Lectins

What are lectins?

Lectins are carbohydrate-binding proteins found in plants, animals, bacteria, viruses, and fungi (1, 2).

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What do lectins do?

In plants, lectins act as a chemical defense mechanism against pathogens, insects, and plant-eating animals (3, 4).

In animals, lectins enable cell-cell contact by binding to carbohydrates found on cell surfaces, which causes them to clump together through a process called “agglutination” (5, 6).

They are known for their ability to agglutinate red blood cells, which is why they’re sometimes referred to as “hemagglutinins” (2).

They’re also involved in many different biological processes, including cell growth, cell differentiation, cell signaling, immunomodulation, and host-pathogen interactions (7).

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Are there different types of lectins?

Lectins are typically classified based on the specific carbohydrates they recognize (2). 

These are some common dietary lectins:

• Peanut agglutinin (found in peanuts) (8)
• Phytohaemagglutinin (PHA) (found in beans, especially red kidney bean) (9)
• Soybean agglutinin (found in soybeans) (10)
• Wheat germ agglutinin (WGA) (found in gluten-containing grains) (11)

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Which foods contain lectins?

Plants, especially legumes, grains, and seeds, contain the highest amounts of lectins (2).

One study tested food samples and found lectins in the following foods (12):

• Legumes: Beans, lentils, peas, peanuts, and soy.
• Grains: Barley, corn, oats, rye, and wheat.
• Nuts: Coconut, hazelnuts, and walnuts.
• Seeds: Caraway, grapeseed, pumpkin, sesame, and sunflower.
• Tubers: Beets, potatoes, and turnips.
• Fruits: Avocado, bananas, blackberries, cantaloupe, pomegranate, raspberries, and strawberries.
• Vegetables: cucumbers, green beans, mushrooms, okra, tomatoes, and zucchini.
• Herbs and spices: Allspice, cocoa, garlic, nutmeg, and oregano.
• Coffee

HOWEVER, the researchers noted that there were SIGNIFICANT variations in the lectin content of the same food item purchased from different stores or from the same store on different days, so this data is not very reliable. Much more research is needed (12).

Although many websites list milk and grain-fed meats as high in lectins, we didn’t find any evidence to support these claims. 

Milk is often listed because Dr. Gundry (author of “The Plant Paradox”) recommends that milk containing the casein a1 protein be avoided because it has “lectin-like” properties, but as far as we know, there is no research to back this up.

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How are lectins digested and absorbed?

Most lectins aren’t digested, because they are very resistant to digestive enzymes (1).

In the intestines, some lectins bind with glycoprotein receptors on epithelial cells and are absorbed along the brush border (13). 

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How do lectins negatively impact health?

Most of the following research is based on animal studies using raw legumes or highly concentrated purified lectins. 

To understand the actual impact on health, we need human studies that use conventionally prepared lectin-containing foods.

1. Lectin poisoning

VERY high levels of lectins, such as those found in raw or undercooked red kidney beans, cause food poisoning-like symptoms, including diarrhea and vomiting (14, 15, 16).

Ricin, a lectin found in castor beans, has actually been used in chemical warfare and can result in death if ingested or inhaled in large enough amounts (17).

2. Impaired nutrient absorption

There are animal studies showing that purified PHA (the lectin found in uncooked beans) impairs the absorption of protein, fat, and vitamins by disrupting the intestinal brush border and reducing the absorptive surface area (1, 18, 19, 20, 21, 22).

Purified tomato lectin mixed with tomato juice, on the other hand, didn’t cause damage to the intestinal lining of rats (23).

However, these results may not have any clinical significance in humans since we do not consume raw beans or purified lectins.

3. Increased intestinal permeability

There is limited evidence to support the role of lectins in increased intestinal permeability and it’s based on animal studies using raw legumes and purified lectins (24, 25).

4. Altered gut bacteria

Several studies have shown that small intestinal bacterial overgrowth (SIBO) of E. coli occurs when rats are fed purified kidney bean lectin (PHA) (21, 26, 27).

The mechanism behind this overgrowth isn’t fully understood, but some have theorized that by reducing the absorptive capacity of the intestines, PHA creates an environment rich in nutrients that allows bacteria to thrive (1).

5. Altered hormone signaling

Pancreatic enlargement has been observed in rats fed large amounts of purified lectins and may be due to increased levels of cholecystokinin (CCK), a hormone secreted by the duodenum that is known to stimulate pancreatic growth (1, 28, 29, 30, 31, 32, 33).

There is also some evidence from animal studies showing that lectins interact with insulin receptors and have insulin-mimicking properties, which may interfere with the regulation of insulin production (20, 34, 35, 36)

6. Immune system stimulation

Research shows that lectins stimulate the immune system and antibodies to specific lectins (peanut, soybean, and wheat germ) have been found in humans (37, 38, 39, 40, 41).

Because lectins share certain amino acid sequences with other proteins, including human tissues, allergic cross-reactivity (when similar proteins cause the same allergic reaction) may occur (42, 43).

Theoretically, this could lead to autoimmune disease in susceptible individuals, but there is no actual research on this topic at this time (42, 43).

7. Parkinson’s disease 

Preliminary research in nematodes found that dietary lectins can be transferred between, to, and along dopaminergic neurons, altering their activity, and is proposed as a possible contributing factor to Parkinson’s disease in some people (44).

