Nightshades contain natural compounds that can increase gut permeability and stimulate immune activity, two things that are already problematic in people with autoimmune conditions. The concern isn’t that nightshades are universally harmful. It’s that the alkaloids they contain may worsen symptoms in people whose immune systems are already attacking their own tissues. The evidence is a mix of lab studies, animal research, and patient-reported outcomes, with no definitive human clinical trials settling the debate.
Which Foods Are Nightshades
Nightshades belong to the Solanaceae plant family. The most commonly eaten ones are tomatoes, white potatoes, bell peppers, eggplant, and chili peppers (including jalapeños). Tomatillos and goji berries also fall into this group. Several spices count too: paprika, cayenne pepper, crushed red pepper, and chili powder are all derived from nightshade plants. Sweet potatoes, black pepper, and mushrooms are not nightshades, despite frequent confusion.
How Alkaloids Affect the Gut Lining
The core concern with nightshades centers on glycoalkaloids, natural defense chemicals the plants produce to deter insects and disease. In potatoes, the main glycoalkaloids are solanine and chaconine. In tomatoes, it’s tomatine. These compounds interact with cholesterol in cell membranes, and at sufficient concentrations, they can punch holes in the cells lining the gut wall.
A study published in the journal Inflammatory Bowel Diseases found that potato glycoalkaloids disrupted the intestinal barrier in a concentration-dependent way, meaning higher amounts caused more damage. Chaconine was more disruptive than solanine, and the two together were worse than either alone. Critically, this effect was more pronounced in animals with a genetic predisposition to inflammatory bowel disease. Animals without that predisposition showed no significant barrier disruption at the same concentrations.
That finding captures the central issue: nightshade alkaloids don’t seem to cause problems for everyone, but they may specifically affect people whose guts are already vulnerable. In autoimmune conditions like rheumatoid arthritis, psoriatic arthritis, and lupus, increased intestinal permeability (often called “leaky gut”) is already a recognized feature. Anything that further weakens the gut barrier could, in theory, allow more undigested proteins and bacterial fragments into the bloodstream, provoking additional immune responses.
Nightshade Compounds Can Stimulate Immune Cells
Beyond gut permeability, some nightshade alkaloids appear to directly activate parts of the immune system. Tomatine, the alkaloid found in tomatoes, has been shown to boost the activity of dendritic cells, which are immune cells responsible for detecting threats and rallying a broader immune response. Research published in Materials Today Bio found that tomatine activates a specific innate immune pathway called the NOD-like receptor pathway, which increases the production of the inflammatory signaling molecule interferon-gamma and ramps up markers on dendritic cells that help them present targets to the rest of the immune system.
This immune-boosting property is so reliable that researchers have explored tomatine as a vaccine adjuvant, a substance that amplifies the immune response to make vaccines more effective. For someone with a healthy, well-regulated immune system, this is likely insignificant at dietary doses. For someone whose immune system is already overactive and attacking their own joints, skin, or organs, even a modest nudge toward more immune activation could be counterproductive.
Capsaicin and Inflammation
Capsaicin, the compound that gives chili peppers their heat, has a more complicated relationship with inflammation. It interacts with substance P, a chemical messenger involved in pain signaling and inflammatory responses. Injecting substance P into rat joints produces pronounced inflammation, and capsaicin can actually suppress that response over time by depleting substance P receptors in the tissue.
This is why capsaicin creams are used for arthritis pain relief. But the initial exposure to capsaicin triggers the release of substance P before eventually depleting it, which can temporarily increase inflammation. For people with autoimmune joint conditions, the question is whether dietary capsaicin from peppers is enough to matter. The answer isn’t clear, but some patients report that spicy foods trigger flares.
What Patients Actually Report
The strongest real-world data comes from a U.S. national survey of people with psoriasis. Of 297 respondents who tried removing nightshades from their diet, 52.1% reported that their skin fully cleared or improved. That placed nightshades third on the list of dietary triggers, just behind alcohol (53.8%) and gluten (53.4%). Nearly 44% of survey respondents also had psoriatic arthritis.
These numbers are self-reported and uncontrolled, so they can’t prove that nightshades caused the flares. People eliminating nightshades may have simultaneously changed other habits. Placebo effects and expectation bias also play a role. Still, a 52% improvement rate across nearly 300 people is hard to dismiss entirely, and it aligns with what rheumatologists and dermatologists hear in clinical practice.
Why the Science Remains Unsettled
The Arthritis Foundation acknowledges the debate directly: there is little scientific evidence on either side, but a lot to be said for lived experience. Older mouse studies suggested solanine damaged the gut lining and worsened intestinal inflammation in colitis. But more recent mouse research has found the opposite, with purple potatoes and goji berries (both nightshades) actually reducing inflammation, improving gut barrier function, and lowering harmful gut bacteria.
Mouse studies rarely translate well to human biology, and these contradictory findings highlight the gap. No randomized controlled trial has tested nightshade elimination in people with autoimmune disease. The nightshade issue also creates a nutritional dilemma. Tomatoes, peppers, and potatoes are rich in vitamin C, potassium, and antioxidants like lycopene. Doctors and dietitians often hesitate to recommend removing them without clear evidence of harm, because the nutritional trade-off is real.
Reducing Alkaloid Exposure Without Full Elimination
If you want to test your sensitivity without completely eliminating nightshades, preparation methods matter. Peeling and boiling potatoes is the most effective way to reduce glycoalkaloid content. Frying and baking are less effective at breaking down these compounds. Green-tinged or sprouted potatoes have dramatically higher alkaloid levels and are worth avoiding regardless of autoimmune status.
Alkaloid concentrations also vary widely by variety. In potatoes, total glycoalkaloid content ranged from 30 mg per kilogram of fresh weight in one variety to over 2,600 mg/kg in another. That’s roughly a 90-fold difference depending on the cultivar. In tomatoes, alkaloid levels are highest in unripe green fruit and drop substantially as the tomato ripens. Cooked, ripe tomatoes contain far less tomatine than raw green ones.
For a more structured approach, the autoimmune protocol (AIP) diet eliminates all nightshades for 30 to 90 days, then reintroduces them one at a time while tracking symptoms. This is the most common method for identifying individual sensitivity. You might find that tomatoes cause problems but bell peppers don’t, or that small amounts are fine but large servings trigger a flare. The goal is to identify your personal threshold rather than assuming all nightshades are equally problematic.