Lectins, a possible cause of diverse digestive issues

A few months ago, a YouTube subscriber asked me to make a video about why I removed the seeds from tomatoes before eating them so in this post I'll explain why. This is a blog about nutrition (and recipes), but today I'll talk a little bit about botany.

Plants and their defense mechanism

Plants are living organism and as well as other living creatures they don't want to die. Unlike animals, plant don't have an immune system to defend themselves from virus or bacteria and they can't move to escape from predators.

To defend themselves from hazards they have two types of defense mechanisms, passive and actives (1). An example of passive defense is when a plant suffers from an infection and in response it generates a necrosis or death of the tissue around the infection, so it doesn't spread. An example of active mechanism is the production of toxic proteins that make sick the ones who dear to eat them.

One of these toxic molecules are lectins, which have a great affinity for carbohydrates, they can detect them and adhere to them. Luckily, there are not many carbohydrates in the body, but the ones there are, are important.

Carbohydrates in the human body

In the cellular wall there are a few glycoproteins and glycolipids, the word glyco comes from glucose which is a simple carbohydrate. Therefore, a glycoprotein or glycolipid is the combination of a carbohydrate plus a protein or lipid, respectively. Having a carbohydrate makes these molecules a perfect target for lectins which adhere to them.

If you remember from the last blog post about the digestive system, it's covered by a mucosa layer which protects us from what we ingest (pathogens and other things).

Unfortunately, this mucosa is made from a glycoprotein called mucin. Mucins form gelatinous liquids, in fact, other body secretions like saliva or vaginal fluids also have mucins in them.

This mucosa layer is more important in the intestine because here is where we find the connection to the rest of the body since here is where the nutrients go to the bloodstream.

In theory what enters the intestine, should come predigested by the stomach acids and the digestive enzymes (you can learn more about enzymes here), but lectins are a though nut to crack and they go straight to the intestines.

Lectins, a though nut to crack

One of the foods with the highest lectin content are beans and only in the United Kingdom there are approximately 100 documented cases of severe diarrhea and vomits due to eating raw or not well-cooked beans (2).

Lectins are heat-resistant, and that is why you need to soak legumes overnight and then cook them thoroughly. Legumes are well known for causing bloating and farts and lectins are the ones to blame. The other issue with legumes is that they contain some highly fermentable compounds that bacteria in the intestine eat. These compounds are known for their acronym FODMAP which stands for fermentable oligo, disaccharide mono and polyols. As you might know if you like beer, an end product of fermentation are gases, those feasting bacteria ferment the undigested food and you end up bloated.

Additionally, to being heat-resistant, lectins are also resistant to proteolysis (protein degradation). As I told you before lectins are proteins and as well as other proteins, they should get broken down into the amino acids that make them, thanks to the stomach acids and the digestive enzymes, but they don't.

What scientist have seen in animal and human models (2) is that only about a 16% to 80% of the lectins in legumes and seeds are digested, compared to the proteins in animal products that are digested between a 85 to 94%. In animal experiments they even die after severe diarrheas and lack of appetite (2). Don't give legumes to your pets!

How lectins damage the digestive tract

As I said the intestinal mucosa is made of glycoproteins which make them a perfect target for lectins and since the stomach acids and the digestive enzymes haven't been capable of breaking them down, they get to the intestine in perfect shape to disrupt the proper functioning of the intestines.

One of the damages lectins do on the intestinal wall is to wear down the microvilli, they shorten them. In the following image I tried to draw what happens.

If the microvilli get shortened, then the efficiency to absorb the nutrients is also shortened (if you want to understand this better go to the post The wonderful digestive system). So, lectins have a negative affect on the nutrient absorption, and this is why people with celiac disease often show certain mineral and vitamin deficiencies before being diagnosed with celiac disease. I haven't told you yet, but gluten is actually a lectin and someday I'll explain more things about gluten, but for now just know it's the most famous lectin out there.

The other harm lectins do is l otro daño que causan las lectinas lo hacen después de atravesar la barrera protectora del intestino. La barrera está allí para protegernos de patógenos que vienen en la comida y los nutrientes pueden pasar a través de la barrera porque la barrera los reconoce y los deja pasar a través de ella, a este proceso se le conoce como endocitosis.

Somehow lectins get through the barrier, the mechanism by which gluten does this was proposed by the researcher Alessio Fassano and it quite aggressive.

