1. What are lectins?
2. What foods contain lectins?
3. Don't we digest lectins?
4. System wide effects of lectins
5. Conclusion
There's a lot of controversy today about grains like wheat and rye. It doesn't take long to find conversation on whether whole grains are better or worse for you, where you should get your fiber, or whether or not you need your six to eleven servings of whole grains in order to promote heart health. Despite all of the debate of the previously mentioned topics (which I hope to touch on in the future), the point of this blog is to discuss lectins. Lectins have been studied for decades, although they have been brought to the public eye over the past few years. Dr. Steven Gundry, a cardiac surgeon, published The Plant Paradox in 2017, where he discusses lectins and the effects that lectins may have on human disease.
What are lectins?
Simply put, lectins are defined as plant proteins that are able to bind to sugar molecules that are located on human cells (1). A few examples of foods that contain lectins include kidney beans, peanuts, wheat, lentils, and tomatoes. As you can see, lectins have been and are still a very common component of our modern diet.
Lectins were first discovered by Stillmark in 1888 when he noticed that extracts from castor beans caused the agglutination (clumping together) of red blood cells. Lectins may bind to simple carbohydrates like glucose, galactose, and fructose, although they prefer more complex sugars called oligosaccharides that are present in many organisms including humans (1). So, we know that lectins like to bind to sugar molecules, but where are these sugar molecules located? They are found on the surface of the cells that line the gastrointestinal tract, from mouth to anus, especially in the small intestines.
Before we get too far ahead, know that lectins can be classified into 4 main groups, based on what molecules they prefer to bind to.
1. Legume lectins: These are found in peanuts, lentils, kidney beans, and peas. Most of their lectin concentration is in the seed (like the bean of the plant that we eat), and these lectins are in large part destroyed or inactivated through the cooking process (1).
2. Mannose-binding lectins are present in shallots, onions, leeks, and garlic. Many of this type of lectin are able to withstand high temperatures, meaning cooking does not destroy or inactivate these lectins, which we consume when these plants are eaten (1).
3. Chitin-binding lectins are found in crops like wheat, tomatoes, and potatoes although in relatively low concentrations. Additionally, these are stable at high temperatures which means they are likely present even after cooking.
4. Type 2 ribosome-inactivating proteins (RIP) are the type of lectin found in elderberry and castor beans. The lectin in castor beans are potentially lethal if ingested, however properly processed castor beans through heating eliminate the potentially harmful lectins (1).
What foods contain lectins?
Although I eluded to a few foods above that contain lectins, let me provide a more expansive list before we move on so you have a good idea of the foods that contain the effects that we will discuss as we go. As of 2009, there were 119 known dietary lectins, although not all identified lectins interact with humans the way that they interact with bacteria or other animals (4). The list below is taken in large part from Dr. Gundry's "no list" based on foods that people should avoid that contain plant lectins (5).
Refined starchy foods:
Pasta, rice, potatoes, chips, milk, bread, tortillas, pastry, flours from grains, cookies, crackers, cereal, sugar, agave, splenda, Sweet n Low, diet drinks
Vegetables: tomatoes (unless peeled, deseeded), cucumbers (unless peeled, deseeded), peas, sugar snap peas, legumes, green beans, chicke peas, soy, tofu, edamae, sprouts, lentils
Nuts & Seeds: pumpkin, sunflower, chia, peanuts, cashews
Fruits & Veggies: all fruits except in season fruit, ripe bananas, zucchini, pumpkins, squashes, melons, eggplants, bell peppers (unless peeled, deseeded)
Non-Southern European Cow's Milk Products (A1 Dairy) -see blog on this
Greek yogurt, yogurt, american cheese, ricotta, cottage cheese, kefir, casein protein powders
Grain or soybean-fed fish, shellfish, poultry, beef, land & pork
Grains: Whole grains, wheat, oats, quinoa, rye, blugur, brown rice, white rice, barley, buckwheat, kashi, spelt, corn, popcorn, wheatgrass, barley-grass
Oils: Soy, grapeseed, corn, peanut, cottonseed, safflower, sunflower, partially hydrogenated vegetable or canola
Don't we digest lectins?
