significance is still incomplete.

Antinutrients are compounds found in the foods we eat. They are mainly found in products of the plant kingdom, but protease inhibitors, for example, can also be found in products of the animal kingdom. Antinutrients are a double-edged sword: on the one hand, some have health-promoting effects, but at the same time, they interfere with the availability of essential nutrients in the body. The NEPGa project aims to combine nutritional quality with the environmental impact of the products. In life cycle calculations that consider nutrition, antinutrients create challenges because some reduce the breakdown or absorption of nutrients in digestion, causing inaccuracy in the nutrition calculation. Additional challenges come from the effect of food processing on the activity of antinutrients. For example, some lectins and protease inhibitors are sensitive to heat that destroys them, while others withstand high temperatures better. Differences between varieties and growing conditions cause variations in the amounts of antinutrients. Thus, determining general rules is not straightforward. Below is detailed information about antinutrients that affect bioavailability and how they can affect nutrition calculations.

Protease inhibitors

Plants produce many different protease inhibitors to fight against plant-eating insects and other animals. Trypsin, chymotrypsin, and elastane are digestive enzymes produced by the pancreas that, together with pepsin, break down proteins from food into small peptides and amino acids. These small peptides and amino acids can be absorbed from the digestive tract into the bloodstream. If the function of enzymes that break down proteins is blocked, the amount of available protein from food decreases. Plants contain inhibitors for all the proteases mentioned above. Some of the inhibitors are not specific and can inhibit several proteases at the same time. Significant amounts of protease inhibitors are obtained from cereals and legumes. Protease inhibitor activities are often measured in vitro, so these measurements are not directly proportional to what happens in digestion. However, they can be used to study the effect of different treatments on inhibition activity and to compare the differences between other food substances. In addition, the so-called digestion mimicking system can be used to measure the degree of protein breakdown. So how much do you need to eat beans, for example, to prevent the absorption of proteins? It is estimated that you should eat about 100 grams of raw soybeans a day and about 200 grams of lentils to inactivate all trypsin and chymotrypsin in the digestive tract. However, legumes are generally eaten cooked, which leads to a problem that complicates nutrition calculations. As a rule, the raw ingredients of the food have been processed in some way. Such processing methods include, for example, boiling, cooking in the microwave or oven, frying, soaking, fermentation, and sprouting. This applies especially to legumes and grains, from which a significant part of the inhibitors would be obtained. However, not all treatments affect inhibitors in the same way. An example can be the chickpea: both raw and soaked, the protease inhibitor of the chickpea remains active, but when cooked, the inhibitory activity disappears completely. On the other hand, if chickpeas are roasted at 120 degrees for fifteen minutes, only about 30% of the inhibition activity is lost. Legumes can also be eaten raw in some cases, like green peas. However, beans at different stages of development may have different amounts of protease inhibitors, so direct conclusions cannot be drawn based on studies that used ripened beans. Differences between varieties may be significant within a species and bring more uncertainties. Bread can be taken as another example. The trypsin-inhibiting effect of wheat bread remains the same as in flour when the activity has been measured with in vitro digestion tests. In the same experiment, the trypsin-inhibiting effect in rye bread decreased compared to rye flour.