Do you remember the tight junctions from the blog post of the digestive system? These are like seams that keep together the intestinal wall’s cells. Gluten rips off those seams and separates the cells, this way the inside of the body gets exposed to pathogens as well as undigested food that shouldn’t go to the bloodstream. This puts us at risk from suffering infections and inflammation. If the word inflammation doesn’t ring a bell, you should read the blog post What is and what causes inflammation, where you'll see that almost every disease is caused by a underlying inflammation.

If you don't believe what I'm saying, check how the blood test for food intolerances are done. What they measure are the antibodies generated against things that shouldn’t be in the blood. If they find antibodies for gluten for example, it means that there is gluten in the blood.

There are other negative effects produced by lectins, for example the inhibition of certain enzymes in the intestines, if you want to go in depth about this, check the scientific paper number 2, the reference is at the end of this post. There are more papers about lectins, one of them was pretty interesting about its link with Parkinson's disease, but to the date it's just a study in rats, if you want to know more check the scientific paper number 3.

Vegetable sources of lectins

Almost all vegetables have lectins, what's important is to know which ones have more and in what part of the plant are lectins accumulated. As I told you lectins are part of the plant’s defense system, hence what's logic is to find lectins in the skin of the fruits because those protect the inside. Another place where lectins accumulate are the seeds, seeds are like the plant's babies, they are the future of the specie, so lectins accumulate there as well. There are some lectins that don't affect humans, but they do other animals, for example sweet potato only affects rabbits (4).

Another factor to consider is that today's crops are not natural, they are centuries of human artificial selection. To get more profits farmers select the seeds from the plants that are more resilien. If they can resist more, it means they know how to defend themselves and how do plants do that? with more lectins among other things. So, today's crops have more lectins than they used to have.

Here you have a list of plant products that have lectins:

• Tomatoes
• Potato
• Beans
• Green peas
• Carrot
• Soy
• Beer
• Blackberries
• Wheat germ
• Rice
• Corn
• Peanuts
• Mushrooms
• Avocado
• Beetroot
• Leek
• Cabbage
• Tea
• Parsley
• Oregano
• Spices
• Nuts

Other lectin sources

Until now I've just talked about plant lectins, but there are non-plant lectins (5) and not all of them are bad. Actually, humans have lectins, because lectins are pretty useful, they recognize cells by the carbohydrates in them, so they are somehow a detection system.

Non plant lectin sources:

• Bacteria
• Protozoa
• Fungi
• Virus
• Algae
• Animals

Those who come from germs (bacteria, protozoa, fungi, and viruses) are there to let the germ know which cells to attack when they get into a host. There's not much information about the algae ones. In animals they basically help to recognize some kind of cell and then trigger a process.

Conclusions

I got surprised doing this research, I knew lectins were problematic, but I didn't know it was so troublesome. It has all the sense anyway; nature is a give-and-take between those who eat and are eaten, and plants had to have a way to defend themselves.

I think we should lower the consumption of vegetables; they don't provide us with high quality nutrients nor high levels of them and in some cases, they contain antinutrients like lectins, phytates and oxalates.

Even so, I don't think we should eliminate them 100%, we just need to be smart and eat them properly, remove the seed and the skin.

We are not all sensitive to the same lectins, from try and error you should sort which vegetables get along with your tummy. If something gets you diarrhea, smelly farts, vomits, nausea or bloating, there don't eat it.

I don't eat legumes and I won't eat them again in my live since they give nausea, vomits and diarrhea, but I eat other vegetables. The amount is also important if I eat half an avocado I end up like balloon, but if I eat a fourth of an avocado once in a while, I'm fine.

As I always say, listen to your body, learn what's best for it, it'll let you know. Believe me, mine is pretty good at giving me stomach aches and smelly farts hahaha.

This post is long enough, until next time!

References

1. Peumans, W. J., & Van Damme, E. J. M. (1995). The role of lectins in plant defence. The Histochemical Journal, 27(4), 253–271. doi: 10.1007/bf00398968
2. Vasconcelos, I. M., & Oliveira, J. T. A. (2004). Antinutritional properties of plant lectins. Toxicon, 44(4), 385–403. doi:10.1016/j.toxicon.2004.05.005
3. Singh, R. S., Tiwary, A. K., & Kennedy, J. F. (1999). Lectins: Sources, Activities, and Applications. Critical Reviews in Biotechnology, 19(2), 145–178. doi: 10.1080/0738-859991229224
4. Nachbar, M. S., & Oppenheim, J. D. (1980). 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(11), 2338–2345. doi: 10.1093/ajcn/33.11.2338