This article summarizes the toxicity of the food that comes from species of Solanum genus, focusing on potatoes, tomatoes and eggplant (or aubergine if you will). I still cannot believe that these food sources have become so popular over the hundreds of years even though the studies regarding their extracts are so limited (or I can’t seem to be able to find any good ones). Let’s just say these food sources are certainly not the most appealing to my eyes, but a majority of people would argue their points how brilliant these are even though that’s far from reality.
One of the main concerns for the nightshade family is the presence of steroidal glycoalkaloids. These alkaloids are present in all parts of the plants in varying concentrations. They serve as a defense against pathogens and predators that quickly regret considering eating them. This is interesting because it has been shown that glycoalkaloids are less toxic to animals than they are to humans and yet humans seem to love consuming food containing them. They are the reason why your meal might taste bitter, so that should dissuade you to eat it, just like curcurbitacins in overripe or decorative squashes. Sadly, this is not always the case and people continue to eat them despite the bitter flavor.
Steroidal glycoalkaloids are often associated with a lethal dose of 3-5 mg/kg 1, but this is obviously different for each individual glycoalkaloid. They are inhibitors of acetylcholinesterase, making them similar in action to other nerve agents and venoms. Their effects might even be compared to strychnine. Additionally, they are disruptors of cell membranes, and negatively impact calcium metabolism and transport of ions through membranes. In my opinion, they can induce poisoning even at low doses, it’s just that the human body doesn’t perceive it as much as higher doses. Symptoms of poisoning are mainly related to gastrointestinal tract (e.g. gastroenteritis, discomfort, decreased gastric emptying, burning sensation in mouth, gastrointestinal bleeding, diarrhea, nausea, vomiting) and central nervous system (increased heart rate, low blood pressure, respiratory distress, convulsions, paralysis, headaches, coma, death) 1,2,3.
Potatoes (Solanum tuberosum)
There are several misconceptions that are associated with potatoes. Almost everyone knows the dangers of eating green potatoes, but not everyone knows that this extends to sprouted potatoes as well. Instead of throwing the sprouted potatoes away, many people just cut off the sprouts and use them anyway. Another mistake that people make is that they think solanine is the only toxic compound in potatoes since it’s the only one that’s talked about. Moreover, a large number of consumers really enjoy eating potato skin and claim that it’s really healthy for you which is not true at all. The concentration of glycoalkaloids seems to decrease by 70% upon removal of the peel 2. And the worst of them is that they think solanine can be thermally degraded, which is true, but not for temperatures that are commonly used in household cooking. It has been shown that no process of cooking, dehydrating, freezing or frying had any impact on the glycoalkaloids present in potatoes 2. While there might be a chance that bleaching or washing your potatoes may reduce the content of glycoalkaloids, this does not fully diminish the presence of glycoalkaloids 2. There are several sources that state that boiling or frying potatoes does reduce the glycoalkaloid content, this might also be due to them shrinking, so technically they would be right.
Approximately 95% of the glycoalkaloid content in potatoes is composed of α-solanine and α-chaconine, while the remaining 5% belongs to ß-/γ-solanine and ß-/γ-chaconine.
The upper limit of glycoalkaloids was stated to be 200 mg/kg for fresh potatoes 1. Typical potato contains approximately 12-20 mg/kg of glycoalkaloids, but there are also sources that claim that this value can be as much as 75 mg/kg. 2,3 When it comes to α-solanine, most of the commercially sold potatoes contain 40-100 mg/kg. This is where a lot of disconnects are visible. How can the content of α-solanine be higher than the range specified for glycoalkaloids? While there may be potato cultivars that only contain 12-20 mg/kg glycoalkaloids, there are also varieties that crossed the upper maximum limit of 200 mg/kg of glycoalkaloids 2. The maximum dose of α-solanine that humans can tolerate was determined to be 1 mg/kg and potentially acute or lethal doses may be in the range of 2-6 mg/kg 2. So if you consume two kilograms of potatoes, they would roughly amount to 80-200 mg of solanine, which is more than enough to produce symptoms of acute intoxication if you weigh 80 kilograms.
The half-lives of α-solanine and α-chaconine have been determined to be 21 hours and 44 hours, respectively 3. There are also studies that claim these are much lower, but this highly depends on the test subject used.