Phytic acid

Phytic acid precipitates divalent minerals such as calcium, iron, and zinc. It also negatively affects the absorption of proteins and starch. Phytic acid is found especially in legumes and grains, where it acts as a phosphorus reserve. The amounts of phytic acid vary according to species, growing conditions, and soil. For example, the amount of phosphorus fertilizer affects the amount of phytic acid. Phytic acids also have favorable health properties, but because of their effects on mineral absorption, it would be important to consider phytic acid in nutrition calculation. Taking phytic acid into account is essential regarding iron intake, especially if the diet shifts towards a plant-based diet. In cereals, phytic acid is mainly found in the hull layer, so its amount can decrease during grinding and de-hulling. In beans, phytic acid can be enriched in protein fractions due to phytic acid binding to proteins. However, removing the skin can also reduce the amount of phytic acid in beans. In some cases, such as lupine seeds, removing the husk can even increase the phytic acid content compared to the whole grain since most of the phytic acid is located in the inner layer of the seed. Soaking in water, sprouting, and fermentation significantly reduces phytic acid. For example, malting barley destroys phytic acid almost completely. On the other hand, heating is generally not a very effective method for reducing the amount of phytic acid. In addition, mainly bacterial or yeast-derived phytase enzymes can be used to break down phytase. Considering the effect of phytic acid on the absorption of nutrients, especially iron, zinc, and calcium, is essential in evaluating nutritional quality. In this case, the target for calculations should be a ready-to-use product because processing significantly affects the amount of phytic acid. It is difficult to determine ready-made calculated values ​​due to the many variables affecting the concentrations of phytic acid.

Tannins

Condensed tannins belong to phenols, which have health-promoting properties. On the other hand, the undesirable properties of tannins are related to their ability to bind to proteins, starch, and other macromolecules, making their absorption difficult. Tannins interfere with the absorption of some minerals and vitamins, such as vitamins A and B12 and iron. The concentration of tannins can vary greatly depending on the plant variety used.

Lectins

Lectins are common compounds produced by plants that are resistant to the conditions and enzymes of the digestive system. Lectins bind to sugars, specifically to a particular sugar or several different sugars with different affinities. Uncooked beans can cause symptoms similar to food poisoning because of the lectins. Lectins have also been found to have health-promoting properties as with other antinutrients. The effect of lectins on nutrient absorption is mainly indirect. When lectins bind to the epithelial cells of the digestive tract, they interfere with the transfer of nutrients to the bloodstream. Continuous exposure to lectins has been found in laboratory animals to reduce the intestine's surface area. However, lectins can be destroyed by sufficient heating. On the contrary, cooking too short or at too low a temperature may increase the activity of some lectins. When eating cooked, lectins hardly matter much when calculating nutrient intake. This is important because beans and grains contain many lectins, but they are mainly eaten cooked, in which case the lectins are inactivated. Instead, lectins may play a role in the nutritional calculations of raw or partially cooked foods. Such could be, for example, nuts, but it is not easy to assess whether the amounts available have a real meaning in a balanced diet.

Oxalates

Oxalates occur in food in either soluble or insoluble forms. Oxalates contribute to the formation of kidney stones, and therefore excessive intake of oxalate should be avoided. In terms of nutritional calculation, soluble oxalate is essential because it can bind calcium, iron, and magnesium, preventing the absorption of these nutrients. Plant oxalate concentrations are usually relatively low, but some plants contain significant amounts. Such plants used in Finland are, e.g. spinach, rhubarb, amaranth, tea, buckwheat, and chocolate. Boiling reduces the amount of soluble oxalate by breaking the cell, allowing the oxalate to dissolve in the boiling water. Another possible way to minimize oxalate concentration in industrial use is using an enzyme that breaks down oxalate. Fermentation may also reduce the amount of oxalate in some cases.

Evaluating the importance of antinutrients to nutrient availability is, therefore, not quite simple. Different products differ significantly in raw materials and manufacturing methods, so a straightforward universal calculation method cannot be used. We are also partially tackling this challenge within the NEPGa project. Although there is still not enough information, some of the effects of antinutrients can possibly be taken into account already.

 

The following sources have been used in writing the blog:

Feizollahi E., et. al. (2021) Food Research International 143:110284

Kårlund A., et. al. (2021) Healthcare 9:1002

Vasconcelos I. M. & Oliveira J. T. A. (2004) Toxicon 44:385-403

Kostekli M. & Karakaya S. (2017) Food Chemistry 224:62-68

Frias J., et. al. (2000) Eur. Food Res. Technol. 210:340-345

Chung K-T., et. al. (1998) Trends in Food Science & Technology 9:168-175

 

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