Making Sense Of Conflicting Claims From Diet Gurus – Dr. Stephan Guyenet and Dr. Mario Kratz

Ari: Well said. I agree. Particularly for certain subsets of people who — I have seen people who have reported that they’ve had longstanding gut issues, that veganism made them feel terrible, and they react horribly to many different kinds of plant foods. For that subset of people, excluding plant foods may very well be a huge breakthrough for them feeling better.

Let’s get into Dr Gundry’s book, The Plant Paradox, on lectin. First of all, can you guys talk about what lectins are?

Mario: I was a primary viewer of The Plant Paradox, so I’m going to take that here primarily, but Stephan, feel free to chime in anytime. Lectins are just proteins that are very ubiquitous in nature. Humans make them, animals make them, plants make them. There’s a lot of different structure. Lectins are not one family that’s defined by its structure. It has a lot of different structures, but one thing they all have in common is that they bind sugar. They have at least one binding side for sugar, many times more binding sides. Commonly, lectins are referred to as sticky proteins.

They’re proteins that if you added them to something biological, say to a cell culture, they stick to stuff, because say cells, for example, express proteins on their surface, and these proteins often have sugars, let’s call them sugars for simplicity sake, sugars attached on their surface. That allows these lectins to bind to these sugars. Sometimes people even say lectins are a type of class of molecules that can help us read what is sometimes called the glyco code, the code of sugars attached to molecules on the surface. Because these sugars actually do affect sometimes the function of these proteins and so forth. They’re very specific, usually, every lectin is very specific for one particular kind of sugar.

Ari: What claims are made about lectins in the book, in The Plant Paradox?

Mario: The central theme is that The Plant Paradox claims that lectins from particular plants, and specifically plants that we have not been able to evolve to throughout evolution, so plants that were introduced into the human diet just in the last 10,000 years or so, that we are not adapted to dealing with them, and that they are causing a large number of diseases. There’s pages and pages of listing, I think, 50 or so diseases ranging from autoimmune disease to diabetes, obesity, heart disease, all kind of Parkinson’s, Alzheimer’s.

Basically, it’s, you could almost say the vast majority of most chronic diseases that we’re all at some point suffering from, the claim is that lectins play a role or a causal agent in the development of those diseases. The argument, I think, because it’s a complex topic, to break it down a little bit for people following this discussion, is such that the argument is there’s lectins in these plants, and we’ll get to which plans are particular critical. Then we eat them, they get into our gastrointestinal tract. These lectins are not digested properly. They get into a bloodstream.

There they attach to cells that have all these different– Specifically to specific cells based on what sugar they’re specific for. Then the immune system reacts basically to those lectins and causes inflammation in those areas, and destruction. That then causes, depending on which cell and which tissue type has been bound to by the lectin, causes destruction in those types of tissues. To maybe take this apart a little bit, so to start from the beginning, the plants that are particularly the focus here in this book, are those, as I said, that have just been introduced into the human diet in the last 10,000 years or so.

Particularly legumes, they are by far the richest source of lectins, whole grains, nightshade family vegetables. Those are tomatoes, eggplants, potatoes, peppers. Cucumber family vegetables; squashes, pumpkins, cucumbers, and so forth. Those are the core foods that according to The Plant Paradox have the highest concentration of plant lectins that our bodies are not well adapted to. That part is definitely true. If we look actually at the evidence, there’s huge evidence and showing very nicely, actually, we don’t know the concentrations in many cases, but lectins are very abundant, particularly in legumes and grains.

That part is definitely true. Then the issue is that when we eat those foods, the claim is we may be ingesting those. Here’s, I think, where we need to take a closer look at the evidence, because when you, say, eat a legume, you typically cook it. If you, say, take a kidney bean out of a package, you really cannot eat it raw. That’s actually an interesting tidbit that maybe many people are interested in. If you were to try to eat, say, raw kidney beans, they’re really highly toxic, and you may end up in a hospital. In other words, the argument that lectins are toxic to people definitely holds water.

It’s true. We should not be eating large amounts of these lectins. That’s exactly why humans have developed techniques to cook these foods. If you cook them properly, so,

to stick with this example, kidney beans, if you soak them and then cook them long enough, or particular if you cook them in a pressure cooker, you reduce 98%, 99% of the lectins in the kidney beans. I’d say there’s some lectins potentially left, small amounts, very small amounts, and we can discuss later whether that may be an issue. First thing for people to understand is yes, lectins are toxic. They’re not desirable.

At the very least, they’ll give us some digestive issues. We can get diarrhea, abdominal cramping, if you eat lectins. Humans usually have evolved ways to process foods in a way that dramatically reduces the concentration of lectins in all of those foods. Actually, I will take that back, it’s not all of those foods. In legumes, we usually cook them. If you cook them long enough and they’re ready for you, you feel like you can eat them, their lectin content is usually very, very low, a small fraction of what it was at the beginning. For grains, similarly. If we soak or ferment a grain and then cook it, again, it reduces the lectin content to almost entirely.

