Lectins are proteins that facilitate cell to cell binding. Letins accomplish this by weakly joining to carbohydrate signal molecules on the surface of cells, and when two such conjugations are made, the cells become joined through an intermediary lectin. The addition of a number of lectins to join two cells can be analogous to that of velcro, whereby the conjugations are individually weak, but provide strong adhesive force when large numbers of conjugations occur. Plants contain many lectins, which are also called haemagglutinins or phytohaemagglutinins. Plant lectins have been shown to have biological effects in mammals. For example, lectins can bind to enterocytes in the gut, causing gastrointestinal symptoms. Also lectins can modulate the immune systems of experimental animals and may be allergenic. The role of lectins in pathological conditions is of interest to nutritionists, and a number of studies have investigated the role of plant lectins in human metabolism and nutrition.
Because of their wide ranging effects in plants, it is though that plant lectins may have wide ranging effects in humans. Their ability to bind to the gastrointestinal tract has resulted in speculation that they may be involved in some cases of celiac disease or inflammatory bowel disease. However, lectins appear to be very widely distributed in edible foods, and most people do not seem to be adversely affected by reasonably high intakes of plant lectins. A good example of a high lectin food in human nutrition are legumes, which have a very low nutritive value on account of the very high content of lectins. Recommendations are therefore to cook legumes, a process which denatures the lectins causing them to become biologically inert. However, reports suggest other commonly eaten foods contain plant lectins, many of which are commonly eaten raw. For example, one study reviewed the nutritional literature to identify edible plant sources of lectins relevant to human nutrition1.
The list or foods that included significant amount of lectins included tomatoes, potatoes, string beans, carrots, zucchini, green peas, soybean sprouts, mung bean sprouts, lentil sprouts, cantaloupe, grapes, cherries, pomegranates, raspberries, blackberries, wheat germ, corn, wheat bran, garlic, marjoram, allspice, peanuts and mushrooms. With regard to mushrooms, the lectins in certain varieties of mushrooms are now recognised as having beneficial immune system modulatory effects in humans, and research is ongoing in this area. In particular the lectins in mushrooms may have particular cancer inhibitory effect by specifically adhering to cancer cells and stimulating immune system recognition of the cancer cells. This research has called into question the traditional view that lectins are disease causing agents. The widespread nature of lectins in our natural diet suggests that lectins are inert at worst, but may be highly beneficial as disease preventive agents at best.