The Lectin-Free Diet: Are Lectins Bad For Your Health?

Ross Carver-Carter Relationship counsellor for humans and their microbes

A growing and vocal movement are claiming that lectins, a protein found in many plant foods, are responsible for everything from inflammation and obesity to autoimmune diseases and even cancers. Find out if lectins are harmful to your health and whether there is any evidence for the benefits of a lectin-free diet.

Table of contents:

What is a lectin?

Lectins are proteins that bind with carbohydrates (such as sugars), like lego pieces clicking together. The word “lectin” originates from the Latin word “lego”, which means “to choose”. They are a type of “anti-nutrient”- a category fibre belongs to as well- and are found in all plant foods; beans, grains and pulses have the highest concentrations in their raw state.

When high levels of some active lectins are consumed, they induce nausea, vomiting, gas and stomach pains. In animal and in-vitro studies, they have been shown to bind with the intestinal wall of mammals and communicate with cells, triggering an immune response. Other animal studies have also demonstrated their ability to reduce nutrient absorption. To date, there have been very few human studies.

It is thought that lectins evolved in plants to defend against animal predation; it is tempting to joke that whilst our mothers might want us to eat our greens, our greens don’t want to be eaten.

The lectin-free diet

The anti-lectin diet was popularised by the US cardiologist Stephen Gundry in his New York Times Bestselling book The Plant Paradox. Gundry cites lectin-rich plant foods as a major cause for everything from obesity and inflammation to cancer and autoimmune diseases. He also champions it as an effective weight-loss plan.

Some foods that the lectin-free diet exclude are:

  • Melons
  • Tomatoes
  • Aubergine
  • Potatoes
  • Lentils
  • Edamame
  • Cashews
  • Green beans
  • Ripe bananas
  • Whole grains

Gundry emphasises the supposed inflammatory effects of lectins, arguing that they damage our intestinal tract through their “sticky” nature, increasing intestinal permeability and lowering microbiome diversity. He encourages people to avoid lectin-rich plant foods, such as nightshade vegetables, pulses and beans, promising it will reduce lethargy, brain fog, joint pain, autoimmune disorders and more.

These are bold claims, but is there any evidence behind them?

Gundry published a case study trialling his diet in 102 patients, 95 of which saw a reduction in biomarkers of inflammation. According to his report, 80 of the 102 participants saw significant improvements in autoimmune diseases, so much so that they stopped taking their medication.

Whilst it sounds compelling, Gundry’s study holds little weight due to the lack of controls; without a comparable group who did not follow the diet, we can’t even establish correlations, yet alone causation. The research paper was not peer reivewed either, significantly undermining it's reliability.

Lastly, the study did not account for confounding variables, so the reduction in inflammation might have resulted from the foods they were eating, as opposed to the lectins they were not. This is why case-studies are one of the lowest forms of observational evidence on the hierarchy of evidence.

The hierarchy of evidence

Are lectins bad for you?

In animal studies, it has been shown that lectins interfere with the absorption of nutrients like zinc, calcium, phosphorus and iron. To date, there have been very few human studies, although it is well documented that raw kidney beans have a high level of lectins called phytohaemagglutinin which can induce food poisoning. As little as four to five raw kidney beans can cause diarrhoea, vomiting and stomach cramps.

The good news is that lectins are heat sensitive, so cooking kidney beans and other foods with a high lectin content destroys most, if not all, of these toxins. That’s the catch with the lectin-free diet; whilst some foods are toxic in their raw form, the poison is in the dose and cooking reduces the lectin content to negligible amounts.

Have you ever eaten a raw kidney bean? I doubt it, and with good reason. They have the consistency of rocks and a disagreeable taste which would make Bear Grylls grimace. It has long been known that raw legumes like beans can induce sickness when consumed raw. The human species discovered this long ago, hence why we soak and boil them before eating.

Did you know☝ The deadly poison Ricin is a legume lectin found in the seeds of the castor oil plant

Most cases of lectin poisoning occur when people accidentally undercook kidney beans, usually by not soaking or boiling them for sufficient time. When properly prepared, most, if not all, lectins are deactivated and rendered harmless.

Likewise, the canning process has been shown to reduce the lectin levels of navy beans to as low as 0.1% of their original level. This is because the canning process is a cooking process.

Many lectin-rich foods, like bell-peppers, goji berries, courgettes, quinoa, aubergines, tomatoes and nuts are great sources of fibre, B-vitamins and other nutrients. Far from harming your gut, as Gundry claims, these are prebiotic rich foods that you will see recommended on your Atlas health report. In short, the health benefits of these foods far outweigh any harm trace amounts of lectin will cause.

An informative breakdown of the lectin-free diet

Some lectins are also anti-microbial and antioxidant, whilst other plant lectins may be able to slow the absorption of carbohydrates, preventing high insulin levels and blood sugar spikes.

