Lectins have 2 roles, an immune role and a food storage role.
They are specific carbohydrate binding proteins known for their ability to agglutinate erythrocytes (red blood cells) in vitro. When ingested, they are inflammatory, can cause diarrhoea and interference with nutrient absorption, they stimulate class II HLA antigens on cells that do not normally display them and as such can be involved in auto-immune disease mechanism (Type 1 diabetes, Rheumatoid Arthritis, Coeliac’s…). They are classed as anti-nutrients.

Different lectins have different levels of toxicity, and not all lectins are toxic. Some have indeed been researched and used to enhance the effectiveness of cancer drugs in specific cancers. They are present in most plants, especially seeds and tubers like cereals, potatoes and beans. They are common in the nightshade family of foods: potatoes, tomatoes, aubergines and peppers. Pulses are a particularly high source with castor bean having one of the deadliest type of lectin: Ricin. This one can kill humans fairly quickly.

Plants create a whole communication network with the various micro-organisms found in the soil in order to respond quickly to predator threat. Plants also communicate with each other, one plant will deliberately lower its lectins to become a target to the predators in order to give time for the other plants to increase their levels of lectin enough to be safe from attack. When trees are being devoured by an overgrown population of insects, they wake up fungi that will specifically decimate these insects. And so the cycle carries on and nature always finds overall balance when left to its own device.

In the plant, the expression of lectins is up-regulated in response to biotic (living organism) or abiotic (eg wind, frost) stress. From damaged self or from pathogens (Lipopolysaccharides-LPS), the damaged cells now have exposed sugars that the lectins will bind to. This then triggers a signalling cascade that alerts and triggers the plant’s immune system.
As the plant is growing, the levels of lectin increase steadily to a level where the plant can then fully respond and interact with its environment. It self-regulates the levels until the point at which the seeds are mature. Until then, the levels of lectins will be kept high throughout the plant to stop predators from eating the fruits or seeds before they are mature enough for reproduction. This is why eating an unripe green apple will give you tummy ache.
When the seeds are mature, they have a skin that is resistant to digestive enzymes and a reserve of lectins which acts as nitrogen storage for the plant to start growing on its own and develop roots. The plant also uses colour to signal to the predators that it has dropped its lectin levels and the fruit is now edible. If the skin is broken when the seed is eaten, it can be digested by the predator, however unbroken, the seed tends to pass intact at the other end and hopefully can now find its way into the soil. It feeds on its own nitrogen/lectin reserve to sprout and grow roots.
The plant, now devoid of its seeds, tends to concentrate levels of other anti-nutrients such as phytic acid and oxalic acid. These can also be challenging as they interfere with mineral absorption. Oxalic acid binds to calcium to form calcium oxalate, 80% of kidney stones are calcium oxalate based. People who are lacking Oxalobacter bacteria in their microbiome are more susceptible to kidney stones as the bacteria stops the oxalic acid from binding to the calcium and therefore prevents stone formation. Oxalic acid in high amount is toxic to humans as we cannot detoxify it efficiently. Mature green leafy vegetables tend to be a high source, the darker, the older the leaf, the higher the content, eg: kale, cabbage, spinach, dandelion. Fermenting or cooking are efficient ways to disable those anti-nutrients. Rhubarb leaf is one of the highest source of oxalic acid and is therefore not edible, even if cooked or fermented!

Once the seed has formed its roots, it can now extract nutrition from the ground, the sprouted leaves allow photosynthesis, the plant is independent and growing. The lectin levels are now at their lowest and will start to gradually increase as the plant grows. This is the most vulnerable time for the plant because they are at their most delicious to all the predators. If you ask your slugs and snails, they will confirm. The cycle then starts again, the plant gradually increases its lectin levels for its own protection.

