Nightshade vegetables include tomatoes, capsicum, chilli peppers and eggplant. Photo / 123rf
The so-called nightshade family of vegetables has been linked to swelling, poor gut health and arthritis. Here’s how the science stacks up.
Tomatoes, capsicums, eggplants, potatoes – to most people, these are just staple veg that form the basis of many healthy dinners. But for those who delve down the wellness rabbit hole, these foods are reportedly detrimental to our health, causing a range of issues from arthritis to inflammation.
“Many people with [autoimmune diseases] are sensitive to nightshades,” functional medicine doctor Datis Kharrazian posted to his 36,000 Instagram followers. “This is especially true if they have joint pain.”
Kharrazian went on to claim that proteins found in the so-called nightshade family of vegetables could cause problems for patients with rheumatoid arthritis, and urged practitioners to sign up for a two-day course at his research institute to learn more.
As well as social media, celebrities have also stirred up distrust against these foods. The personal chef for the legendary American football player Tom Brady was famously quoted as saying that his client studiously avoided nightshades because of their supposed inflammatory properties.
Despite these claims, according to Tim Spector, an epidemiology professor at King’s College London, Telegraph columnist and co-founder of nutrition science company Zoe, there is little evidence that nightshades actively drive inflammation in the average person. In fact, Spector says, his data suggest that the opposite is true.
“Zoe’s research has shown that one of the best things you can do for your health is to consume a diverse range of plants, including nightshades,” he says. “This will actually lead to improved gut health and lower inflammation.”
So why is this group of foods being demonised?
To understand where this all began, nightshades refer to the 2000 or more plants within the vast Solanaceae plant family. “The term nightshades includes lots of things that we eat commonly all the time,” says Bridget Benelam, a senior nutrition scientist for the British Nutrition Foundation.
Here are some of the most commonly consumed nightshades:
The origin of the somewhat sinister name is unclear, but it may stem from the fact that some Solanaceae plants – not ones that are typically on our plates – were historically used in medicinal and hallucinogenic potions, and it may be that this murky past has inadvertently tainted some standard foods.
There are claims that proteins in nightshades called lectins, which are also found in many beans, can trigger an immune response that promotes inflammation.
However, there is no research in humans which has ever found that the lectins present in tomatoes or potatoes can be problematic. Even in mice, the evidence is limited: one study found that tomato lectins could bind to the gut wall in the rodents, but didn’t show that they caused any damage. According to Benelam, the main evidence that lectins can be problematic relates to beans – which are not nightshades – but this applies only if you’re consuming them in large amounts.
Others highlight a chemical called glycoalkaloids, bitter-tasting chemicals that are produced by nightshade plants as a natural defence against insects and fungi. Over the years, a few studies in mice have suggested that glycoalkaloids might exacerbate or induce gut issues. One experiment, which fed fried potato skins to groups of rodents engineered to have gut issues resembling inflammatory bowel disease, concluded that the glycoalkaloid content within the skins had aggravated the inflammation.
However, experts say that it’s a very big leap to conclude that the same is happening in humans, not least because the concentrations of glycoalkaloid are highest in the leaves of nightshade plants, and present only in much smaller amounts in the edible food. “Like anything else, the dose is important,” says Spector. “Certain chemicals found in these vegetables could be toxic if consumed in vast quantities, as with most chemicals, but are actually beneficial when you’re just enjoying them as part of a meal.”
Charlie Lees, a professor of gastroenterology at the University of Edinburgh, agrees, arguing that if they were really causing inflammation in a lot of people, we would see this in population studies, while the opposite is the case.
“There’s no robust evidence over quite a long period of time that they cause gut inflammation,” he says. “In fact, these foods are part of a Mediterranean-style diet. If you look at most dietary advice for the general population, and people with inflammatory bowel disease, this diet is what people are advised to have as being the best for a healthy, diverse gut microbiome and supporting good bowel function.”
Yet some patients, particularly those with certain autoimmune conditions such as rheumatoid arthritis, are convinced that eliminating nightshades has alleviated some of their symptoms.
