This is because plants may contain antinutrients.
These are plant compounds that reduce the absorption of nutrients from the digestive system.
They are of a particular concern in societies that base their diets largely on grains and legumes.
This article reviews several simple ways to reduce the amount of antinutrients in foods.
In some cases, they can be eliminated almost completely.
What are Antinutrients?
Antinutrients are plant compounds that reduce the body�s ability to absorb essential nutrients.
They are not a major concern for most people, but may become a problem during periods of malnutrition, or among people who base their diets almost solely on grains and legumes.
However, antinutrients aren�t always �bad.� Under some circumstances, antinutrients like phytate and tannins may have some beneficial health effects as well (1, 2, 3).
The most widely studied antinutrients include:
- Phytate (phytic acid): Mainly found in seeds, grains and legumes, phytatereduces the absorption of minerals from a meal. These include iron, zinc, magnesium and calcium (4).
- Tannins: A class of antioxidant polyphenols that may impair the digestion of various nutrients (5).
- Lectins: Found in all food plants, especially in seeds, legumes and grains. Some lectins may be harmful in high amounts, and interfere with the absorption of nutrients (6, 7).
- Protease inhibitors: Widely distributed among plants, especially in seeds, grains and legumes. They interfere with protein digestion by inhibiting digestive enzymes.
- Calcium oxalate: The primary form of calcium in many vegetables, such as spinach. The calcium bound to oxalate is poorly absorbed (8, 9).
Bottom Line: The most important antinutrients are phytate, tannins, protease inhibitors, calcium oxalate and lectins.
Beans and other legumes are often soaked in water overnight to improve their nutritional value (10).
Most of the antinutrients in these foods are found in the skin. Since many antinutrients are water-soluble, they simply dissolve when foods are soaked.
In legumes, soaking has been found to decrease phytate, protease inhibitors, lectins, tannins and calcium oxalate.
For example, a 12-hour soak reduced the phytate content of peas by up to 9% (11).
Another study found that soaking pigeon peas for 6-18 hours decreased lectins by 38-50%, tannins by 13-25% and protease inhibitors by 28-30% (12).
However, the reduction of antinutrients may depend on the type of legume. In kidney beans, soybeans and faba beans, soaking reduces protease inhibitors only very slightly (13, 14, 15).
Not only is soaking useful for legumes, leafy vegetables can also be soaked to reduce some of their calcium oxalate (16).
Soaking is typically used in combination with other methods, such as sprouting, fermenting and cooking.
Bottom Line: Soaking legumes in water overnight may reduce phytate, protease inhibitors, lectins and tannins. However, the effect depends on the type of legume. Soaking may also decrease oxalates in leafy vegetables.
Sprouting is a period in the life cycle of plants when they start emerging from the seed. This natural process is also known as germination.
This process increases the availability of nutrients in seeds, grains and legumes (17).
Sprouting takes a few days, and may be initiated by a few simple steps:
- Begin by rinsing the seeds to remove all debris, dirt and soil.
- Soak the seeds for 2-12 hours in cool water. The soaking time depends on the type of seed.
- Rinse them thoroughly in water.
- Drain as much water as possible and place the seeds in a sprouting vessel, also called a sprouter. Make sure to place it out of direct sunlight.
- Repeat rinsing and draining 2-4 times. This should be done regularly, or once every 8-12 hours.
During sprouting, changes take place within the seed that lead to the degradation of antinutrients such as phytate and protease inhibitors.
Sprouting has been shown to reduce phytate by 37-81% in various types of grains and legumes (18, 19, 20).
There also seems to be a slight decrease in lectins and protease inhibitors during sprouting (21).
You can find detailed instructions on other websites. For example, Sprout People has excellent information on how to sprout various types of beans, grains and other plant foods.
If you need to buy a sprouting vessel or a sprouter, then there is a wide selection on Amazon.
Bottom Line: Sprouting reduces phytate in grains and legumes, and may slightly degrade lectins and protease inhibitors.
Fermentation is an ancient method originally used to preserve food.
It is a natural process that occurs when microorganisms, such as bacteria or yeasts, start digesting carbs in food.
Although food that becomes fermented by accident is most often considered spoiled, controlled fermentation is widely used in food production.
Food products that are processed by fermentation include yogurt, cheese, wine, beer, coffee, cocoa and soy sauce.
Another good example of fermented food is sourdough bread.
Making of sourdough effectively degrades antinutrients in the grains, leading to increased availability of nutrients (17, 22, 23).
In fact, sourdough fermentation is more effective at reducing antinutrients in grains than yeast fermentation in typical bread (24, 25).
In various grains and legumes, fermentation effectively degrades phytate and lectins (26, 27, 28, 29).
For example, fermenting pre-soaked brown beans for 48 hours caused an 88% reduction in phytate (30).
Bottom Line: Fermentation of grains and legumes leads to a significant reduction in phytate and lectins.
High heat, especially when boiling, can degrade antinutrients like lectins, tannins and protease inhibitors (14, 31, 32, 33).
One study showed that boiling pigeon peas for 80 minutes reduced protease inhibitors by 70%, lectin by 79% and tannin by 69% (12).
Additionally, calcium oxalate is reduced by 19-87% in boiled green leafy vegetables. Steaming and baking are not as effective (34, 35).
The cooking time required depends on the type of antinutrient, food plant and the cooking method. Generally, a longer cooking time results in greater reductions of antinutrients.
Bottom Line: Boiling is effective at reducing various antinutrients, including lectins, tannins, protease inhibitors and calcium oxalate.
Combination of Methods
Combining many methods can reduce antinutrients substantially, sometimes even completely.
As an example, soaking, sprouting and lactic acid fermentation decreased the phytate in quinoa by 98% (36).
Similarly, sprouting and lactic acid fermentation of corn and sorghum degraded phytate almost completely (37).
In addition, soaking and boiling pigeon peas led to a 98-100% reduction in lectins, tannins and protease inhibitors (12).
Bottom Line: The most effective way to reduce antinutrients in plant foods is to combine several different elimination strategies. Combining methods may even degrade some of the antinutrients completely.
Below is an overview of the main antinutrients and effective ways to eliminate them.
- Phytate (phytic acid): Soaking, sprouting, fermentation.
- Lectins: Soaking, boiling, heating, fermentation.
- Tannins: Soaking, boiling.
- Protease inhibitors: Soaking, sprouting, boiling.
- Calcium oxalate: Soaking, boiling.
Take Home Message
Antinutrients can significantly reduce the nutritional value of many plant foods.
Luckily, they can be degraded with a few simple methods such as heating, boiling, soaking, sprouting and fermenting.
By combining different methods, many antinutrients can be degraded almost completely.
By Atli Arnarson, PhD