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.