α-solanine is very thermally stable, decomposition might occur at temperatures 260-270 °C. The same probably applies to other glycoalkaloids due to their insoluble nature, but I am not exactly sure, you can definitely expect thermal stability up to 150 °C. While some studies have found that frying potatoes in oil at high temperatures of oil removes some of the glycoalkaloid content, I am still skeptical to believe that they are highly fat-soluble.
Saturated cholesteranol is the main precursor of α-solanine and α-chaconine as it is cyclized to solanidine and then further glycosylated to α-solanine and α-chaconine inside the tubers 4. This process naturally occurs in potatoes, but there seems to be a lot of evidence suggesting that these two constituents get converted back to solanidine in gastrointestinal tract. Solanidine is the product of hydrolysis of α-solanine and possibly α-chaconine as well.
Toxicity:
α-solanine has been proven to be teratogenic, inducing malformations of the central nervous system, e.g. exencephaly, encephalocele and anophthalmia 2. There has been a link between consumption of potatoes stored over winter and anencephaly along with spina bifida 2. α-solanine has also induced several defects of organ systems in fish embryos 2.
α-solanine bioaccumulates in the body up to 24 hours after ingestion which makes its teratogenic effects either linked to the acute poisoning or accumulation. It has been found toxic to murine stem cells, making it a considerable risk for humans as well 2. It also affects morphology of cardiac cells and causes necrosis of the gastrointestinal tract 2. α-solanine should be poorly absorbed from gastrointestinal tract and reaches peak concentration after 12 hours 5, but yet again there are sources that state otherwise. At high (and most likely even low) doses it seems to target liver, spleen, kidneys and intestines 5.
Hamsters fed α-solanine and α-chaconine experienced fluid filled stomach and small intestines which proves that glycoalkaloids act as gastric emptying inhibitors in order to increase the serum concentration 6.
Several studies have proven that α-solanine and α-chaconine by themselves are less toxic than when they are acting synergistically.
Solanidine has been shown to bio-accumulate in human tissues, especially liver causing its enlargement. The accumulated content of solanidine may be activated during states of metabolic stress and cause negative health effects 7. According to the same study that used intravenous administration, the rate of excretion of solanidine might only be 2% per day 7. Solanidine has also been detected in serum of people before their midday meal 8, further indicating that solanidine is not just limited to gastrointestinal tract, but it might be targeting red blood cells. Abortifacient effects of solanidine have also been observed in mice fed solanidine for 14 days 9. Solanidine and tomatidine (glycoalkaloid from tomatoes) have also been implicated as scar pruritogens, exacerbating conditions of patients with scars 10. Even though solanidine has been shown to be less toxic than its glycoside derivatives α-solanine and α-chaconine, its accumulation and slow excretion make this compound an unknown that should be more investigated.
Potatoes might also contain antinutrient lectins that can destroy epithelia of gastrointestinal tract, but these seem to be destroyed with heat 11.
In summary, eating potatoes every day might not be the smartest thing to do, especially if they are your guilty pleasure.
Eggplant (Solanum melongena)
Compared to edible potatoes, this vegetable (which is botanically a fruit) is, in my opinion, even more deadly than potatoes. In terms of toxic compounds that you can ingest from this fruit these would be α-solanine, α-chaconine, solanidine, solamargine, solasonine and solasodine. Solamargine and solasonine are the highest in concentration and extremely underresearched. Solasodine (which is naturally formed from cholesterol) is the product of hydrolytic removal of a side chain from solasonine or solamargine. The main problem with eggplant is that there is a higher potential for synergistic toxicity due to the presence of solamargine and solasonine.
Solasonine content in ripe fruit seems to be around 74.7 mg/kg and the more bitter the fruit, the higher the concentration of glycoalkaloids 1. What is really interesting, unlike the tomato, the concentration of steroidal glycoalkaloids seems to increase with ripening.
The glycoalkaloid content of different eggplant varieties may reach 62.5 to 205 mg/kg. This range is similar to potatoes with the difference that eggplant’s toxicity is less predictable than potato’s toxicity due to limited studies done on solamargine, solasonine and solasodine.
Toxicity:
Solamargine and solasonine cause liver damage and heart damage, making eggplant more of a threat to organs. 4 Solamargine and solasonine seem to target organs just like other glycoalkaloids, so does this mean that they accumulate as well?
Solamargine is twice as teratogenic as solasonine. 12
Solamargine is most likely transformed quickly in the body 3, but the study regarding this used intravenous administration so I am not sure about its oral administration.
