“We calculate that 99.99% (by weight) of the pesticides in the American diet are chemicals that plants produce to defend themselves….We estimate that Americans eat about 1.5g of natural pesticides per person per day, which is about 10,000 times more than they eat of synthetic pesticide residues.”

From the paper Dietary pesticides (99.99% all natural)

This is part seven of my series on small intestinal bacterial overgrowth (SIBO) and a continuation of the previous post on dietary factors predisposing to it.

So what are plant lectins and why should you care?

Lectins are sugar-binding proteins that have a high affinity for the mucous surfaces that line our digestive tract, the cells that comprise the gut wall and our beneficial gut flora. They also have a high affinity for other cells in the body when they cross the gut wall.

The word lectin comes from the Latin verb “to choose.” Many plants produce these substances to avoid being destroyed by fungus and bacteria or being eaten by insects and animals, and that includes us.

Many people are largely ignorant of the fact that plants are not helpless organisms just waiting to nourish us. Unlike animals that can bite, claw, gore, run, slither, hop, swim or fly away when threatened, plants can’t do any of these behaviors. So over millions of years of evolution, mother nature has given them highly advanced and effective defense mechanisms, some visible, some not. Lectins fall into the latter group. They are part of the 99.99% all-natural pesticides mentioned in the above quote.

Lectins exist in many foods. However, many of these are harmless to humans or readily inactivated by cooking. But there are two families of lectins that have toxic effects in humans if they appear in our food.

Lectins found in legumes (beans and peanuts) and gluten grains are the most problematical. Whole-gluten grains like whole-wheat is a particularly rich source of a lectin called wheat-germ agglutinin or WGA for short. WGA is found in highest concentration in the germ of the plant. This makes the most evolutionary sense as the seed of the plant contains the genetic material necessary to propagate the species.

Lectins have shown very active biological properties when studied in animals and humans and entire books have been written on the subject. It is not my intention to summarize all these findings here. Rather, I want to concentrate on those factors that are directly relevant to acquiring small intestinal bacterial overgrowth.

But first, let me clear something up. Recently, there has been talk in the “Paleo Diet” and health blogosphere that concerns about lectin toxicity in the diet are overblown and should not unduly worry anyone because cooking inactivates them.

Yes, wet-heat cooking does indeed inactivate these pesticides. Subjecting beans to prolonged, high-heat cooking renders them a harmless and nutritious, albeit fiber-rich and gassy, food. And subjecting wheat-based foods to wet heat as in boiling pasta or noodles also inactivates WGA. So far so good.

However, dry-heat cooking methods as exists in baking or breakfast cereal manufacturing does not seem to inactivate them.

In 1980, an article was published in the Journal of Clinical Nutrition that sought to quantify just how prevalent lectins were in the United States food supply. Entitled Lectins in the United States Diet it stated:

“The survey of the edible portions of fresh and processed foods reported here found lectin activity in about 30% of the food stuffs tested, including such common foods as salad ingredients, fruits, spices, dry cereals and roasted nuts. Moreover, a review of the literature uncovered 53 additional edible plants in which phytohemagglutinins have been identified. While, in most cases, the significance of the latter is somewhat obscured since the nonedible parts of the plant were tested, nevertheless, it is quite apparent that exposure to dietary lectins is a frequent and widespread event.

Although both cooking and the normal digestive processes might be expected to blunt or abrogate dietary lectin activity, this need not necessarily be the case. Liener has pointed out that dry heat may not completely destroy lectin activity. This phenomenon is clearly illustrated in the finding of hemagglutinating activity in the processed wheatgerm, peanuts, and dry cereals that we tested. Similar findings for wheat germ have been presented by Brady et al. In addition, several of the lectins have been found to be resistant to proteolytic digestion e.g., wheat germ agglutinin, tomato lectin, navy bean lectin and when looked for, have been recovered intact in stool. It can be concluded that at least some lectins in foodstuffs will survive one or both degradative processes to interact with cells, secretions, and mircoflora of the digestive tract resulting in, as yet unknown, functional consequences.”

This exposure has undoubtedly increased since this research was first conducted because of the dramatic increase in the availability of processed whole-gluten-grain food products and the incessant recommendation by dietitians, nutritionists, doctors and other health “experts” to eat more of them. It’s almost impossible today to buy a wheat-based cereal that isn’t whole-grain.

