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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. Green potatoes contain elevated levels of these toxins which may cause some health problems.
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).
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)
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, 6)
The maximum safe level of glycoalkaloids is established at 20mg/100g of potato (fresh weight). (7)
Here are some common examples of the contents of total glycoalkaloids of which majority are solanine and chaconine: (8)
- 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)
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.
How to avoid poisoning from green potatoes?
- 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.
NUTRITION FACTS VS NUTRITION MYTHS
You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.