Potatoes and other nightshade vegetables—and actually most vegetables—have yet another complicated class of molecular ingredient: lectins. These are proteins that bind to carbohydrates. Like glycoalkaloids, lectins probably evolved as a plant self-defense mechanism.1 The immune system is is an enormously complex defense, and lectins “are commonly used in direct defense against pathogens and in immune regulation.” That includes viruses, fungi, and bacteria. Humans even have lectins to help neutralize the bacteria that cause gonorrhea and listeriosis.2
Sometimes called an anti-nutrient, lectins are a hot topic in the DIY diet & health world. The popular 2017 book The Plant Paradox, by Steven Gundry, focuses on lectins. You can now even buy supplements that claim to limit lectin damage. Gundry draws from a lot of interesting science in his diet rationale, and it’s beyond the scope of this essay to analyze that body of work. It may or may not be worth noting the overlap with the nightshade hypothesis. It’s not worth noting since many more foods also contain lectins, and cooking largely destroys them. It’s also considered by most nutritionists to be a fad diet largely unsupported by the evidence. But it’s worth noting because tomatoes, potatoes, peppers, and eggplant all contain lectins.
Like many fads, the concern with lectins is built around a framework of truth—see above re the biological role of lectins. But on the topic of nightshades there is an intriguing bit of research from the Central Food Technological Research Institute (CFTRI) in Karnataka, India that looked at how potato lectins may influence the immune system. The immune chemistry is frankly beyond my capacity to interpret and explain, but involves the classic allergy immunoglobulin IgE and its activation of two kinds of immune cells (basophils and mast cells).
Fortunately, the authors interpret their own findings: “This may explain why certain atopic subjects (suffering from allergic rhinitis, asthma or both) experience adverse reactions upon consumption of foods prepared with potato and avoid eating potato-based foods, although they are not truly allergic to potato. A similar, but biologically relevant, clinical scenario may be occurring in the case of consumption of tomato, as tomato lectin behaves in an identical fashion to potato lectin, and that tomato is consumed to a greater extent than potato in raw form.”3
An atopic reaction is where one part of your body is exposed to a substance, but another part of the body reacts. Atopy is one of the things that makes food reactions so hard to diagnose. How are you supposed to know that the grapefruit that you just ate is what activated the histamine response in your forehead, particularly when other things also trigger a similar histamine reaction?
What this lectin finding suggests is immune activation via IgE that is different from classic allergic IgE activation. It’s not proof of anything, but it does demonstrate that the immune system is far more complex in its reaction to food than we used to suspect. It makes it even harder to study, but also indicates a need for further study.