In a paper found on the Whole Grains Council website entitled Are We There Yet? Measuring Progress on Making At Least Half Our Grains Whole we learn that:

  • Consumption overall rose 20% from 2005 to 2008, after remaining steady from 1998 to 2005.
  • 18 to 34 year olds, as a group, increased the most, with consumption rising 38% from 2005 to 2008.
  • 60% of Americans consumed at least one whole grain product during a typical two-week period in 2008, up from 35% in 2006.

These figures apply to all whole-grains, not just gluten grains, so the percentage increase in these grains would be lower. In this paper, we also learn that of the products available on the market in 2008, granola, hot cereals, cold cereals, granola bars and bread where the items most likely to be eaten in whole-grain form.

The reality is that no one has any clue whatsoever as to how much of this toxic crap is in the food supply, but apparently the folks at the Whole Grain Council don’t care. The increase in whole-gluten-grain consumption is absolutely fabulous news to them! Otherwise their website wouldn’t cite all those golly gee wilikers epidemiological and prospective-cohort studies.

I just want to interject here that unfortunately for whole-grain advocates, these types of studies are by their nature incapable of proving anything other than that it’s relatively easy to hoodwink the general public and news reporters. That is why studies like these are carefully worded to say whole-grain consumption is associated with this or that positive health outcome.

These types of studies are inherently rife with confounding variables and sketchy food questionnaires filled out by participants who are apt to stretch the truth or simply can’t remember what they had for breakfast over the past two years, as if anyone could.

Association is not causation. Only controlled clinical trials can determine whether eating whole grains is truly healthy. Anyone who claims differently is being disingenuous to put it charitably.

Rant over.

I need to make one other clarification before proceeding. The findings I refer to were conducted in animals using raw bean or wheat lectin which are pretty much interchangeable as they attach to the same gut structures and intestinal flora.

This obviously doesn’t mimic how humans consume these foods in real life. No one is eating uncooked legumes (except for peanuts) or gnawing on a stalk of wheat. If they are they have bigger problems to contend with.

But that doesn’t mean that these research findings are irrelevant. Far from it. While human exposure to these natural pesticides is at far lower concentrations than what was used in animal studies (or at least I hope so), the impact on gut health from low-grade, chronic exposure would be expected to be the same although not manifest as quickly.

In a culture where wheat is the “king” of grains and consumption of processed-wheat products is pervasive, exposure to lectins begins at a very early age and continues for a lifetime in most. Meal after meal, day after day, year after year and the damage begins to pile up.

Lectins, inflammatory gluten peptides, gluten opioids, adenosine and increased zonulin production really make this noxious grain and its siblings a true biohazard of the first order for the gut. Throw in other gut-destroying practices like repeated courses of antibiotics, drugs (both licit and illicit) and alcohol and the negative impact skyrockets.

Lectin’s known effects on gastrointestinal function

Lectins are extremely efficient in attaching themselves to the mucosa of the small intestine. The interior lining of our small intestine is called the sugar coat for a reason. Here, cells and bacteria present a surface coating that consists of sugar molecules attached to either protein (glycoprotein) or fat (glycolipid) creating a kind of “slime” that serves both a protective and attachment function.

In lectin-fed rodents, lectins bind to and strip away the mucous coat exposing the underlying cells to the contents of the lumen. They also inhibit repair by blocking mucus secreting cells from producing this important protective lubricant.

Rodents fed lectins are incapable of properly utilizing the protein content of their diets. If sufficient quantities of lectins are present in their food, the rate of protein breakdown and loss from their body exceeds the amount of protein taken in.

Lectins bind to the epithelial cells of the small intestine resulting in disfigurement and damage to the digestive brush border.

Lectins cause rapid cell division, growth and turnover in the cells lining the small intestine. Because of this, these cells are too immature to properly digest food or maintain proper intestinal barrier function.

In rodents, lectins consistently increase the size of the pancreas.

Lectins bind to and interfere with cells responsible for the production of various gut-satiety hormones. Satiety hormones tell your brain, gee, I’m really full and can’t eat another bite. When they aren’t produced or produced in lower amounts than normal, hunger is a constant companion. I’ll explore this topic in a future series on gut health and weight regulation.

Lectins bind to and interfere with the gut-associated lymphoid tissue or GALT system. This is the gut’s immune system that protects against infections from pathogens.

Finally, lectins have been shown to directly disturb gut flora populations in animals.