Solamargine and solasonine seem to be water soluble to some degree compared to solasodine which isn’t soluble in water. 4
Tomato (Solanum lycopersicum)
Tomato plants contain the alkaloids called α-tomatine, dehydrotomatine, tomatidine and esculeoside A. Tomatine (normally formed from cholesterol) is mostly degraded with the ripening process with roughly 5 mg/kg of tomatine remaining in ripe tomatoes. Esculeoside A might also be produced from tomatine as the fruit matures. 3
α-tomatine affects cell membranes as well, but it might not be as toxic as other glycoalkaloids since its complex with cholesterol might not be absorbed well from the intestines. 3 Tomatine seems to be degraded with longer storage, freeze drying or storing it at low temperatures. One table spoon of ketchup was found to contain 0.13 mg of α-tomatine, which is so much better than the concentrations of glycoalkaloids in potatoes and eggplant.
α-tomatine and α-solanine have the potential to cause arrest of heart activity at high doses and increase heart rate at lower doses 13 .
There are several questions that I haven’t been able to answer: What happens to solasodine in human body? Does it behave like solanidine and accumulates in the body? What are the retention times for solamargine and solasonine? What about the toxicity of ß- and γ- compounds?
References
1. Bagheri, M., Bushehri, A. A. S., Hassandokht, M. R. & Naghavi, M. R. Evaluation of Solasonine Content and Expression Patterns of SGT1 Gene in Different Tissues of Two Iranian Eggplant (Solanum melongena L.) Genotypes. Food Technol. Biotechnol. 55, 236–242 (2017).
2. Ordóñez-Vásquez, A., Aguirre-Arzola, V., Garza-Ramos, Angélica De la, M., Hugo Urrutia-Baca, V. & Suárez-Obando, F. Toxicity, Teratogenicity and Anti-cancer Activity of α-solanine: A Perspective on Anti-cancer Potential. Int. J. Pharmacol. 15, 301–310 (2019).
3. Brunton, N. & Siddique, M. A. B. Food Glycoalkaloids: Distribution, Structure, Cytotoxicity, Extraction, and Biological Activity. (2019). doi:10.5772/intechopen.82780
4. Al Sinani, S. S. S. & Eltayeb, E. A. The steroidal glycoalkaloids solamargine and solasonine in Solanum plants. South African J. Bot. 112, 253–269 (2017).
5. Nishie, K., Gumbmann, M. R. & Keyl, A. C. Pharmacology of solanine. Toxicol. Appl. Pharmacol. 19, 81–92 (1971).
6. Langkilde, S. et al. A 28-day repeat dose toxicity study of steroidal glycoalkaloids, α-solanine and α-chaconine in the Syrian Golden hamster. Food Chem. Toxicol. 47, 1099–1108 (2009).
7. Claringbold, W. D. B., Few, J. D. & Renwick, J. H. Kinetics and retention of solanidine in man. Xenobiotica 12, 293–302 (1982).
8. Harvey, M. H., McMillan, M., Morgan, M. R. A. & Chan, H. W. S. Solanidine is Present in Sera of Healthy Individuals and in Amounts Dependent on their Dietary Potato Consumption. Hum. Toxicol. 4, 187–194 (1985).
9. Friedman, M., Henika, P. R. & Mackey, B. E. Effect of feeding solanidine, solasodine and tomatidine to non-pregnant and pregnant mice. Food Chem. Toxicol. 41, 61–71 (2003).
10. Alonso, P. E. & Rioja., L. F. Solanidine and tomatidine trigger scar pruritus. Burns 42, 535–540 (2016).
11. Hoskins, F. H. Food Toxicants, Naturally Occurring. in Kirk-Othmer Encyclopedia of Chemical Technology 1–23 (John Wiley & Sons, Inc., 2009). doi:10.1002/0471238961.0615150408151911.a01.pub2
12. Blankemeyer, J. T., Mcwilliams, M. L., Rayburn, J. R., Weissenberg, M. & Friedman, M. Developmental Toxicology of Solamargine and Solasonine Glycoalkaloids in Frog Embryos. Food Chem. Toxicol. 36, 383–389 (1998).
13. Ventrella, E. et al. Cardioactive properties of Solanaceae plant extracts and pure glycoalkaloids on Zophobas atratus. Insect Sci. 22, 251–262 (2015).
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