Food Sensitivities: Lectins

Part 2 of the Food Sensitivity Series

Lectins are a current hot topic on the nutrition front, being blamed for problems ranging from gastrointestinal distress to obesity, diabetes, and autoimmune diseases.  The premise is that grains, beans, and other cultivated agricultural products from the past 10,000 years are relatively new to humans and our bodies have not evolved to digest them (or, rather, their lectins) properly.  Truth, pseudoscience, or something in between?  A complicated topic, so here goes…

So, what are lectins?  Around 1900, they were first described as agglutinins, proteins that cause clumping reactions. In fact, heme-agglutinating proteins are famous lectins that cause red blood cells to stick together and enable the differentiation of the A, B, and O blood types*.[1]  The word, lectin, was coined in 1958 and is biochemically described as a large protein that seeks to bind to a specific sugar or carbohydrate that occurs freely or is bound to another molecule, including another protein.

Over the past 50 years*, research has found that lectins are common, occurring in all organisms.  They facilitate recognition, communication, regulation, protection, and other functions depending on the organism’s needs.[2]  Bacterial and viral lectins* promote their adhesion to host tissues, allowing for the passage of infective agents or bacterial toxins into the organism.[3]  In animals, including humans, lectins also play a role in immunity, cell signaling, and control of cell growth and death.  Lectins play a defensive role in plants, causing severe gut and other reactions against insects and other animals that eat their leaves, roots, fruits, and seeds. In many plants, the lectins are found primarily in fruit and vegetable peels, seeds, and seedlings. 

Whether a food-related lectin will sicken an animal depends on how much is eaten, how often it’s eaten, the age or ripeness of the plant part, and whether the animal has the right conditions or enzymes in its gut to render the lectin harmless.  In humans, some, but not all, lectin proteins are susceptible to being denatured (rendered ineffective) by stomach acid, digested by protein-breaking enzymes, or broken down by gut bacteria.  Evolutionary adaptations in some classes of animals enable them to tolerate or disarm the lectins in a desired food that might sicken another animal group.  For instance, a bird might safely ingest a fruit that a mammal can’t.

 Humans expanded their diets over the past 10,000 years, shifting from hunter-gatherers to farmers, growing fruits, vegetables, and grains as well as raising poultry and livestock.  Over time, they developed ways to make these agricultural foods more digestible—soaking and draining, sprouting, peeling, deseeding, fermenting, nixtamalization, and cooking.  In most cases, applying wet heat by boiling or pressure-cooking works better than dry heat methods like roasting.  Traditional combinations of foods may also lead to lectins binding to sugar or carbohydrate components within a dish, rendering them less troublesome. 

 While most humans tolerate many dietary lectins in moderate amounts, some experience gastrointestinal (GI) upset with nausea, vomiting, abdominal pain, and perhaps diarrhea developing hours after ingestion of the noxious protein.  In some cases, these symptoms persist for several days to weeks despite reinstating a previously tolerated diet.   

So how do lectins wreak their havoc?  Problems arise when a lectin binds with its desired sugar which is already attached to an important protein in the body.  If the lectin binds to a receptor protein (think of it as a lock), the lectin (a key) might replace a legitimate molecule, but still cause a reaction to occur (the key can enter and turn).  Or it might block a legitimate signaling protein, thus preventing an important process (the key enters, cannot turn, but blocks the correct key).  Receptor proteins are designed to bind to hormones or signaling agents and can be fooled by the masquerading lectin.  Fortunately, lectin-binding to a protein site is usually reversible over time.

One example is when certain lectins bind to a specific receptor within the cell membrane of your gut’s epithelial lining, “the key turns,” and a communication molecule, zonulin*, is released.  Zonulin then stimulates loosening of the tight junctions between cells lining the gut, thus “opening the gates” for large molecules like partially digested proteins, bacterial toxins, or even whole lectins themselves to sneak past the epithelial cells and infiltrate the body.[4]   This illustrates one of the pathways to increased gut lining permeability, also called “leaky gut” by the mainstream media.   “Leaky gut” is thought to be responsible for prolonged undiagnosed abdominal discomfort and, perhaps, for chronic inflammation and autoimmune effects*.[5] 

Foods containing the most potent lectins include grains, especially wheat; legumes such as beans, peanuts, and lentils; and nightshade family members such as tomatoes and potatoes.  Four of the most studied dietary lectins are wheat germ agglutinin, soybean agglutinin, kidney bean phytohemagglutinin, and peanut agglutinin. 

Wheat germ agglutinin (WGA) is found primarily in the wheat germ portion of the wheat kernel.  The entire grain is used for whole wheat flour and cereals whereas most of the germ and bran covering are removed before milling white flour.  WGA is resistant to cooking, stomach acid denaturization, and digestion.  Recommendations for sensitive individuals include trying a wheat-free diet or, at least, avoiding wheat germ and whole wheat products.

Soybean agglutinin (SBA) is also resistant to heat for deactivation.  It is found in the skins covering soybean seeds and to a lesser extent in the seed.  Removing the hulls can help and fermentation also seems to reduce SBA concentration, so tempeh and miso may be better tolerated than edamame or tofu.

