Antinutrients, which may be natural or synthetic, occur in nearly all foods. Thus, the chemical composition of a food isn't always a good indicator of how much of a nutrient you'll actually get from eating it. Legumes contain lectins and trypsin inhibitors that interfere with digestion. Tannins in tea and other plants chelate and reduce absorption of iron and zinc. Phytic acid, an antinutrient found in many grains, chelates iron, zinc, calcium, magnesium and other metals.
Antinutrients typically don't cause a problem in a person's diet unless he or she eats only a few different foods. Also, cooking food or other processes such as fermentation and malting may increase the nutritive content of foods by inactivating or reducing levels of antinutrients.