There are some foods obviously that we eat that are not maybe prepared that way. Peanuts come to mind because very often we may be eating peanuts just briefly dry roasted. Dry roasting is likely to not entirely destroy the lectins to the same degree as we see, say, with cooking or pressure cooking kidney beans. Others are, say, tomatoes or peppers, because the lectins are actually in the skin or the seeds. Say, with tomatoes, you usually do eat the whole tomato, which includes the skin and the seeds. If you eat that raw, you will be exposed to some tomato lectin. Maybe let’s keep that in mind that usually we will have some exposure to some of these lectins.

Ari: There’s debating a few different directions to go from here. Let’s talk about what you just mentioned a minute ago, which is, is that a risk? Is it a risk to consume some small amount of lectins? Let’s say someone’s thinking after listening to what you just said, that beans have these toxic compounds called lectins. Even if I cook them, they’re still 1%, 2%, 3% of these poisonous lectins in there. Am I going to be harmed by that? What does the research say about consuming a very small amount of lectins, and the potential for harm from that? Maybe connecting that to the broader literature around populations that consume large amounts of lectin-rich foods, and long-term health outcomes.

Mario: Short answer is we really don’t know. There’s lots of research to be done, I feel. The biggest issue that I think I had with the book is that claims were made that were partially supported by something in the literature, cell culture experiments, mostly animal experiments in mice. Then the claims were exaggerated. They were translated to human disease where in usually most cases there was absolutely no evidence linking them. Let’s use one specific example. Let’s use maybe peanuts because I mentioned that earlier. Peanuts are often eaten just dry-roasted.

The degree to which lectins are reduced in a food are related to how long it’s cooked, the pressure which it’s cooked, and if it’s wet versus dry-cooked. A peanut basically being roasted briefly and not with water, and not with pressure, you would expect there’s some percentage of the peanut lectins still in the peanut when we eat it. There’s good research for most lectins actually, that lectins are poorly digested. That’s an important finding actually that I found very interesting because when I was in graduate school in nutrition 25 years ago, I was taught proteins are digested in the stomach.

What arrives in the small intestinal tract is just basically amino acids or little, shorter snippets, what we call polypeptides, not the intact immunogenic protein anymore. It looks very clearly there’s good evidence for peanut lectin, good evidence for tomato lectin, good evidence for garlic lectin that they pass through the gastrointestinal tract mostly undigested. You can detect them in the stool in animals. There’s good reason to think this could be true in humans as well. I think that’s already an interesting thing that certainly distinguishes lectins as a somewhat different type of protein.

There seems to be something in its evolution that it has evolved to be resistant to digestion. Peanut lectin, I think we can fairly assume, will get to our small intestinal tract, to the site where it’s absorbed, in a way that is fairly intact. Now, the book basically says lectins in general trigger what is called leaky gut. That would include that the epithelial cells in the gut basically open up a little bit, and larger molecules can just enter. Again, I’d say there’s evidence for this in animals, no evidence in humans whatsoever. For peanut lectin, we can clearly say that it does enter the bloodstream because there is at least one paper I’m aware of.

Then in which we can find intact peanut lectin actually in the blood that you can detect with antibodies. Most people actually carry antibodies against peanut lectin in their blood. There’s clearly been exposure in those people to intact immunogenic peanut lectin. Immunogenic, meaning it’s an intact protein that our immune system can react to and bind, create an immune response to. All of those facts we know for peanut lectin, and they’re all very interesting actually. I would say I think that already is a cause for concern.

Beyond that though in humans, we know nothing. There’s no link ever that I’m aware of, linking peanuts to — I’ve looked quite thoroughly actually, even yesterday night, still to see how– Because I’ve reviewed The Plant Paradox three years ago, so I wanted to make sure maybe there’s some killer new science that we now have. There’s nothing that I could find that actually says, “Now, if this peanut lectin gets into the blood, what does it do there?” The Plant Paradox would argue now, it can maybe attach to some tissue. There’s again, evidence from cell culture that peanut lectin can attach to the cartilage and joints.

What you would expect then, if that were to happen, is that the peanut lectin attaches to the cartilage and joints, and the immune system attacks. Somehow in the process of the inflammation that develops, partly destroys the joint. Maybe that could be one of the contributing factors for something like rheumatoid arthritis. Then what we would expect to see is that peanut consumption would at least loosely, at some point, be associated with rheumatoid arthritis. I’m not aware of any such study. I’ve looked specifically for those types of links.

As we know, particularly in observational studies in nutrition, if there’s one critique, then that we may be looking at too many of these links. You can find association studies for almost anything. Vitamin K is linked to Alzheimer’s or osteoporosis. People look at everything. If it’s positive, it’s going to be sure to be published. I think actually that is somewhat of a concern with nutritional epidemiology. In spite of that, I have not been able to find really any evidence that that’s the case. That’s an important one. Because it gets into the blood and the immune system may respond to it, it’s a large stretch then to now say, it is the cause of an autoimmune disease.