I want to put forward the hypothesis that lectins are uniquely toxic to our beneficial gut flora. As disturbed gut flora seems to be a necessary condition for the initiation of infection in the small intestine, this is not a minor issue.

This theory revolves around the fact that lectins, like WGA, have a high affinity for gram-positive bacteria. Beneficial Lactobacillus and Bifodobacterium species are of this type.

In order to explain this, I need to get a bit technical here, but I’ll do my best to simplify this for you.

So what are gram-positive bacteria?

Well, there are two major families of bacteria and they are distinguished by the difference in the structure of the membranes that enclose them. Gram-positive bacteria derive their name from the fact that their membranes will retain a crystal-violet dye when stained in a laboratory thus appearing blue under a microscope. Gram-negative bacteria will not retain the dye and instead appear red or pink.

The reason this is so is because gram-positive bacteria have a thick, exposed cellular wall called the peptidoglycan. How’s that word for a mouthful? Gram-negative bacteria, however, have a thin peptidoglycan layer sandwiched between an inner membrane and an outer membrane consisting of very toxic, inflammatory and potentially deadly lipopolysaccharides or LPS.

It’s these LPS fragments that are increasingly seen by medical researchers as the cause of diabetes, heart disease and other inflammatory disorders once they gain access to our livers and bloodstream in appreciable amounts. The medical term for this is endotoxemia.

Some of the gram-negative bacteria you may have heard of include Camplybacter, E. coli, H. pylori, Klebsiella, Salmonella and Yersinia.

In the following illustration, you can see the thicker and more exposed peptidoglycan layer of gram-positive bacteria on the left (light violet color) versus the thinner and less exposed peptidoglycan layer of gram-negative bacteria on the right:

Courtesy of Medical Microbiology

Bean lectins like phytohaemagglutinin or PHA, which is found in red kidney beans, and wheat germ agglutinin found in gluten grains are specifically designed to attach to cellular structures that express N-acetyglucosamine. If this name sounds familiar, it’s because the word glucosamine is part of it. Glucosamine is what many take to treat aching joints.

Because peptidoglycan has two sugar components with one of them being N-acetyglucosamine, these lectins attach to it rather readily. Because the peptidoglycan area of gram-positive bacteria is large and exposed, these plant lectins readily attach to it.

Conversely, because the peptidoglycan membrane in gram-negative bacteria is thinner and protected behind an outer lipopolysaccharide layer, these lectins do not bind to them or disturb them in any way.

We know this to be the case in regard to wheat germ agglutinin because a 1990 paper titled Alternate Gram Staining Technique Using a Fluorescent Lectin states:

“We have developed an alternative gram staining technique which is simpler and faster, requires fewer reagents, is easier to interpret, and, in our hands, is less susceptible to errors. This technique takes advantage of the selective binding of a lectin, wheat germ agglutinin, to N-acetyglucosamine. This molecule is a prominent component of the peptidoglycan layer found in all eubacteria except Mycoplasma spp. In gram-positive bacteria, the peptidoglycan layer is the outer portion of the cell wall. The exterior layer of gram-negative bacteria is a membrane which covers the peptidoglycan layer. Thus, a large molecule such as a lectin should be able to attach to the peptidoglycan layer of the gram-positive bacteria but should not be able to penetrate the outer membrane and thus could not attach to the peptidoglycan of gram-negative bacteria.”

Very true indeed! However, while the authors of this paper are thrilled with their discovery, I’m not. Why?

Well, the third word in wheat germ agglutinin is of course agglutinin. An agglutinin is a substance that can clump or coagulate other substances like red blood cells and bacteria. Were you to inject WGA directly into your bloodstream it would cause blood clotting by binding together red blood cells.

Bacteria that are agglutinated, whether beneficial or pathogenic, die. They die for two reasons: first because when clumped together they are deprived of space and nutrition to survive. That is why our antibodies agglutinate harmful bacteria as a first step in their clearance from our bodies. Secondly, agglutinated bacteria is seen by our immune system as foreign and destroyed.

The good news is that harmful gram-positive bacteria are likely destroyed when eating any appreciable amount of legume or gluten lectin. The bad news is that these lectins are also harmful to beneficial gram-positive gut flora.

Because gram-negative bacteria are not affected by these lectins, they are more likely to proliferate when beneficial gram-positive gut flora populations are reduced or absent.

