Nightshades. To Eat, Or Not To Eat?
"Are nightshades bad?" is a question I often get from clients. Although no concrete evidence suggests that we need to avoid them, some individuals with psoriasis, irritable bowel disease (Chron's disease and/or ulcerative colitis) and an autoimmune disease called rheumatoid arthritis have reported reduced symptoms with lower intakes or when they eliminated nightshades from their diet altogether.
The first time I heard any suggestion about nightshades being "bad" was the fall of 2017. Tom Brady, a quarterback for the New England Patriots at the time, was eliminating nightshades as part of a recommended diet plan. Admittedly, it's hard to argue against this type of approach when an athlete is at the top of their game. However, dietitians look at the research and evidence which then determines dietary recommendations. Depending on the topic, sometimes I go a bit further and ask additional questions like "Who is profiting from x, y, z?", or, "What politics are involved in promoting or discouraging a particular food or food group?". Evidence can be manipulated when there is potential for profit with fad diets so it's important to ask these questions. This didn't seem to be one of those situations, so I stuck with the research which is extremely limited.
Nightshades are a family of plants, Solanaceae, that includes edible and inedible varieties. They contain a compound called glycoalkaloids. You might see these referred to as "alkaloids" for short in some articles. The purpose of glycoalkaloids is to serve as a natural insecticide for the plant. Solanine is a type of glycoalkaloid and can be dangerous to humans in large quantities. It's the amount of solanine that makes a particular plant edible or toxic and they are often found in higher concentrations in the stems and leaves rather than the flesh of the fruit or vegetable.
The edible nightshades are potatoes (white, not sweet potatoes), bell peppers, eggplant, tomatillos, and tomatoes. Paprika and cayenne pepper, derived from peppers, are also in the edible group. Belladonna, mandrake and tobacco are examples of the poisonous, inedible nightshades.
Solanine increases with greening of the peel due to chlorophyll synthesis so avoid eating potatoes when their peels have a green hue. If you still have concerns, you can remove the skin of the potato and any sprouting spots. This has been shown to lower the glycoalkaloid content by 20% to 58%. Is it necessary to remove the skin if it's not green? No. Based on data that is available for potatoes, for example, it is not likely that we would consume too many glycoalkaloids when eaten in its natural form.
Glycoalkaloids, and therefore nightshades, have been reported to cause or exacerbate inflammation in the body. These reports are testimonials rather than research-based evidence. Some individuals with rheumatoid arthritis or irritable bowel disease (Crohn's disease or ulcerative colitis) have reported reduced symptoms after reducing or eliminating these foods from their diet, but this hasn't been part of a long-term or large-scale controlled study. Meaning there wasn't what's called a control group to compare results from those receiving nightshades and those that weren't. Research thus far is limited to mice and test tubes.
Allergy or Sensitivity?
You can consult your doctor if you think you have an allergy to these foods. It is possible to test for a nightshade allergy by having a skin prick test. Note that a food allergy and food sensitivity are different in that a food allergy triggers the immune system and can be severe (i.e. an anaphylactic response). Sensitivity symptoms, although uncomfortable, are not life-threatening. These can include itchy eyes, skin rashes, bloating, nausea and/or loose stools. If you think it's less severe and more like a sensitivity, a dietitian like myself can guide you through an elimination diet. A dietitian can help ensure you're receiving nutrients from other foods that you would be missing out on from eliminating nightshades.
What's the takeaway?
Unless you have a compromised intestinal tract or immune system, the nutrients in nightshades like vitamins (A, B6, and C), biotin, potassium, manganese, lycopene and fiber outweigh any risk of glycoalkaloids which has yet to be demonstrated in large-scale studies. The foundation of a healthy diet is plant-based foods which includes nightshades. More research is needed before recommendations to reduce or eliminate them can be made.
Misinformation about nutrition is everywhere. It's on television, on the radio, in magazines, on billboards and as hearsay from friends and family. It can also be tempting to follow what our favorite professional athlete is doing! Trying to make sense of all of the information available can be confusing and overwhelming. Ask questions, look at the research and consult a dietitian. A healthy, well-rounded diet doesn't have to be complicated and it can definitely include nightshades.
This post is intended to serve as an informative piece on nightshades and also in defense of them. My hope is to steer people away from labeling food as "bad." Foods are typically nutritious and beneficial for us, or they're not. I'm thinking of some roasted nightshades versus that sugary mocha chocca frotha latte that might have been part of someone's breakfast or mid-afternoon treat! Most people can benefit from including more fruit and vegetables rather than reducing them.
Nightshades can be enjoyed and celebrated in many different ways. I have potatoes and tomatoes most days as part of my breakfast. I'm also a fan of tomatoes in sandwiches, salads, pasta sauce and salsa! I love sun dried tomatoes and hummus on toast as another breakfast option. Sliced bell peppers are one of my favorite snacks and one I often recommend as a non-starchy snack option for Type 2 diabetics. Eggplant is usually one of my favorite side dishes at my local Mediterranean restaurant along with hummus and falafel. I have a variety of tomatoes and bell peppers in my garden this year and I can wait to try them!
If you would like to read about glycoalkaloids in potatoes in more detail, I found this article to be helpful and informative: A Review of Occurrence of Glycoalkaloids in Potato and Potato Products