We are amid the season of edible nightshades. Recipes showcasing the bold flavors of tomatoes, peppers, eggplant, and potatoes dominate menus from spring to fall. Though often praised for their nutrient content, a dark shadow lingers over these popular fruits and vegetables. They’re associated with a host of ailments and are accused of worsening inflammation, but do they warrant the concern?
Edible nightshades belong to the Solanaceae family, same as the poisonous deadly nightshade herb (Atropa belladonna). Meaning “beautiful lady” in Italian, Atropa belladonna has held cultural prominence since antiquity. A few drops in the eye cause pupils to dilate, thought to enhance one’s appeal. Though potentially lethal, titrated doses have a hallucinogenic effect, making it a once-important player in rituals and orgies. This spicy history makes edible nightshades boring in comparison, but their familial ties come with a tainted reputation. A deep dive into botany can help clarify the confusion.
Solanaceae plants contain solanine, a bitter-tasting glycoalkaloid known to produce a physiological response in humans. It protects the plant against harmful fungi, but is deadly when consumed in large quantities. Atropa belladonna is especially potent. Cases of toxicity show it inhibits nerve impulses responsible for involuntary muscle movement, such as that of the lungs and digestive and urinary tracts.
As for edible nightshades, the solanine content weighs in at a lower, nontoxic level after ripening (toxicity occurs at 2 to 5 mg solanine per kg of body weight—about 136 mg for a 150-lb person). The average eggplant contains 11 mg, meaning an adult would need to consume more than 11 eggplants in a single sitting to reach the lethal dose. There’s controversy over whether tomatoes contain any solanine, with researchers now identifying tomatine as the potential irritant in question. Potatoes, on the other hand, aren’t completely out of the woods. Though extremely rare, cases of solanine toxicity have been documented. The risk can be reduced by avoiding potatoes that have sprouted, taste bitter, or have a greenish hue, a sign of solanine build-up.
Still, even if deemed safe, worry remains over the possibility that trace amounts of solanine can aggravate autoimmune and bowel diseases. Thus far, research is limited. Studies have linked potato glycoalkaloids to increased inflammation and intestinal permeability in mice with inflammatory bowel disease, but the effects on humans are unknown. Reports linking nightshade consumption with arthritis are mostly anecdotal and unproven. In fact, the Arthritis Foundation recommends including nightshade vegetables, particularly colorful bell peppers, as part of a nutritious, antioxidant-rich diet.
Lectin, a carbohydrate-binding protein found in edible nightshades, also gives eaters pause. Lectins are resistant to human digestion and travel through the gut unchanged, with side effects ranging from gas and bloating to vomiting and diarrhea. Animal and cell studies have shown lectins interfere with mineral absorption and damage the intestinal flora. In a human study presented as a poster session, a diet eliminating lectin-rich, gluten-free foods improved biomarkers of leaky gut, and identified that another protein, aquaporin, also may cause harm. However, it’s worth noting that myriad foods (including grains and beans) were removed from a small number of participants’ diets, making it difficult to isolate nightshades as the offender. If clients are wary, cooking methods such as boiling and stewing vastly reduce lectin activity, as do soaking and sprouting beans and grains before cooking.
With all the fuss, why should clients even attempt to eat nightshades? Because, if tolerated, they’re nutrition powerhouses.
Tomatoes are rich in lycopene, an antioxidant and scavenger of free radicals used to prevent and combat cancer. Sliced bell peppers provide 1,490 mcg beta-carotene per cup, promoting a healthy immune system. Various varieties of potatoes contain vitamins A, B, C, and E, along with iron and zinc, and, with their availability and affordability, are a convenient way to get critical nutrients to people across the world. Lastly, the high phenolic content of eggplant ranks it among the top 10 vegetables for antioxidant capacity (with garlic topping the list).
Ultimately, there’s insufficient evidence to link edible nightshade consumption with autoimmune and bowel diseases or increased inflammation. However, it’s humbly acknowledged that current data are limited, so RDs should be careful not to disparage claims of intolerance. Instead, dietitians can guide patients through an elimination diet with careful reintroduction to determine whether nightshades are of a concern or a valuable part of a balanced diet. As part of the elimination diet RDs can provide a list of alternative foods to incorporate during the trial period to ensure they include vital nutrients.
— Bethany Oxender, MS, RD, CSOWM, is an author, registered dietitian, and certified specialist in obesity and weight management. She’s a contributor to the Academy of Nutrition and Dietetics’ Food & Nutrition Magazine. For a list of her work, please visit www.bethanygrey.com. She can be found on Instagram and Twitter as @bthnygrey.