While our gastrointestinal tracts are able to break down many molecules, lectins can be very resistant to acid and enzymes lending to their inability of being digested in the digestive tract (1). Roughly 90% of kidney bean lectins and 60% of elderberry lectins were recovered in their active form after being exposed to the digestive tract (1). The result is that many lectins are able to remain undigested when passing through the digestive tract. What this means is that these lectins are able to interact with the cells that line the gastrointestinal tract by binding to the carbohydrate (sugar) molecules on the intestine cells that they have an affinity (a liking) towards (1). The sugar molecules that lectins can bind to in the gastrointestinal tract are located on hormone receptors, growth factors and cytokines, transport proteins, brush-border membrane enzymes, glycolipids, gangliosides, and mucins (1). All this means is that there's a lot of nice places for lectins to bind to the intestines.
Recap: lectins are proteins found in plants like wheat, kidney beans, peanuts, and tomatoes that bind to sugar molecules. The sugar molecules that they like to bind to are found throughout the human digestive tract, especially on the intestines. So why is this worth talking about?
System wide effects of lectins
To start, we mentioned that lectins can bind nearly anywhere through the gastrointestinal tract from mouth to anus, but most of the binding occurs in the small intestines due to the nature of the carbohydrates expressed on the cells in this region (1). When a lectin binds to the surface of an intestinal cell, it is able to communicate with the cell and initiate a response.
This response alters gene expression so that the cell produces molecules like cytokines, growth factors, hormones, and enzymes that may elicit a system wide effect (1). Furthermore, these inflammatory and endocrine molecules created by the effected cells can interact with neighboring cells, eliciting a local effect (1). Lectins not only interact with the surface of cells, they may be taken up into the inside of cells through a process called endocytosis and create a biological response (1).
The systematic responses to lectins are vast and impressive. Wheat Germ Agglutinin (WGA), the lectin from wheat, has been shown to be able to activate NF kappa beta proteins which are associated with both acute and chronic inflammatory disorders including neurodegenerative diseases, autoimmunity, and inflammatory bowel diseases (4). Soybean agglutinin, the soybean lectin, has been demonstrated to increase intestinal growth, destruct the spleen and kidney, and reduce insulin levels (4). Lectins are also able to decrease natural killer (NK) cells by disrupting the intestinal microbiome, which are a huge component of the body's defense system to pathogens like viruses.
System wide, lectins are able to alter the metabolism of carbohydrates, proteins, and fats, enlarge internal organs and tissues, as well as alter hormones and immune system function (2). Not only may they possess these effects towards humans, lectins are also considered insecticidal to certain insect pests although much less is known about this area of study (2).
So if you're tracking with me, you get the idea that lectins are capable of producing a lot of potential effects throughout the body! Although the first place that lectins interact with is the digestive tract, their effects do not end there. They are able to be taken up into the general circulation and elicit effects elsewhere in the body ranging from joints to other organs. Lectins are able to impart an effect on the immune system, endocrine system, digestive system, and are thought to play a role in many different disease processes. In the following blog, we will dive into a few different organ systems and diseases and see how lectins might play a role in their aggravation or development, as well as cover steps on how to decrease lectin consumption.
Conclusion
Lectins are plant proteins found abundantly in our modern diets. Sources of lectins include wheat, corn, tomato, and kidney beans among many others. We are not able to easily digest lectins, and these lectins like to interact with sugar molecules located in the gastrointestinal tract, specifically in the small intestines. After binding to and interacting with our intestinal cells, they are able to initiate a wide variety of system wide effects including initiating the immune system and altering the endocrine system. In the next blog series, we will cover a vast array of diseases, how lectins influence the health of your body, and how to decrease your dietary lectin consumption. In Part 2 we will discuss the specifics of how lectins cause inflammation.
1. Aricigil, Selma and Pryme, Ian F. 2015. Potential Beneficial Effects of Dietary Plant Lectins on Health. Natural Products: Research Reviews. 2: 1-27.
2. Vasconcelos, IM and Oliverra, JT. 2004. Antinutritional properties of plant lectins. Toxicon. 44(4): 385-403.
4. Hamid, Rabia and Masood, Akbar. 2009. Dietary Lectins as Disease Causing Toxicants. Pakistan Journal of Nutrition. 8(3): 293-303.
5. Dr. Steven Gundry. 2017. Dr. Gundry Approved Foods (a print-friendly list). Retrieved: May 20, 2019. From: https://gundrymd.com/plant-paradox-shopping-list/
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