Such anecdotes are familiar to Rosalind Fallaize, an associate professor at the University of Hertfordshire and registered dietitian who studies people living with psoriasis, an autoimmune condition characterised by the formation of plaques on the skin. Fallaize has seen surveys in both the UK and the United States showing that significant proportions of psoriasis patients are convinced that eliminating nightshades has helped reduce the severity of their plaques in some way. Likewise, patients with irritable bowel syndrome (IBS), inflammatory bowel disease (IBD) and rheumatoid arthritis have also claimed that cutting out nightshades has reduced stomach pain and improved bowel habits.
While Fallaize hasn’t seen any hard data to back this up, she also believes that nutrition experts need to listen to what patients are saying. “I don’t think there is really any strong evidence here, but patients are still reporting eliminating these foods and having perceptions of benefit,” she says.
So what might be going on? Because there are no readily available tests which can easily measure inflammation levels before and after eliminating nightshades, Fallaize says it’s hard to assess. A 2023 paper by gastroenterologists at the University of Pittsburgh suggested that nightshades might be driving the over-activation of a particular population of immune cells in the gut. However, in the absence of any hard data, Lees isn’t convinced.
“I think it’s tenuous and speculative,” he says. “Maybe at the same time [as eliminating nightshades], they’re exercising more and looking after their sleep better and things that can truly help prevent gut inflammation.”
Lees believes that in some cases, people with conditions such as IBS or IBD may be perceiving benefits from eliminating nightshades, because they may have also excluded high-Fodmap foods (for example, beans, lentils, artichokes, asparagus, onions and garlic). This collection of foods contains short-chain carbohydrates and is known to trigger various digestive symptoms in susceptible people because the small intestine absorbs them poorly.
“For people who have irritable bowel syndrome, there are dietary triggers, including Fodmaps,” says Lees. “When these fermentable sugars are not dealt with properly by the gut microbiome, they persist and produce a lot of gas, which can cause bloating and diarrhoea.”
While Spector believes it could be plausible that a few people may have a rare allergy or food intolerance in response to nightshades, he suggests that people look for other, more likely explanations before actually eliminating any of these foods. “It’s worth understanding the whole dietary pattern and other exposures before demonising the entire food group,” he says.
Fallaize says she’s concerned that strict elimination diets, especially those involving staple plant foods, could lead to nutritional deficiencies which ultimately worsen the underlying disease process and place people at greater risk of other chronic health conditions over time.
At the same time, she’s acutely aware that many patients suffering from debilitating autoimmune conditions are desperate and willing to try anything for symptom relief, and so she believes there is a need for dietitians and nutrition experts to offer practical advice. While she doesn’t advise eliminating nightshade foods, her recommendation for people who are determined to try is to eliminate one thing at a time, wait for up to six weeks to assess whether it’s making any difference, and make notes on what else may have changed in that period, whether stress levels or sleeping patterns, and consider how that might also be having an impact on their symptoms.
“I’ve spoken to people who are eliminating dairy, nightshades, sugar and alcohol, and if you’re doing all that at once, how do you know which one is actually helping you?” she says. “Nightshades is one of these grey areas in nutrition where we don’t have the evidence, but that doesn’t mean we still can’t be supporting patients with their thought process. If we don’t give them the answers, they will go to social media and people without any qualifications to find them.”
Which vegetables are often mistaken for nightshades?
Several vegetables that are often mistaken for nightshades include sweet potatoes, mushrooms, onions, courgettes and cucumbers. Black pepper is also sometimes confused with nightshade capsicum, but is not a nightshade itself.
What are the health benefits of nightshades?
Nightshade foods are recommended because they’re packed with beneficial macronutrients and micronutrients. Tomatoes, for example, contain ample amounts of vitamin C, which is good for immune function, while eggplants are a useful source of beneficial forms of dietary fibre.
Are beans nightshades?
No. Beans belong to a different family, the legume family. They have been conflated with nightshades as they both contain lectins, which can cause digestive issues in some people, but only when ingested in very high quantities.
What are the symptoms of nightshade intolerance?
Some people, particularly those with autoimmune conditions, believe that nightshades can exacerbate joint pain and other symptoms of their disorder. However, researchers say there is currently no proof for this, and suggest that other explanations such as the wider dietary pattern, stress or sleep disruption may be responsible.