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What are the health benefits of lectins?

Lectins have a few health benefits, including:

1. May help prevent cancer 

There is strong evidence from animal and in vitro studies that some lectins have the ability to inhibit tumor growth by stimulating apoptosis, and scientists are experimenting with ways to use them in new anti-cancer drugs (45, 46, 47, 48, 49, 50).

This might partially explain why, in observational studies, legume consumption is inversely associated with cancer risk (51, 52, 53, 54).

2. Delayed carbohydrate absorption

High lectin foods like legumes and whole grains typically have a low glycemic index (55).

One study found that kidney bean lectin added to white bread reduced the rate of starch digestion in vitro, which suggests that lectins might be part of the reason that high lectin foods tend to have a lower glycemic index (55, 56).

3. May fight HIV/AIDS 

Many lectins have antiviral effects that may protect against HIV infection in vitro, but research in humans is needed (2).

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What is a lectin-free diet?

It doesn’t actually exist!

Following a diet that is 100% free of lectins is not possible, because most foods contain at least small amounts (2).

Instead, consumption can be reduced by eliminating foods high in lectins or using food preparation strategies to decrease lectin content.

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Who popularized lectin-free diets?

In 2017, Dr. Steven Gundry, a cardiothoracic surgeon, wrote a book called “The Plant Paradox: The Hidden Dangers in “Healthy” Foods That Cause Disease and Weight Gain” (57).

His “Plant Paradox Protocol” diet requires that the following foods be eliminated (58):

• Grains and pseudo-grains
• Legumes (including beans, soy, and peanuts)
• Cashews
• Melons
• Nightshade vegetables
• Squash
• Milk products containing casein A1
• Grain-fed or soybean-fed meats

In 2018, Gundry conducted a study of 102 patients with autoimmune disease and found that 95 of those patients experienced “complete resolution” of autoimmune and inflammatory markers after 9 months on a lectin-limited diet (59).

He has presented these findings, along with several other studies, at conferences but none have ever been peer-reviewed or published in full (58, 59, 60, 61).

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Are low-lectin diets ever warranted?

There is very weak evidence (with a high potential for bias) to suggest that reducing consumption of foods high in lectins (grains, legumes, etc.) as part of an elimination diet may be useful for patients with autoimmune disease (59).

Theoretically, a low-lectin diet might be helpful as part of a gut-healing protocol, because it eliminates the negative effects lectins might have on intestinal cells and gut microbiota. 

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Does blood type impact sensitivity to lectins?  

There have been claims that people should avoid different lectin-containing foods based on their blood type.

This comes from the idea that certain lectins react with the carbohydrates found on the surface of red blood cells.

But most lectins actually react with all ABO blood types, and the small amounts we get from food aren’t enough to cause significant issues in our blood (12).

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What are some strategies for reducing lectins in food?

If you want to reduce the amount of lectins in your food, try some of the following strategies:

1. Soaking legumes

Legumes soaked in water for 12 hours result in complete elimination of lectin activity when cooked (62).

Unsoaked legumes, on the other hand, only show a 58-96% reduction in lectins after cooking (62).

Adding baking soda to the soaking water has often been recommended, but this doesn’t seem to reduce lectins more effectively than plain water (62).

2. Sprouting legumes

There is conflicting evidence regarding the effects of sprouting on lectins.

One study found that sprouting soybeans for 42 hours at 77°F resulted in a 60% decrease in lectin activity, but another study showed an INCREASE in lectins after 24-96 hours of sprouting (62, 63).

During germination, lectins are typically broken down into amino acids to be used by the growing seedling (64).

3. Cooking legumes

Cooking unsoaked beans in water significantly reduces, but does not completely eliminate, lectins (62).

To inactivate all lectins, legumes must first be soaked, then heated in water to reach a temperature of 212°F for at least 10 minutes (65).

This can be achieved through boiling on the stove or using a pressure cooker.

Make sure that beans are fully cooked because partially cooked beans actually have an even higher lectin content than raw beans (15).

Dry heat cooking methods (baking, roasting, etc.) aren’t effective at destroying lectins (65).

4. Fermenting legumes

There is limited evidence that fermentation reduces the lectin content of legumes by very small amounts (66, 67).

One study found that lectins decreased by 19% when legume flour was mixed with water and allowed to ferment naturally for 48 hours (67).

5. Removing seeds from fruits and vegetables

The highest concentrations of lectins are usually found in plant seeds, so removing the seeds from fruits and vegetables (tomatoes, cucumbers, etc.) is one way to reduce your lectin intake (2).

6. Refining grains

Lectins in grains are mainly found in the bran and germ, so choosing grains that have been refined (white rice, white bread, etc.) is another way to reduce lectin intake (68, 69, 70).

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Can supplements help reduce lectin absorption?

Maybe, for certain lectins.

Lectin shield is a supplement designed and promoted by Dr. Gundry that he claims will bind the lectins in your diet so they don’t interfere with your digestion, energy, and overall health.

The main ingredient, N-acetyl D-glucosamine, has been shown to bind dietary lectins (wheat germ agglutinin), but this research was in vitro, not in humans (75, 76).

The other lectin-binding ingredients, such as D-mannose, sialic acid, okra fruit, and larch arabinogalactan, have been shown to bind to specific types of lectins that are found in humans, microbes, and poisonous plants, but not common dietary lectins (71, 72, 73, 74).