Lectins have different levels of toxicity and are eaten in different amounts according to food type, season and preparation. This explains different levels of reaction from people. Ultimately lectins are meant to cause disruption and trigger the immune system. Studies have shown that food lectins get past the gut wall and can deposit themselves in distant organs. For example, wheat gliadin binds to human intestinal mucosa and glomerular capillary walls in the kidneys amongst other cells. They trigger the immune system, this is not without collateral damage, it causes inflammation and increased leakiness of the gut.
Furthermore gluten triggers the production of Zonulin, a compound that regulates gut leakiness. Studies done by Dr Alessio Fassano have shown a clear correlation between gluten and gut leakiness. Solanines are another anti-nutrient, they are a glycoalkaloid poison found in the nightshade family of foods, these are also thought to increase permeability of the gut wall. This happens for everybody, hence the importance of eating a little of lots of different things. Part of the problem with junk food is that 80% of it is made from just 4 ingredients: highly processed fat, highly refined sugar, refined salt and soy, mostly GMO. To those, chemicals are added for texture and flavour.

Gut dysbiosis where the gut lining is abnormally porous for various reasons allows food particles and bacterial fragments to pass from the gut into the blood stream in excessive amounts and in a form that is potentially not fully digested. This may be a factor in how lectins travel and deposit in distant organs.
Depending on your genetics, the potential risk of developing certain auto-immune diseases such as Coeliac’s disease, rheumatoid arthritis, Ankylosing spondylitis is increased. Fassano’s studies showed that HLA DQ2/DQ8 is expressed in 99.4% of people who develop Coeliac’s disease. Gluten avoidance is currently the only treatment for Coeliac. A trial of gluten avoidance in children with IgA Nephropathy reported reduced proteinuria and immune complex levels. People who are HLA B27 are more at risk of developing Ankylosing Spondylitis (AS). A dietary approach removing starch from the diet of AS patients is currently trialled.
Because lectins are proteins that bind to specific carbohydrates, they can be blocked by simple sugars and oligosaccharides. These natural compounds are potentially exploitable as drugs should lectin induced diseases be identified.

Just like the plant subjected to biotic and abiotic stresses becomes stronger, a little lectin in our diet is manageable and only makes us more resilient to our environment. A leaky gut would change that by increasing the amount of exposure and could become overwhelming. Removing all lectins forever is not the solution, restoring appropriate gut wall permeability is essential.

Plants are now grown in tunnels in cities where the light, the food, the environment is ultra-controlled to make the perfect plant. A plant that has never been stressed will not provide the same nutrition, nor the same challenges to create resilience and health ultimately.

The body has a tolerance level for everything and can cope with a certain amount. That tolerance is determined by many factors and is highly individual, commensal bacteria being one of the major player for tolerance. Certain bacteria will allow a more efficient handling of toxins. There are many examples in the animal kingdom of species that have evolved to tolerate, even thrive in a toxic environment simply by gaining the ability to disable the poisons through their own bacteria. This evolutionary symbiosis can only be created through regular exposure, it is essential to become an integral part of the environment.

Lectins are inherent to the plant kingdom, to understand them is key to health. To avoid them fully is impossible and would leave very little available to eat indeed.
So, in short, eat the plants in season, at the right time. Ferment, soak and sprout, cook when appropriate. Enjoy a variety in moderation. Gut permeability regulation is essential, the microbiome is its biggest ally, inflammation its biggest enemy. Foods are a major determinant of the quality of the microbiome.

And in summary: a young green leaf has low lectin and anti-nutrient levels. The older and the darker the leaf, the higher the levels of both lectins and anti-nutrients. Lectins are also upregulated when the plant is under biotic or abiotic stress. A fruit with unripe seeds has high lectin levels, a fruit where the seeds are mature for reproduction has low lectin levels but the seeds have high lectin levels for energy storage. A young sprouted seed has its lowest lectin levels.

soaking, sprouting, cooking, fermenting are all methods of disabling lectins. Lectins have different levels of toxicity, some plants may require more than one of these processes to make it safe to eat.

And basically, don’t eat green apples with white pips, green tomatoes, old spinach or kale unless you cook them or ferment them first. Use baby spinach or young kale raw in your smoothies.