Medical nutrition & nightshades: A look at the evidence

Nightshade fruits and vegetables — tomatoes, peppers, eggplant and potatoes — have sparked debate for decades. The point of contention? Whether or not these foods can cause flares in some people with autoimmune disease, including inflammatory forms of arthritis such as rheumatoid arthritis and psoriatic arthritis. Here is what primary care physicians need to know to clear up patients’ misconceptions.  

Learn more about this topic in the HealthCert Professional Diploma program in Medical Nutrition Management – online nutrition training for GPs.

Are nightshades toxic?

When people talk about the toxins in nightshades, they are often referring to glycoalkaloids and alkaloids like solanine, capsaicin, and nicotine, which act as natural pesticides (1). Many so-called health gurus prescribe eliminating all nightshade vegetables in your diet because they may be toxic to those who consume it.

While cases of solanine poisoning have happened – the best-known incident was in the late 1970s, when a group of schoolchildren were poisoned by green potatoes (2) – they are rare and usually linked to improper storage or preparation of food. For most people (i.e., those without diagnosed allergies), nightshade plants are tolerated well by the digestive system and provide a range of beneficial vitamins and minerals. A group of pigments called anthocyanins (responsible for the red, purples, and blues in fruit and vegetables) have been shown in human clinical trials to exert powerful antioxidant effects (3, 4).

Inflammation and autoimmunity

People with autoimmune diseases such as rheumatoid arthritis and inflammatory bowel disease may worry that these foods can lead to gut problems and, thus, worsen inflammation, but there's no robust evidence supporting this claim. While some older studies have indicated that the glycoalkaloids found in nightshades might exacerbate gut lining permeability and inflammation, these were conducted in mice models (5). Conversely, recent research shows that nightshades could even reduce inflammation, due to their high levels of antioxidants (6, 7).

There is also confusion about lectins, an “anti-nutrient” that are present in high amounts in nightshades and which have been cited as a major cause for obesity, chronic inflammation, and autoimmune diseases (8). Most healthy people can tolerate these during the digestive process with no problem, with studies showing that consuming wholegrains and legumes in an overall healthy dietary pattern can help to prevent disease (9-11). However, others with bowel syndrome (IBS) may complain of the gastrointestinal effects of lectins, although it’s difficult to determine if they are responding specifically to the lectins in isolation (8).

For most people, there’s no need to avoid nightshades, as there isn’t enough robust evidence to link them to negative health effects. However, just like any food, it is possible to be intolerant or allergic to them. Primary care physicians should work closely with patients to identify root causes of symptoms.

Learn more with the online HealthCert Professional Diploma program in Medical Nutrition Management.

– Lynette Law, Accredited Practising Dietitian

Read another blog: The role of nutrition in endometriosis

References

1. Chowański S, Adamski Z, Marciniak P, Rosiński G, Büyükgüzel E, Büyükgüzel K, et al. A Review of Bioinsecticidal Activity of Solanaceae Alkaloids. Toxins (Basel). 2016;8(3).
2. Solanine poisoning. British Medical Journal. 1979;2(6203):1458-9.
3. Gonçalves AC, Nunes AR, Falcão A, Alves G, Silva LR. Dietary Effects of Anthocyanins in Human Health: A Comprehensive Review. Pharmaceuticals (Basel). 2021;14(7).
4. Wallace TC, Slavin M, Frankenfeld CL. Systematic Review of Anthocyanins and Markers of Cardiovascular Disease. Nutrients. 2016;8(1):32.
5. Iablokov V, Sydora BC, Foshaug R, Meddings J, Driedger D, Churchill T, et al. Naturally occurring glycoalkaloids in potatoes aggravate intestinal inflammation in two mouse models of inflammatory bowel disease. Dig Dis Sci. 2010;55(11):3078-85.
6. Hellmann H, Goyer A, Navarre DA. Antioxidants in Potatoes: A Functional View on One of the Major Food Crops Worldwide. Molecules. 2021;26(9).
7. Przybylska S, Tokarczyk G. Lycopene in the Prevention of Cardiovascular Diseases. Int J Mol Sci. 2022;23(4):1957.
8. Petroski W, Minich DM. Is There Such a Thing as “Anti-Nutrients”? A Narrative Review of Perceived Problematic Plant Compounds. Nutrients. 2020;12(10):2929.
9. Aune D, Norat T, Romundstad P, Vatten LJ. Whole grain and refined grain consumption and the risk of type 2 diabetes: a systematic review and dose-response meta-analysis of cohort studies. Eur J Epidemiol. 2013;28(11):845-58.
10. Hosseinpour-Niazi S, Mirmiran P, Fallah-Ghohroudi A, Azizi F. Non-soya legume-based therapeutic lifestyle change diet reduces inflammatory status in diabetic patients: a randomised cross-over clinical trial. Br J Nutr. 2015;114(2):213-9.
11. Tosti V, Bertozzi B, Fontana L. Health Benefits of the Mediterranean Diet: Metabolic and Molecular Mechanisms. J Gerontol A Biol Sci Med Sci. 2018;73(3):318-26.