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Ask Tufts Experts May 2017 Issue

Q. What are lectins, and should we avoid beans, seeds and grains since they contain lectins?

A. George S. Ellmore, PhD, an associate professor in the department of biology at Tufts University, responds: "A lawyer might answer this question with, 'It depends.' Lectins are proteins from plants used to store nitrogen and provide instant defense against animals trying to eat the plant since they can cause digestive upset, particularly when eaten raw. Lectins are found in roots, stems, leaves and seeds of many plants. The part of these plants that humans commonly eat is the seed (from a botanical standpoint). Lectins have been detected in the seeds of about 500 species of plants, including legumes (beans), nuts, oilseeds (such as sunflower seeds) and grains, such as wheat, rice, barley and rye.

"Once eaten by an animal, lectins bind to the sugar portions of the animal's intestinal wall where they interfere with digestion and nutrient uptake. That is why lectins are sometimes called ‘antinutrients.’ However, after edible seeds are prepared by methods using heat [especially moist heat], they contain very little lectin. Yet, they provide concentrated packets of protein, fat, carbohydrate and fiber, all of which are nutritionally beneficial.

"For example, raw soybeans contain 10 to 20 grams (g) of lectin per kilogram (kg) of soybeans, but when heat-processed to make soybean meal or soymilk, lectins drop to 2 to 4 g per kg. Surprisingly, at that low level, lectins improve the health of the gut microbiota [bacteria and other microbes] in ways that may ease food allergies and prime the human immune system.

"While raw seeds use lectins to resist pests, cooked beans and grains have supported the diets of some of the longest-lived humans on the planet. Therefore, we should celebrate these heat-treated foods in our diet."

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