Foods in the nightshade family, like potatoes and tomatoes, are blamed for a host of health issues by so-called wellness gurus, but is there any scientific evidence behind the claims? Nutrition scientist Tim Crowe finds out.
Heard of nightshades? They’re a group of plants that include some very commonly eaten foods, including tomatoes, eggplants, capsicums and potatoes. These are staple foods eaten all around the world, but if you go digging into health and wellness advice on the internet, you’ll come across all sorts of alarming health warnings – such as digestive issues and inflammation – about nightshades. But what does the science say about health harms from eating nightshades, and what about the many health benefits that they also give you?
What are nightshade vegetables?
Nightshades are plants that belong to the Solanaceae family. This is a pretty big family and many of the plants in it, such as tobacco, are considered inedible. But there are some foods in the family which are staples eaten the world over, like tomatoes, white potatoes, eggplant and peppers.
Nightshades and glycoalkaloids
Nightshades naturally contain potentially harmful compounds called glycoalkaloids, and this is where they get their fearsome reputation. Glycoalkaloids help plants defend themselves against insects, disease and some plant-eating animals. Because these compounds can be toxic to predators, you can see why there are health concerns about these foods. One of the main glycoalkaloids in the edible nightshades is a compound called solanine. Solanine, a neurotoxin found in the skin of potatoes that are turning green, can cause digestive problems like stomach upset, nausea and vomiting. Cases of poisoning, though, are rare and you’d have to eat a lot of green potatoes to have an effect. One estimate is that it would take almost 500g of fully green potatoes to make a small adult sick.
Nightshades and inflammation
Gut issues from glycoalkaloids aside, probably the most common alleged health issue you’ll read about nightshades has to do with inflammation. People love to label foods as inflammatory, yet there are very few foods that are clearly inflammatory or anti-inflammatory; the evidence just doesn’t go that deep to make such simplistic statements. It is whole dietary patterns and lifestyles that are linked to inflammation, not single foods. Think of a Mediterranean-style diet as a classic anti-inflammatory diet. The opposite of this is a typical Western diet, high in added sugar and highly processed food, which is linked to inflammation. The amount of solanine, which certainly can act as an inflammatory irritant in the gut if you eat enough of it, is incredibly low in nightshades. And it is the dose that matters. If your diet isn’t rich in green potatoes, then you’re good to go.
Nightshades and health: it’s complex
The problem of zeroing in on one component of a food, such as solanine, is that it neglects the food’s benefits. For example, there is evidence that eggplants have anti-inflammatory properties thanks to the interplay between glycoalkaloids, antioxidant compounds and vitamins. At least one study has shown that solanine has anti-inflammatory properties and could be a valuable compound in the treatment of inflammatory diseases, while others indicate that it may help inhibit the growth of cancer cells and could be a potential anti-cancer treatment. But other studies suggest nightshades could be a trigger for people with inflammatory bowel disease and irritable bowel syndrome. Most of the research is in early stages and almost all is in cells grown in a laboratory or in animals. But it shows that things are a lot more complex than simplistic advice to cut nightshades out of your diet.
Nightshade sensitivity is likely a thing.
With nightshades there is research, and then there is personal experience. It does seem that certain people, particularly people with autoimmune diseases such as rheumatoid arthritis, fibromyalgia, and lupus, may find that nightshades aggravate their symptoms. While most of the evidence here is anecdotal, some people do report unpleasant digestive symptoms or joint-related pain when they eat nightshades. Whether it is real or imagined, if a person finds they feel better for eating less of these foods then that is a good thing. If you suspect you have a sensitivity or intolerance to nightshades, it may be helpful to talk with a dietitian about eliminating them from your diet in a controlled way, to see if it helps with any symptoms you may be experiencing. But that doesn’t mean that everyone has to stop eating nightshades. Just because some people have anaphylactic reactions to peanuts, we don’t tell everyone to avoid them.
Nightshade benefits
Edible nightshades are mostly low in kilojoules and super high in a whole range of beneficial nutrients. This makes them an excellent choice for people trying to maintain a healthy weight and keep healthy. They’re also a great source of dietary fibre, which can help support a healthy gut.
Potential harms
Some people, anecdotally, report that eating nightshades aggravates autoimmune condition symptoms and digestive issues in irritable bowel syndrome or inflammatory bowel disease.
The bottom line
Nightshade vegetables are not only safe to eat for almost anyone, but they also offer many health benefits. And while it is true that some people may have a sensitivity or intolerance to nightshades, for the majority of people they are a healthy and delicious addition to the diet. So if you haven’t already, consider adding more nightshades to your diet and experience the many health benefits they have to offer.
References:
Im et al. 2016. In Vitro antioxidative and anti-inflammatory activities of the ethanol extract of eggplant (Solanum melongena) stalks in macrophage RAW 264.7 cells, Food Agric Immunol, 27(6):758-771.
Koelman et al. 2022. Effects of Dietary Patterns on Biomarkers of Inflammation and Immune Responses: A Systematic Review and Meta-Analysis of Randomized Controlled Trials, Adv Nutr, 13(1): 101-115
Kuang et al. 2023. Nightshade Vegetables: A Dietary Trigger for Worsening Inflammatory Bowel Disease and Irritable Bowel Syndrome? Dig Dis Sci. 68:2853–2860.
Luo et al. 2021. A narrative review of the antitumor studies of solanine. Trans Cancer Res. 10(3).
US Forest Service. The Powerful Solanaceae. Available at fs.usda.gov/wildflowers/ethnobotany/Mind_and_Spirit/solanaceae.shtml
Zhao et al. 2018. Steroidal alkaloid solanine A from Solanum nigrum Linn. exhibits anti-inflammatory activity in lipopolysaccharide/interferon γ-activated murine macrophages and animal models of inflammation, Biomed. Pharmacother. 105: 606-615.
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