Green potatoes are poisonous.

Are green potatoes toxic?

Pawel Malczewski
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Short summary

A green potato indicates that it may contain a high level of naturally occurring toxic chemicals called glycoalkaloids. If this level is high enough, green potatoes become poisonous and result in the loss of consciousness and even death. Fatal poisonings from green or incorrectly handled potatoes are not uncommon. For a quick answer click here.

Explanation

Potatoes and other nightshade vegetables contain toxins called steroidal glycoalkaloids. Two of the most abundant toxins are solanine and chaconine that guarantee a natural protection from insects and fungus.

Note: digestive issues associated with eating raw potatoes have different origins such a presence of resistant starch or contamination of potatoes with bacteria present in the soil, and are not as severe as in the case of green potatoes  (read more..)

Why are green potatoes toxic?

When potatoes are subjected to stress such as sunlight (in the field or during storage) they accumulate chlorophyll, visible as the green patches on the peel (dark-skinned potato varieties may camouflage it). The green color usually extends to the flesh of the potato. This exposure to the sun is a stress factor, which causes the amount of the above-mentioned toxins to increase in the potato. (1)Friedman M, McDonald GM. Postharvest changes in glycoalkaloid content of potatoes. Adv Exp Med Biol. 1999;459:121-43. Available here. (2)Kuiper-Goodman T, Nawrott PS. Solanine and chaconine. IPCS Inchem. Available here. (3)Storey RMJ, Davies HV. Tuber quality. The Potato Crop. World Crop Series 1992, pp 507-569. Available here.

There are other stress factors that can cause a similar effect on the toxins: mechanical damage, improper storage or when sprouting due to high temperature.
The amount of toxins in the potatoes depends largely on how the growers handle them and how potatoes are stored by retailers.

Peeling removes only up to 30% of these toxins. Any form of cooking does not change the amount of these toxins in the potatoes. Once the levels of toxins increase, they are harmful regardless of how you cook them. (4)Woolfe J. The Potato in the Human Diet. June 2009. Cambridge University Press. Chapter 5. Available here.

There have been cases of poisoning (sometimes fatal) every year from potatoes with high levels of these toxins. Please note that the green color is only an indication that the particular potato was exposed to the sun and therefore can be toxic. Toxins may be present in other potatoes, not necessarily of a green color.

Although poisoning from these chemicals is mostly related to eating toxic potatoes, higher amounts of these toxins may be also present in unripe (green) tomatoes. However, reports of poisoning are rare. Studies show that the amount of toxins in green tomatoes is safe, but the Canadian government advises minimizing the exposure by reducing the consumption of green tomatoes and products made of green tomatoes, such as relishes, and never consuming other tomato green parts, such as leaves or steams. (5)Stimekova E, Horcin V. Determination of solanine in tomato cultivars. Article first published online: 25 AUG 2006. Available here. (6)Health Canada. Food and Nutrtion. Glycoalkaloids in Foods. Available here.

The maximum safe level of glycoalkaloids is established at 20mg/100g of potato (fresh weight). (7)Health Canada. Food and Nutrtion. Canadian Standards (Maximum Levels) for Various Chemical Contaminants in Foods. Available here.

Here are some common examples of the contents of total glycoalkaloids of which majority are solanine and chaconine: (8)Kuiper-Goodman T, Nawrott PS. Solanine and chaconine. IPCS Inchem. Available here.

  • whole potato (or tuber): 4.3-9.7 mg/100g
  • potato flesh: 1.2-5 mg/100g
  • skin: 15-30 mg/100g
  • potato that is poisonous (tastes bitter or is green): 25-80 mg/100g
  • peel from the poisonous potato: 150-220 mg/100g

Symptoms of poisoning from green potatoes

The symptoms of low-grade poisoning, which usually start from minutes to 2 days after eating the toxic potato, include diarrhea, vomiting, severe stomach upset and pain.

The symptoms of severe poisoning have a longer incubation period and are characterized by neurological symptoms such as drowsiness, confusion, apathy, weakness and impaired vision. There have been reports of unconsciousness and death. (9)McMillan M, Thompson JC. An Outbreak of Suspected Solanine Poisoning in Schoolboys: examination of criteria of solanine poisoning. PubMed. Volume 48, Issue 2, 1 April 1979. Available here.

It has been found that if potatoes are fresh, were handled correctly by the cultivators and were not exposed to the sun, contain the normal glycoalkaloid levels and if consumed daily as a staple diet do not pose any risks.

How does the poisoning work?

Toxins, such as solanine, prevent the breakdown of a neurotransmitter called acetylcholine in the body. It results in an accumulation of acetylcholine in the areas where nerve endings interact with muscles, preventing the nervous system from functioning properly, therefore affecting the organs.

Conclusion

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Here are a few methods to reduce the risk of getting poisoned: (10)Health Canada. Food and Nutrtion. Glycoalkaloids in Foods. Available here. (11)Health Canada. Food and Nutrtion. Natural Toxins in Fresh Fruit and Vegetables. Available here.

  • Buy only fresh potatoes, avoid potatoes that look old or are already sprouted and definitely do not eat sprouts growing on potatoes;
  • Store potatoes in a cool, dry place, away from the sun or artificial light;
  • Avoid potatoes that have been damaged, have green patches, bruises, cuts or with any parts rotting (read more..);
  • Peel off all the potato skin where most toxins are contained;
  • Do not eat bitter tasting potatoes, since it is an indication of a high amount of toxins present.

References   [ + ]

1. Friedman M, McDonald GM. Postharvest changes in glycoalkaloid content of potatoes. Adv Exp Med Biol. 1999;459:121-43. Available here.
2. Kuiper-Goodman T, Nawrott PS. Solanine and chaconine. IPCS Inchem. Available here.
3. Storey RMJ, Davies HV. Tuber quality. The Potato Crop. World Crop Series 1992, pp 507-569. Available here.
4. Woolfe J. The Potato in the Human Diet. June 2009. Cambridge University Press. Chapter 5. Available here.
5. Stimekova E, Horcin V. Determination of solanine in tomato cultivars. Article first published online: 25 AUG 2006. Available here.
6. Health Canada. Food and Nutrtion. Glycoalkaloids in Foods. Available here.
7. Health Canada. Food and Nutrtion. Canadian Standards (Maximum Levels) for Various Chemical Contaminants in Foods. Available here.
8. Kuiper-Goodman T, Nawrott PS. Solanine and chaconine. IPCS Inchem. Available here.
9. McMillan M, Thompson JC. An Outbreak of Suspected Solanine Poisoning in Schoolboys: examination of criteria of solanine poisoning. PubMed. Volume 48, Issue 2, 1 April 1979. Available here.
10. Health Canada. Food and Nutrtion. Glycoalkaloids in Foods. Available here.
11. Health Canada. Food and Nutrtion. Natural Toxins in Fresh Fruit and Vegetables. Available here.

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