Heard of nightshades? They are a group of plants that include some very commonly eaten foods. Tomatoes, eggplants, capsicums and potatoes are all nightshades. 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 about nightshades. Digestive issues and inflammation feature prominently. In this blog post, I’ll explore the evidence for any health harms from eating nightshades and put this into context for the many health benefits that they also give you.
Nightshades are plants that belong to the Solanaceae family. This is a pretty big family with many of the plants in it considered inedible. Tobacco is a nightshade for example. But there are some foods in the family which are staples eaten the world over. Tomatoes, white potatoes, eggplant and peppers feature prominently.
Where the term nightshade came from is unclear. But it could be related to their ‘dark and mystical’ past as some nightshades were rumoured to have been used as narcotics and hallucinogens. So, the word translates as ‘shade that is active in the night’ where shade here means ghost or spirit.
Nightshades contain glycoalkaloids
Nightshades naturally contain potentially harmful compounds called glycoalkaloids. And this is where they get their fearsome reputation. Glycoalkaloids are there to help the plant defend itself against insects, disease and some plant-eating animals. Because these compounds can be toxic to predators, you can see where the health concerns about these foods come in.
One of the main glycoalkaloids in the edible nightshades is a compound called solanine. Solanine is found in the skin of potatoes that have started to turn green. Solanine is a neurotoxin that can cause digestive problems like an upset stomach, nausea and vomiting. Cases of poisoning though are rare and you’d have to eat a lot of green potatoes. One estimate is that it would take almost 500 grams of fully green potatoes to make a small adult sick.
Yet there are plenty of plants that contain compounds that can be potentially harmful to us humans if eaten in high enough amounts. For example, just because some species of mushrooms are toxic doesn’t mean that all mushrooms are off the menu.
And then we have the large family of polyphenols. Several polyphenols give foods such as broccoli, spinach and even coffee their bitter taste to stop them from being eaten. Sensing bitterness is a primal way for our bodies to detect potential poisons, as many of these bitter chemicals are part of a plant’s defence system. Yet eating small amounts of these substances can stimulate the body’s defence system. This enhances our antioxidant system to protect against disease.
So, it does seem curious that poor old nightshades get singled out here because they contain a potentially harmful class of chemicals.
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. I’ve already done a whole blog post on the topic of inflammation so you can check that out here for all the background you’ll need.
People love to label foods as ‘inflammatory’ as a simple way to give advice on how to cut out food from your diet to improve your health. 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.
Dietary patterns and lifestyles trump individual foods. And it is whole diets and lifestyles that are linked to inflammation. 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 and which is linked to inflammation.
But is there any merit in tarnishing the nightshades as inflammatory foods? No. 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 here 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 just homing in on one particular component of a food such as solanine to make a judgement that a food is unhealthy neglects the positive health benefits of the food. Take eggplants for instance. There is evidence that eggplants have anti-inflammatory properties thanks to the interplay between glycoalkaloids, antioxidant compounds and vitamins.
And at least one study could show that solanine has anti-inflammatory properties with the authors going on to state that it could be a valuable compound in the treatment of inflammatory diseases. And probably worth mentioning that several studies suggest that solanine may help inhibit the growth of cancer cells and could be a potential anti-cancer treatment.
To counter the positive links, there is research to suggest that nightshades could be a dietary trigger for people with inflammatory bowel disease and irritable bowel syndrome. Potentially the alkaloids in nightshades may further aggravate the intestinal lining of these people. But more research is needed in humans before recommendations to eliminate nightshades can be made.
For everything I’ve just written in this section, don’t get too excited about it either way. As most of the research I’ve profiled is in very early stages and almost all of it is in cells grown in a laboratory or in animals. But it serves to show that things are a lot more complex than simplistic health and wellness advice telling you to cut nightshades out of your diet.
Nightshade sensitivity is likely a thing
For nightshades, there is research, and then there is personal experience. And 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. This allows you to do it 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. I mean, just because some people have severe anaphylactic reactions to peanuts, we don’t then tell everyone to avoid them. You can make a case for any food as having some level of adverse problems in a subset of people. Nightshades appear to be no different.
And here’s another big plus of nightshades: they’re all 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. And have I mentioned that they’re a great source of dietary fibre, which can help support a healthy gut?
What it all means
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.