The Mediterranean diet, which has been consistently associated with reduced risk of heart diseases, is full of lectins. If lectins were as harmful as Gundry claims they are, surely this would not be the case. In comparison, no human studies have shown that lectin-rich foods (when properly prepared) cause any adverse health effects.

In fact, the Blue Zones project- a group researching long-living populations- notes that plant foods high in lectins are a staple food in places of longevity.

Moreover, there are studies showing that some lectins have potential medical applications. For example, the lectins found in soybeans are being studied for their potential in cancer treatment.

IBS and lectins

If you suffer from a digestive issue like IBS, lectin-rich foods might exacerbate symptoms. In these cases, It could be wise to reduce or avoid certain lectins. This should only be done after consulting with a nutritionist and with their supervision.

All plant foods contain lectin, albeit at differing levels, so whilst you might want to avoid kidney beans, you should be able to tolerate other foods much better, such as cucumber or tomatoes. The levels of a lectin called Phytohaemagglutinin (PHA) are measured in hemagglutinating units (Hau). According to the FDA, raw kidney beans contain between 20,000 and 70,000 Hau. This is reduced to as little as 200-400 Hau when they are cooked.

In contrast, one Indian study estimates that lentils have merely 500-600 Hau when uncooked. Despite this significant difference, both are simply listed as “No” foods on Gundry’s website.

The lectin-free diet: fad or fact?

Stephen Gundry’s approach to nutrition is dangerously reductive, categorising foods as either “good” or “bad” when the reality is far more nuanced. His website even contains a list of “yes” and “no” foods- it doesn’t get more black-and-white than that.

The current evidence does not support the claims Gundry makes. Firstly, there are no human studies to show that long-term consumption of lectin-rich foods has any adverse health effects, assuming they are sufficiently cooked. If the evidence does come to light, we’ll be the first ones to tell you.

Secondly, Gundry’s simplistic categorisations entirely ignore the nutritional content of food beyond their lectin content. Take goji berries, which earn a place on his “no” list. They have been shown to alleviate oxidative stress and eliminate free radicals, preventing damage to DNA and proteins. They also have antiaging and anticancer properties.

Gundry’s own study has no case controls, meaning we can draw very few conclusions from it. Whilst anecdotal evidence abounds from those who have followed the diet, this is not reliable; the lectin-free program also disallows processed foods: who is to say the health benefits observed aren't from this?

Additionally, Gundry presents the lectin-free diet as one-size-fits-all. In reality, our bodies are unique, something our diets should reflect. Worryingly, he also places a significant emphasis on weight loss, which in and of itself is no marker of health.

Ultimately, the lectin-free diet is far more fad than fact, overstating the health risks of lectins and relying upon a dangerously reductive approach to food. In most healthy individuals, high-lectin foods are tolerated with no side effects. Whilst it is important to know how to prepare these foods properly, this is not a reason to avoid them.

How to safely prepare high-lectin plant foods

Kidney beans

Red kidney beans have the highest lectin content when raw and are responsible for a large proportion of lectin-related food poisoning. Whilst you might not deliberately eat raw beans, you could accidentally undercook them. To avoid this, let’s take a look at the best preparation methods for these beans to ensure you do not suffer digestive upsets.

The World Health Organisation (WHO) gives the following advice on preparing kidney beans: “Lectins are destroyed when the dried beans are soaked for at least 12 hours and then boiled vigorously for at least 10 minutes in water.” The FDA recommend that consumers boil beans for 30 minutes to ensure they reach a sufficient temperature.

Make sure you drain the water you soaked the beans in. Canned beans have already undergone this process and can be safely consumed, though check the label just in case.

The Food and Drug Administration (FDA) warns that cooking red kidney beans for less than 10 minutes below boiling temperature can increase the toxicity five-fold. Do not cook raw beans in a slow-cooker as the heat is insufficient to remove lectins. Likewise, soaking alone will not remove all the lectins.

The lectin-free diet also warns against goji berries, potatoes, tomatoes, melon, seeds, lentils, edamame, peas and zucchini, among other fruit, vegetables and legumes. For most individuals, eating raw tomatoes, edamame or goji-berries will not cause any issues. As for lentils, make sure you soak and boil them sufficiently. They have far lower levels of the lectin PHA than kidney beans, but it is still essential to ensure they are cooked thoroughly.

The takeaway

  • Lectins are proteins that bind with carbohydrates
  • Plant lectins evolved to defend against predation
  • In animal studies, lectins have been shown to reduce nutrient absorption and induce inflammation
  • Raw beans, especially red kidney beans, have high levels of lectin and can induce food poisoning
  • Plant lectins are heat sensitive and water-soluble
  • Soaking and boiling high-lectin foods, like lentils and pulses, reduces most lectins
  • The benefits of high lectin foods outweigh the dangers
  • If you have IBS and find that lectin-rich foods trigger this, speak to a nutritionist before cutting them out

☝️DISCLAIMER☝This article is for informational purposes only. It is not intended to constitute or be a substitute for professional medical advice, diagnosis, or treatment.

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