Phytohemagglutinin (PHA) is found in large amounts in both kidney and cannellini beans and severe GI distress can occur if they are not properly cooked.  For these particular beans, boiling for at least 10 minutes before reducing the heat to finish cooking will deactivate the PHA.  Pressure cooking is also safe for making bean dishes or processing canned beans.  Other dried beans and green beans contain much smaller amounts of PHA and are tolerated after traditional cooking methods.  The lentil lectin is also easily deactivated by cooking.  (I wrote about beans last July.)** 

Peanut agglutinin (PNA) is found in both peanuts and peanut oil.  PNA is resistant to most heating methods except for prolonged roasting or boiling for over an hour.  Consuming raw peanuts or large amounts of peanuts may produce negative GI effects from the PNA.  Drinking an animal-based milk with your PB&J may reduce PNA’s effectiveness by providing another source of that lectin’s desired sugar, galactose to tie it up.

The tomato lectin resides primarily in the seeds and skins.  Removing these helps as does cooking the fruit.  If your garden is overproducing, make tomato sauce, straining out the seeds and skins rather than trying to eat them all in salads.

Potatoes also can bother some folks and their lectin is reduced by cooking.  Eating potatoes every day can add to a lectin load.

The cucurbit lectin is found in melons, cucumbers, pumpkins, zucchini, and summer and winter squashes.  It is found mainly in the seeds and to a lesser extent, the skins.  Removing the seeds or eating very young cucumbers, summer squashes, and zucchinis (immature seeds) helps.

Despite the potential ill effects from lectins, there are wide variations in individual sensitivity to lectins based on dietary choices, age, genetic background, gut microbiome, concurrent medical issues, and medications.  Combinations of foods may worsen or improve GI responses.  A food diary may help sort this out. 

So, are there ways of improving tolerance to lectins?  A thick GI tract mucous layer is protective; however, older men, postmenopausal women, and those who don’t drink enough may have a thinner mucous layer along their GI tracts.  With less trapping and sugar binding of lectins by the diminished mucous, lectins have easier access to cell membrane receptor binding sites.

Certain foods and medications may affect how lectins are dealt with.  Acid-blocking agents reduce stomach acid which impedes your first line of defense, so take only if necessary.  Antibiotics might reduce the good bacteria in your gut which could remove another line of defense, so take them only when your doctor prescribes them.  Avoid eating ultra-processed foods since their very refined ingredients, especially wheat and soy, may make these lectins more available. 

Studies on how to pre-emptively denature, block, or bind injurious dietary lectins[6] in the GI tract are ongoing. Some involve using whole foods (stock made with bovine cartilage, conch, or certain mushrooms) or supplements made from shellfish shells containing N-acetyl glucosamine.  Consuming these products may nourish the good intestinal bacteria, improve the mucous layer, and possibly bind some of the heat and acid resistant lectins like WGA.[7]  Further investigations could look at other simple and complex dietary sugars found in seaweed, okra, cranberries, and other fruits that can be used in dishes to bind lectins before they are eaten.[8]  (Unfortunately, some of those last foods are FODMAPs.**)

With all the negative media attention, one might be convinced that all dietary lectins are problematic.  On the contrary, some food-based lectins have been found to have antioxidant properties, positive immunological (antiviral and antibacterial) attributes, and toxic effects on tumors.[9]  It is thought that some Traditional Chinese Medicine herbal remedies work to prevent or reduce bacterial or viral infections by using plant lectins to bind these microbes.  Further studies are needed to elucidate whether more beneficial abilities can be harnessed.  

In the meantime, if some of the lectin-containing foods bother you, try the recommendations.  Keeping a food diary allows you to notice trends.  Follow along with my series on Food Sensitivities to find out more.  And, as usual, eat a varied diet and consume each food in moderation.

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[1] Victor Nizet, Ajit Varki, and Markus Aebi, “Microbial Lectins: Hemagglutinins, Adhesins, and Toxins,” in Essentials of Glycobiology, ed. Ajit Varki et al., 3rd ed. (Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press, 2015), http://www.ncbi.nlm.nih.gov/books/NBK453032/.

[2] Nathan Sharon and Halina Lis, “History of Lectins: From Hemagglutinins to Biological Recognition Molecules,” Glycobiology 14, no. 11 (November 1, 2004): 53R-62R, https://doi.org/10.1093/glycob/cwh122.

[3] Nizet, Varki, and Aebi, “Microbial Lectins.”

[4] Alessio Fasano, “Zonulin, Regulation of Tight Junctions, and Autoimmune Diseases,” Annals of the New York Academy of Sciences 1258, no. 1 (July 2012): 25–33, https://doi.org/10.1111/j.1749-6632.2012.06538.x.

[5] Karin de Punder and Leo Pruimboom, “The Dietary Intake of Wheat and Other Cereal Grains and Their Role in Inflammation,” Nutrients 5, no. 3 (March 12, 2013): 771–87, https://doi.org/10.3390/nu5030771.

[6] Martin Nachbar and Joel Oppenheim, “Lectins in the United States Diet: A Survey of Lectins in Commonly Consumed Foods and a Review of the Literature,” The American Journal of Clinical Nutrition 33 (December 1, 1980): 2338–45, https://doi.org/10.1093/ajcn/33.11.2338.

[7] Anna Shmagel et al., “The Effects of Glucosamine and Chondroitin Sulfate on Gut Microbial Composition: A Systematic Review of Evidence from Animal and Human Studies,” Nutrients 11, no. 2 (January 30, 2019), https://doi.org/10.3390/nu11020294.

[8] Hamid and Masood, Dietary Lectins as Disease Causing Toxicants.

[9] Rabia Hamid et al., “Lectins: Proteins with Diverse Applications,” Journal of Applied Pharmaceutical Science, n.d., 11.