One of the questions I have often been asked involves the safety of eating "nightshade vegetables." This typically means tomatoes, peppers, eggplants, and potatoes. In particular, people who have autoimmune conditions are warned against eating them, as it is claimed that components of these foods can induce or exacerbate inflammation. The "autoimmune protocol" in particular excludes "nightshades." The reasons given are that they contain alkaloids, lectins, and capsaicin.
Another allegation: "Nightshades of all types were considered inedible prior to the 1800s, because some varieties, such as belladonna or 'deadly nightshade' (Atropa belladonna) were known to be toxic and used as a poison in ancient times"… "Nightshade vegetables contain enough toxins to cause inflammation in some people, particularly those with autoimmune disease." (source)
Is this really true?
Well, for one thing, Italians have been eating tomatoes since the 1500s. And people in the Americas were eating them, and other "nightshades" such as peppers and potatoes, for much longer than that.
Like so many misconceptions, there is a grain of truth. Yes, nightshades do contain alkaloids, lectins, and capsaicin. Most vegetables do (with the exception of capsaicin). But there is no evidence that those in the "nightshades" we eat are actually harmful. In fact, many substances, especially in tomatoes and peppers, are anti-inflammatory and/or antioxidant. Studies linking alkaloids found in "nightshades" to harm date from the early 2000s and were either performed in vitro (in a dish or test tube, basically) or used high doses of extracts (rather than whole foods) in mice. There is an absence of follow-up studies, suggesting problems with replication. What we do see now are studies of human diet and correlations with health outcomes. These studies have mostly looked at tomatoes and potatoes and report beneficial effects.
For instance, studies to date have linked tomato consumption with lower incidence of cancer, cardiovascular disease, and "all-cause mortality" (Cheng 2019; Xu 2021; Li 2021). This suggests that a diet including these fruits is likely to be beneficial for autoimmune conditions as well, or indeed any condition involving unrestrained inflammation. It may be important to remember that what we eat is the fruit of the plant. Since the whole point of fruit is to have animals eat it and spread the seeds around, it is not toxic. So even though the leafy parts of plants in the nightshade family are toxic, the fruits are not. But don't make a salad out of tomato greens.
I like this explanation from the Arthritis Foundation:
"They're a big family of around 2,500 plants, diverse enough to include potatoes, tobacco and the hallucinogen datura. Many nightshades are inedible and some, like bittersweet nightshade, a tomato relative, are notoriously toxic. All contain small amounts of a toxic compound called solanine, which helps the plants repel insects and can be poisonous to both humans and animals. Cattle, sheep and pigs are particularly sensitive to solanine and can die from eating the vines and leaves of tomatoes and potatoes. No research suggests this level of toxicity in humans, though you want to steer clear of potatoes that have turned green; they've developed exceptionally high amounts of solanine and aren't safe to eat." (source)
Based on what we know about the importance of gut health for immune tolerance and autoimmunity, and what we know about the importance of diet for gut health, it is clear that diet needs to be a target of interventions for autoimmune conditions. The key thing, though, is that recommendations need to be factual. A real concern is that cutting out "nightshades" means cutting out highly nutritious foods that contain anti-inflammatory and immune-regulatory substances.