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AS A botanist fascinated by the properties of plants, I am always curious when I uncover new claims about them. So when a colleague lamented to me about having to give up eating tomatoes (her very favourite food) over lunch the other day, fearful they would exacerbate her crippling rheumatoid arthritis, I could barely clear my plate before reaching to dig out the studies.

The first thing I discovered was that a link between tomato consumption and this painful, poorly understood degenerative condition wasn’t a new idea at all, just new to me that day. It has been a staple for health writers in newspapers, books and blogs for decades.

It isn’t just tomatoes either. Everything in the botanical family to which they belong, called the nightshades, including potatoes, aubergines, peppers, chillies and crops such as goji berries, is claimed to exacerbate the symptoms of arthritis.

The alleged culprit is a toxic alkaloid compound they apparently contain called solanine. So what does solanine do in this context? To my surprise, I couldn’t find a single scientific paper that addressed the question. In fact, to date there appear to have been no peer-reviewed clinical trials investigating if solanine even has any plausible connection with rheumatoid arthritis to begin with. The only reference I could find was an animal study in the Arab Journal of Nuclear Sciences and Applications, which involved feeding rats a special diet based largely on diseased potatoes.

Potato plants produce solanine to defend themselves against pathogens, so rotting potatoes are likely to have particularly high levels of it. This study did indeed find increased levels of some blood markers associated with rheumatoid arthritis in these rats. But humans aren’t rats and we don’t eat diets based on rotting potatoes. Even if these results were transferable to humans, how do we know it was the spuds that had the effect, and not the pathogen?

The authors’ conclusion, that everyone with arthritis should eliminate every plant in the nightshade family from their diets, isn’t really a conclusion that can be made from testing one crop in a family of some 3000 species.

This highlights a central problem with any rationale behind the claim: solanine isn’t really found in many plants in the nightshade family, at least not at the same levels as in rotting potatoes. It crops up mainly in the green tissues of growing potato plants. In tomatoes, it is found at far lower concentrations, down to barely measurable traces in some varieties. In goji berries, it doesn’t seem to be present at all.

“Over the years, the claim seems to have cycled and been recycled so many times it is now cited as scientific fact”

These plants may contain other related alkaloids, such as tomatine, which may be the source of some confusion. But picking out solanine as a clear culprit is a tough ask based on the evidence. Solanine is also found in plants outside the nightshade family, from apples to artichokes, none of which seems to be in the cross hairs of proponents of this claim. Meanwhile, some wellness writers say that nightshade plants promote inflammation because they are in the same family as the irritant poison ivy. They are not.

When I dug back through the studies, the one that I kept coming back to as the apparent original source of this claim is one from the late 1970s, when a horticultural researcher noticed his arthritis was alleviated when he quit smoking and stopped eating all other related plants.

Yes, tobacco is in the nightshade family too. Encouraged by this personal anecdote, the researcher conducted a postal questionnaire run through magazine adverts, collated the resulting anecdotes and wrote a book based on his idea. Over the years, supported only by this shaky evidence, the claim seems to have cycled and been recycled so many times that it is now regularly cited as scientific fact.

What we do know, by the way, is that evidence does suggest a link between smoking and rheumatoid arthritis. These anecdotes may have been more to do with giving up smoking one toxic nightshade plant than no longer eating a bunch of safe ones. Go figure.

Given this lack of any solid scientific evidence for this belief, the advice from both the US-based Arthritis Foundation and the British Nutrition Foundation is that tomatoes, potatoes and the like aren’t just healthy additions to our diets, but contain potentially anti-inflammatory compounds like carotenes and vitamin C that can help protect our tissues, possibly benefiting those who have rheumatoid arthritis.

In the face of really no good studies, here is my take: if lived experience has shown that eating these plants is a problem for you, of course, don’t eat them. But if you have been frightened off your favourite foods because of a tabloid headline, take it, as it were, with a pinch of salt.