I think that’s important for people to understand. We can do similar things. I’ve done similar things, say, for legumes. We look at legumes, could well be that small amounts of lectins remain in the legumes even if they’re properly cooked, or tomatoes which we often eat raw, as I said earlier. It’s possible, again, that they actually do get into our bloodstream, that they trigger an immune response. I’m very open actually to the possibility because I studied as a researcher for a long time. Low-grade chronic inflammation.

It’s really important for us to try to be open to all different types of sources of that inflammation because really we don’t know in many cases what causes these inflammatory diseases. I’ve really been open to the idea, could this be true? I’m still open. I think it’s really an interesting hypothesis, is how I would frame it. When we look at populations that eat the most, say, tomatoes, or the most legumes, for most health outcomes, including those that according to The Plant Paradox are caused by these lectins, we actually see the opposite of what we would expect if that was true.

Ari: If I can summarize paraphrasing and maybe adding a bit of my own commentary here, someone listening to this might still be very concerned about eating lectin-rich foods, given everything that you just said. They might still be thinking, “It sounds there’s a pretty reasonable case that these lectins could potentially be really harmful and poisonous to us, and it sounds like cooking is not a perfect solution. This other thing about humans probably not digesting a lot of lectins also may not be a perfect solution. Maybe it’s better for me to avoid these lectin-rich foods.”

(So given that someone may still be thinking that), what does the literature say more broadly about populations that consume foods like legumes that are very rich in lectins, or lots of tomatoes, maybe raw tomatoes, things like that? Based on looking at that level of research or randomized controlled studies where they maybe have a group of people with, let’s say diabetes, introduce legumes into their diet, and then they track biomarkers over a few months. What do those kinds of studies tell us about the relationship of lectins to human health?

Mario: It’s very clear that in all the kinds of studies that I’m aware of, observational studies, let’s say, let’s start with that people and populations who eat the most, say, legumes typically have good health outcomes associated with that. People who eat more vegetables, including nightshade vegetables, have better health outcomes associated with that. In intervention studies, people who increase their consumption of legumes, or people who increase their consumption of vegetables, including nightshade vegetables, have improved health outcomes associated with that, including for example, reduced levels of inflammation commonly.

Again, I’m not familiar with any study really that would suggest that, say, eating legumes is the thing that gives you autoimmune disease, and cutting out legumes will cure these autoimmune diseases. What I was just discussing is I feel like in science we need to be open to new ideas. I’ve been dismayed sometimes by scientists who hear something like this about lectins, and without ever looking into the literature they immediately say, “Rubbish. It’s all rubbish. Lectins are great. We should be eating all these foods.” Yes, I’d say we should be eating all these foods. If viewers or listeners want to know, I do. I eat plenty of legumes and vegetables.

Ari: After reviewing this book in-depth, and after spending a lot of time looking at the literature, specifically being open to and trying to find evidence that supported a link between consumption of lectin-rich foods and these various diseases like the claims in the book were making, after spending all that time doing that, you are still eating lots of lectin-rich foods.

Mario: Yes. I just make sure I prepare them properly. I think that’s an important point. I think it’s important for people to understand the science behind this. There is science, mostly in animals, that suggest these lectins, if we eat them, they can get into our bloodstream, that they’re bad. We have no evidence whatsoever in humans. That’s, in my mind, this huge jump that the book makes, where it takes a large body of evidence from cell culture and animals, and translates it to humans. The important part here as well is if you look actually…

I probably read close to 200 papers on lectins, and the animal literature specifically. If you’re a researcher, how do you test with lectins have negative health effects? You order some isolated lectins from the chemical company and put it into the animal’s food. I’m not kidding. Those animals in studies that have shown, for example, that lectins may cause what we call leaky gut, they’re using lectin concentrations that are hundreds of fold higher than what we would ever be expected to eat. It’s almost certain that those findings are not really translatable to the human situation where we’re actually eating food, or mice being fed uncooked beans.

Actually, that’s another very common method. I think we need to be very, very careful. The dose makes a toxin, that’s for sure. If we study a model just to understand some basic science– I’m not saying the model is wrong. I think it’s good that they’re doing these studies, but then if someone comes along and reads these studies and immediately draws conclusions to not ever eat these foods again, even though the concentrations that we would be exposed to in properly prepared legumes, for example, are maybe 1% or less of what is used in these studies.

I think that’s a problem. That way, in the end, we can make the case for almost any food that it has some negative health effects. If we’re focusing on that one negative health effect that’s been shown, let’s say with meat, we can focus on the fat and the saturated fat content and say, “It may raise LDL cholesterol, so better not eat meat.” If we focus on lectins, we’re not going to eat any legumes and vegetables, and in the end we’re going to end up eating nothing. That’s a silly approach, right? What we really need to look at is the totality of the evidence.

The totality of the evidence right now tells us, I’d say with a high degree of confidence, that legumes are good for us if they’re prepared correctly. Whole grains and vegetables, good for us if we prepare them correctly. I think it should be the take-home message of people. As a result, in our review, the scientific claims made in the book didn’t fare all that well. People who are interested in this should really look into it. Just because I’m saying I’m open to these ideas, I’m looking at the literature, doesn’t mean at all that the claims that these lectins actually are the cause of chronic disease, are supported by anything really.