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Nightshades: Friend or Foe?

There are many debates about which foods should be avoided in the diet. Among items like sugar, dairy, processed foods, gluten, grains, and soy, you may come across the idea nightshades should be eliminated, especially if you have arthritis or an autoimmune disease.

Is there any evidence that these vegetables, which often have healthy nutrients and phytochemicals, should be black-listed? Let’s see what the evidence has to say about it.

What are Nightshades?

Nightshade is the term given to a family of plants whose scientific name is Solanaceae. Some of the more than 3,000 species in this family are known poisons, such as belladonna. Others are familiar foods found in diets around the world. The most commonly eaten varieties include:

  • Ashwagandha
  • Cape gooseberry
  • Eggplant
  • Ground cherry
  • Peppers (except peppercorn)
  • Potatoes (not yams or sweet potatoes)
  • Tomatillo
  • Tomatoes

In addition, there are some plants that also contain solanine that some may recommend you avoid if you display a sensitivity, including blueberries, artichokes, and huckleberries.

Why Remove Nightshades?

Those in the anti-nightshades camp generally argue that these plants can be harmful for the following reasons:

  • Alkaloids and glycoalkaloids
  • Lectins
  • Food allergies and histamine

Let’s start by looking at the alkaloids and glycoalkaloids in the plants. These are natural pesticides and include solanine, capsaicin, and nicotine. The hypothesis is that since they act as a protection to the plant, they may be toxic to those who consume it. Some studies have found adverse reactions to some of these alkaloids and glycoalkaloids.

In one mouse study, the natural glycoalkaloids in potatoes increased intestinal inflammation in mice with two different models of IBD. The researchers used fried potato skins that had low to medium levels of glycoalkaloids and potatoes with high levels. One IBD model (deficient of interleukin-10 genes) had significantly elevated levels of IFN-gamma compared to controls after consuming the fried skins with medium glycoalkaloid levels. Those in the other model of IBD (dextran sodium sulfate colitis) also had significantly elevated levels of three pro-inflammatory cytokines (IFN-gamma, TNF-alpha, and IL-17). They also had enhanced intestinal permeability. This inflammatory response increased with higher levels of glycoalkaloid.

An in vitro test on rat and human intestinal mucosal epithelial cells did show an increase in intestinal permeability to potato and tomato glycoalkaloids, especially alpha-tomatine. However, this only shows a potential mechanistic effect and does not demonstrate what may happen in humans consuming the whole plant with its other nutrients.

On the other hand, there is evidence that potatoes may actually be beneficial for IBD. In one study with 103 participants, there was an inverse association between consuming legumes and potatoes and disease relapse. Those in the highest quartile for consuming legumes and potatoes had a 79 percent lower risk of having active IBD. In the same study, there was a positive association with consuming meat, and there was no association with vegetables, cereals, fish, or dairy. Potatoes have also been shown to have other health benefits, in part due to the potassium, vitamin C, and fiber found in them.

There is also evidence pointing to the idea that some of the glycoalkaloids may be beneficial. For example, although associated with causing pain, capsaicin has been shown to be effective in controlling pain as well as having benefits for various health concerns, including obesity, cancer, diabetes, and cardiovascular disease. Some of the steroid glycoalkaloids may also have potential anti-tumor effects. One study in mice found that tomatidine, a glycoalkaloid in tomatoes, could help to reduce atherogenesis.

Some believe that solanine, one of the alkaloids, contributes to inflammation in arthritis. However, the connection between arthritis and nightshades is largely considered anecdotal with limited science. In one study, consuming colorful potatoes actually had an anti-inflammatory effect, at least in healthy men, largely thanks to their high levels of antioxidants.

As you can see, the evidence is mixed in terms of whether alkaloids and glycoalkaloids are harmful. It may be that some varieties are harmful while others have potential health benefits, some of which may exist in the same nightshade plant. There may also be a dose-response relationship. Many of the studies thus far are animal studies or in vitro with the glycoalkaloids or alkaloids isolated from the food, which limits the ability to apply that to real-world scenarios. There is the possibility that some people are more sensitive to the effects, however.

Many nightshade vegetables contain lectins, which some believe are anti-nutrients, as discussed in a previous blog. However, when prepared properly, lectins do not have the same toxicity and do not create the inflammation and intestinal permeability that many claim. That being said, it does not mean that some people do not have an adverse reaction to lectins or need to remove them from their diet from time to time.

People can have a food allergy to plants in the nightshade family. A food allergy to potatoes is possible, with those with atopic dermatitis and other food allergies the most at risk. People also report food allergies to tomatoes and eggplants (aubergines), although the allergy to eggplant may be from the histamine content rather than to the eggplant-specific proteins. As discussed in a previous blog, when you do present with a food allergy, it is beneficial to remove those foods from your diet.

Is There Evidence for the Benefits of Eliminating Nightshades from the Diet?

There have been some studies that point to a potential benefit to eliminating nightshades in the diet, at least for some people. In one study, removing nightshades improved atopic dermatitis in patients. In this cross-sectional study, 169 patients were surveyed about their diet, and 87 percent had removed some type of food from their diet. Those who removed white flour, gluten, and nightshades had the best results compared to others, at 53.6 percent, 54.1 percent, and 54.1 percent respectively. This study was a survey and relied on self-reported outcomes, but it does demonstrate the potential for the removal of nightshades in improving atopic dermatitis for a percentage of people.

A similar study using a survey and self-reported outcomes found that reducing nightshades helped to improve skin in those who have psoriasis, with 52.1 percent of those who had eliminated nightshades from their diet reporting skin improvement, which was the third most effective behind alcohol (53.8 percent) and gluten (53.4 percent). In this study, 41.5 percent of responders did find that making alterations to their diet (adding or removing items) played a more important role in treating their psoriasis than over the counter medication, complementary medications, prescription medications, exercise, and stress reduction.

The autoimmune protocol (AIP) diet, which is popular for those with autoimmune diseases, incorporates the removal of nightshades. In a small study with just 15 patients with IBD (9 with Crohn’s disease and 6 with ulcerative colitis), 73 percent reached clinical remission after the 6-week elimination phase of the AIP diet, and all maintained clinical remission during the maintenance phase of the study (five weeks during which no new foods were introduced). It was performed in adjunct to medical therapy, rather than in lieu of. This diet is meant to be a limited-time, elimination style diet to heal the gut and discover which foods to remove from your diet on a more long-term basis.

Conclusion

The research against nightshades, especially in humans reflecting real-world application, is limited at the moment. Some studies point to a potential problem with nightshades in some populations. Thus, although for most people avoidance of nightshades may not be necessary, there is a section of the population for whom it may be beneficial. Those who will likely benefit the most from a trial elimination are those who have an autoimmune disorder, arthritis, or IBD, but there may be others who also benefit from it, such as those with psoriasis, atopic dermatitis, or some other skin condition.

If you struggle with a chronic illness, have removed the more common triggers from your diet, and consume a lot of nightshades, then you may benefit from a trial to see if eliminating them works. Talk with your nutritionist, dietician, doctor, or other healthcare professional to see if removing nightshades may be beneficial for you.

 

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