In the book Plant Lectins we learn:

“Food and some of its components, particularly the lectins, may directly interact with the bacterial flora or, alternatively, they may indirectly affect bacterial proliferation in the small intestine through interference with the binding of selected species to epithelial tissues. Whichever of these two mechanisms operates, the end result is the potential inducement of selective proliferation of some species of bacteria in the digestive tract, including the small intestine.”

And in animals fed red kidney bean lectin:

“…concurrent with increased toxicity, there is a dramatic overgrowth of Escherichia coli in the small intestine of conventional rats fed on PHA-containing diets…Similar studies with other animal species have fully confirmed the existence of this causative relationship between the presence of PHA in the diet, E. coli overgrowth and toxicity. Although the mechanism of the selective overgrowth and how this affects nutritional efficiency is not clear, one possible mechanism is that the lectin-induced virulence of coliforms [gram-negative pathogens] in the small intestine of kidney bean-fed rats is the result of the elimination of competing species.”

I reiterate again that this is only a hypothesis and has yet to be proven true in humans because no one that I’m aware of has subjected this to clinical investigation. I would like to think this would happen some day soon, but given the immense public and private vested interests involved in the production and sale of wheat and wheat-based products in the United States, I’m not holding my breath.

Nevertheless, I think this hypothesis is highly plausible. Any pesticide designed by nature to protect these plant seeds would do so by disrupting the digestive tract of a predator by simultaneously attacking the mucosa, cells and beneficial gut flora that are responsible for the animal’s digestive capacity, gut-barrier defense and overall health. I can’t think of cleverer way nature could have designed a pesticide than to trash the intestines of a mammalian predator.

Practical applications for your diet

If beans are a large part of your diet, make sure they are cooked at high temperatures for a sufficient length of time. Many people have been made ill eating undercooked red kidney beans. Don’t be one of them. I don’t eat beans in restaurants or out of a can because I have no way of knowing how thoroughly they were cooked. The only beans I eat are those I cook myself, and I would recommend you do the same.

Avoid peanuts as they are high in lectins and dry roasting them does not inactivate these toxins.

As for wheat germ agglutinin, make sure your gluten-food product is subjected to wet-heat cooking. Ready-made processed gluten foods are best avoided. Hell, I think gluten grains are best avoided which is why I don’t eat them. It is possible that sourdough fermentation may neutralize WGA but I haven’t read anything in the literature to that effect. If anyone out there has info on this, please share.

To counter the negative effects of lectins in the diet, consider supplementing with prebiotics and probiotics found in food or supplements. Also consider taking a glucosamine pill with every meal containing beans or gluten grains. This will act as a decoy and bind lectins before they can do any damage to your mucosa, digestive cells and friendly gut flora.

In the next and last post in this series, I’ll talk about treatment options for SIBO…


Bruce N. Ames, Margie Profet, and Louis Swirsky Gold. (1990). Dietary pesticides (99.99% all natural). Proceedings of the National Academy of Sciences USA, (87): 7777-81.

Dalia, A.B. and Weiser, J. N. (2011). Minimization of bacterial size allows for complement evasion and is overcome by the agglutinating effect of antibody. Cell Host & Microbe, 10(5): 486-96.

Hamid R. and Masood A. (2009). Dietary Lectins as Disease Causing Toxicants. Pakistan Journal of Nutrition, 8 (3) 293-303.

Make Half Your Grains Whole Conference. (2009). Are We There Yet? Measuring Progress on Making At Least Half Our Grains Whole. www.wholegrainscouncil.org

Matucci, A., Veneri, G., Dalla Pellegrina, C., Zoccatelli, G., Vincenzi, S., Chignola, R., et al. (2004). Temperature-dependent decay of wheat germ agglutinin activity and its implications for food processing and analysis. Food Control 15(5): 391-395.

Nachbar, M. S. and Oppenheim, J. D. (1980). Lectins in the United States diet: a survey of lectins in commonly consumed foods and a review of the literature. American Journal of Clinical Nutrition 33(11): 2338-45.

Pusztai, A. (1991). Plant Lectins. New York, Cambridge University Press.

Pusztai A., Ewen S.W.B., Grant G., et al. (1993). Antinutritive effects of wheat-germ agglutinin and other N-acetyglucosamine-specific lectins. British Journal of Nutrition, 70: 313-321.

Sizemore, Ronald K. et. al. (1990). Alternate Gram Staining Technique Using a Fluorescent Lectin. Applied and Environmental Microbiology, 2245-47.

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