How to recognize a food sensitivity to nightshade vegetables

Q: Why do I feel better after cutting nightshades from my diet? Should everyone avoid eating them?

A: If you develop inexplicable gastrointestinal symptoms after eating nightshades, which include tomatoes, white potatoes, eggplants and peppers, you may have a food intolerance, or “sensitivity,” to them.

The most reliable way to know if you have an intolerance to nightshades or any other food is to eliminate the food from your diet and see if your symptoms improve. Symptoms of nightshade intolerance include gas, bloating, diarrhea, heartburn, achy joints and severe fatigue. Some people report less abdominal pain and acid reflux and improved bowel habits and even general well-being after removing nightshades from their diet.

That doesn’t mean we should all stop eating them. Nightshades have gotten a bad rap lately on TikTok and celebrity-endorsed detox diets, but cutting them from your diet is unnecessary unless you have an allergy or intolerance. In fact, nightshades form part of one of the world’s healthiest diets: the Mediterranean diet.

So why do some foods, including nightshades, make some people feel ill?

Enter the confusing — some might say exciting — world of food sensitivities.

Identifying food sensitivities

It’s quite common for people to swear certain foods give them unpleasant symptoms. But it can feel frustrating when standard tests come back negative. There might not be a disease to diagnose or large research studies to officially prove why it’s happening, either.

If that sounds familiar, you may have a food intolerance. Examples include diarrhea after eating gluten but not having celiac disease or vomiting every time you have tofu despite a negative allergy test.

Could it be that doctors aren’t looking in the right place at the right time? A study published in Nature in 2021 explored that question. Researchers injected solutions of foods that commonly cause sensitivities, such as wheat or soy, into the colons of healthy people and patients with symptoms of irritable bowel syndrome after eating certain foods.

In healthy people, for the most part, nothing happened.

However, the scientists watched through a colonoscope as the guts of patients with irritable bowel syndrome became inflamed minutes after the injection. Microscopically, these patients also had more mast cells — the immune cells responsible for allergic reactions — near the nerve fibers of their guts. Yet each of these patients had tested negative for food allergies through the usual skin and blood tests.

In how many other people is this visceral reaction occurring? Perhaps the pain many experience after eating certain foods is due to unseen inflammation in the gut.

In other food sensitivities, like to fermentable carbohydrates, or FODMAPs, the foods themselves might cause symptoms because of their effect on intestinal gas.

The truth about nightshades

Now let’s clear the air about nightshades.

Nightshades are members of a plant family that contain glycoalkaloids — potentially toxic compounds that defend plants from pests and herbivores. There are several inedible nightshades such as tobacco plants, mandrakes and, of course, the deadly nightshade (also known as belladonna). These can cause poisoning with symptoms of blurry vision, hallucinations and potentially death and should not be consumed.

On social media, some people claim all nightshades are toxic. But if that were true, most of us would have been in big trouble a long time ago.

The nightshades found in grocery stores do have trace amounts of glycoalkaloids, but not in quantities that could poison us (thank goodness — you’ll never take my ketchup from me alive). Two exceptions are green or sprouting potatoes; these contain a higher concentration of the alkaloid solanine and can be potentially toxic to humans when eaten. If your potatoes have any light green spots or sprouts, cut those parts out before cooking and eating. It’s best to throw out potatoes with large green areas.

And while some studies in mice suggest nightshades may aggravate inflammatory bowel disease, rest assured: Researchers have not concluded that nightshades are implicated in human diseases, including arthritis or osteoporosis, as some have dubiously claimed.

A potentially “anti-inflammatory” diet, which excludes nightshades, has made the rounds online for those suffering from the autoimmune condition rheumatoid arthritis. There have been no studies to date that actually demonstrate such a diet can lower inflammation in these patients.

But can nightshades be the cause of a food intolerance? New research has found that solanine and other proteins in nightshades can activate mast cells, which may be the cause of bothersome symptoms in “sensitive” people.

Rebecca Kuang, an internal medicine physician at the University of Pittsburgh Medical Center, has been analyzing the role of nightshades in gastrointestinal disease. Because of their potential effect on mast cells, it’s possible that nightshades “can trigger GI symptoms of abdominal pain, diarrhea, but also systemic symptoms of tiredness and exhaustion after exposure to these chemicals,” she said in an email interview.

We still need to do much more research in the general population to get clearer answers and one day develop treatments for food intolerance, including for nightshades.

What I want my patients to know

You know your body better than I do. So unless the way you’re eating is harmful, I’ll support you in responding to your symptoms and avoiding foods that provoke them. But I get worried when people start cutting out large groups of foods without consulting with a registered dietitian first. It’s always safest to meet with a specialist who can create a systematic approach to identifying possible sensitivities while ensuring you maintain a healthy, balanced diet.

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