Potatoes that are turning green contain more solanine.
Although potatoes contain a natural pesticide, they are not harmful to humans in the amount we typically consume.
Here’s what Wikipedia says about solanine in potatoes:
Solanine is a glycoalkaloid poison found in species of the nightshade family (Solanaceae), such as the potato (Solanum tuberosum) and the tomato (Solanum lycopersicum). It can occur naturally in any part of the plant, including the leaves, fruit, and tubers. Solanine has pesticidal properties, and it is one of the plant’s natural defenses.
When potato tubers are exposed to light, they turn green and increase glycoalkaloid production. This is a natural defense to help prevent the uncovered tuber from being eaten. The green colour is from chlorophyll, and is itself harmless. However, it is an indication that increased level of solanine and chaconine may be present. In potato tubers, 30–80% of the solanine develops in and close to the skin.
Showing green under the skin strongly suggests solanine build-up in potatoes, although each process can occur without the other. A bitter taste in a potato is another, potentially more reliable indicator of toxicity. Because of the bitter taste and appearance of such potatoes, solanine poisoning is rare outside conditions of food shortage.
Deep frying potatoes at 170°C (338°F) is known to effectively lower glycoalkaloid levels, because they move into the frying fat, as does boiling, because solanine is water-soluble, while microwave cooking is only somewhat effective, and freeze-drying or dehydration has little effect.
I ran some numbers:
Wikipedia says that doses of solanine down to 2 to 5mg/kg of body weight can cause symptoms such as gastrointestinal distress. This site says it takes about 28mg/kg, and that most commercial varieties contain less than 12mg/100g, normally between 2 and 13 mg/100g.
Nutritiondata says that 1 small potato, about 2 inches in diameter, weighs about 170 grams. That would mean a small potato could contain up to 20 mg of solanine (assuming the higher 12mg/100g), but probably contains less.
If a person weighs 120 pounds (54 kg), to feel symptoms they’d have to ingest, at the low end (3.5mg/kg x 54 kg) 189 mg of solanine or the amount in 9.5 small potatoes. Or, using the higher number (28mg/kg x 54) 1512 mg of solanine or the amount in 75.6 small potatoes. A person who weighs more, say 170 pounds (77 kg), they’d have to eat 13 to 108 small potatoes.
These amounts 1) are assuming the highest amount of solanine, and 2) are at one sitting because the kidneys and liver get rid of solanine rather quickly:
First, solanine levels in the blood are low after ingestion due to poor absorption by the gastrointestinal tract. Second, it is removed from the body fairly rapidly in both the urine and the feces, usually within 12 hours, preventing accumulation in the tissues. Third, intestinal bacteria aids in the detoxification by hydrolyzing the glycoside into solanidine (aglycone), which is less toxic than solanine and also poorly absorbed.
So, a 170 pound man might have to eat about 100 small potatoes, in one meal, just to feel something.
Two things you can do to reduce the amount of solanine you’re exposed to:
- Don’t eat greening or sprouting potatoes.
- Boil potatoes and throw out the water.
Fanatic Cook 
I just wanted to thank you for the great blog. My wife and I appreciate the good dietary advice, sprinkled with interesting natural tidbits! Keep up the good work!
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Well, that was nice to wake up to. Thank you, Conor, and your wife too.
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Melinda asked whether green tomatoes are a problem. I didn’t know…
This eHow site says that green tomatoes do indeed contain more solanine that red tomatoes, maybe up to 100 times more.
The amount in a red tomato is hardly anything, 0.5mg/100g. Recall a non-green potato has about 12mg/100g. So red tomatoes are fine.
Compared to a potato (that isn’t green), a green tomato can have up to 4 times the solanine, by weight.
A medium green tomato (123g) could contain (at 500mg/kg fruit) about 62 mg solanine. A 120 pound person might feel something at 3 medium green tomatoes (or 24, using the less conservative figure, and depending on how much solanine the person can metabolize. Older adults probably don’t metabolize as quickly as younger adults.). A larger 170 pound person: from 4 to 35 green tomatoes.
There are a lot of variables here, as you can see. Since there is more solanine in a green tomato, the variables are starting to matter.
Bottom line: If you aren’t boiling them and throwing out the water, I wouldn’t eat more than a couple green tomatoes in a day.
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I’m going to backtrack on what I just said… I saw that this eHow site says another name for solanine is tomatine. Is that so?
I wrote a couple posts about the benefits of tomatine for sarcopenia, e.g. …
Compounds In Green Tomatoes And Apple Peels Stimulate Muscle Growth, Protect Against Muscle Wasting (Sarcopenia)
Perhaps solanine and tomatine are not the same. Perhaps the alkaloid in potatoes is more problematic than the alkaloid in green tomatoes?
Harold McGee wrote about this in:
Accused, Yes, but Probably Not a Killer
Not only benign, but beneficial as McGee’s article and my sarcopenia post describe. Given this, I wouldn’t limit my green tomato consumption. I might even search them out.
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Thanks for responding, even though I misplaced my comment! Personally, I don’t care for green tomatoes, but they’re huge in the south. I looked up tomatine (which I had never heard of–thanks!) and solanine on Wiki, and it says that tomatine is a glycoalkaloid found only in the leaves and stems of tomato plants and that it has multiple health benefits. (https://en.wikipedia.org/wiki/Tomatine) Solanine, though, is a “glycoalkaloid poison found in species of the nightshade family (Solanaceae), such as the potato (Solanum tuberosum) and the tomato (Solanum lycopersicum). It can occur naturally in any part of the plant, including the leaves, fruit, and tubers. Solanine has pesticidal properties, and it is one of the plant’s natural defenses.” Whew! I’ve also heard people question (on similar grounds) tomatillos, used to make salsa verde, as they too are in the nightshade family. But I haven’t come across any concrete info on toxic properties they may have. And of course, there are also eggplants (which sometimes have a green cast to the flesh) and chili peppers in the nightshade family. But thanks for that VERY useful info on green tomatoes!
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Does it say only leaves and stems? It looks like Wikipedia could be updated then. Tomatine is found in tomato flesh too.
Tomatine is a natural pesticide, as is solanine. Both alkaloids. Both harmful to humans although at different doses.
I don’t like the word toxin. All foods contain natural “toxins” if you will. Another word I don’t like is antinutrients. People capitalize on these words to influence our dietary consumption.
Words I do like are “the dose makes the poison.” I like how McGee describes it in his article:
I think it’s neat that what we used to think of as a poison in tomato plants turns out to have benefits.
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Yes, it says leaves and stems for tomatine in tomatoes. And yes, it’s neat that it has benefits!
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I think each person reacts to solanine differently. My husband is highly sensitive to potatoes with high level of solanine, we need to peel every potato we eat to be safe.
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Thanks !!!! For The helpful information
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