Most plants, including many domesticated food crops, are equipped with chemical defenses against being eaten. Humans have learned to love the pungent sulfur compounds in onions and the fiery capsaicin in chile peppers, but some other compounds are more problematic. Many plant crops contain compounds with antinutrient properties, meaning they interfere with the body's ability to extract nutrients from foods.
Except for fruit, most plant foods contain some toxins or antinutrient compounds. In most cases, they occur in negligible quantities, thanks to centuries of selective breeding by farmers. However, many common foods, including staples such as beans and spinach, still contain high levels of antinutrients. Cooking destroys or deactivates the antinutrient qualities of these compounds, permitting your body to absorb the nutrition from them. In many cases, foods with antinutrient characteristics are still healthy and nutritious, even when eaten raw, but informed diners will eat them selectively.
Several types of antinutrient compounds exist. One group consists of specialized proteins called protease inhibitors. These interfere with the action of the enzymes that help your body digest proteins, converting them to fuel and building blocks for the body's own repair kit. Beans and other legumes, cucumbers, radishes, broccoli, spinach and potatoes all contain protease inhibitors. Protease inhibitors aren't a major factor for most Americans because the mainstream American diet is rich in animal proteins and relatively light on raw vegetables. However, some vegetarians and vegans, especially those on raw food diets, should monitor their consumption of these foods.
Lectins are another group of antinutrient proteins, which inhibit absorption of nutrients in a more direct and broad-based fashion. Your small intestine is lined with fine cylindrical structures called villi, which absorb nutrients from the digested foods passing through. Lectins bond to the villi, creating a thin layer of protein that acts like the nonstick coating on your cookware. Food and nutrients pass by without being absorbed and used by the body. In some cases lectins can make their way into your bloodstream, where they cause red blood cells to bond together. They can be responsible for symptoms closely resembling food poisoning.
A variety of other chemical compounds in foods have antinutrient properties. Usually, they work by bonding to one or another nutrient in the food and altering it to a form that your body can't absorb. One example is phytic acid, found in cereal grains, wild rice and some beans. It bonds with calcium, iron and other nutrients and prevents their absorption. Oxalic acid, found in spinach, also bonds iron and calcium. The tannins in tea and some fruit can prevent the absorption of zinc. These compounds all have beneficial effects as well and should not be avoided altogether.