Solanine

Solanine is a glycoalkaloid poison found in species of the nightshade family. It can occur naturally in any part of the plant, including the leaves, fruit, and tubers. It is very toxic even in small quantities. Solanine has both fungicidal and pesticidal properties, and it is one of the plant's natural defenses.

Symptoms
Solanine poisoning is primarily displayed by gastrointestinal and neurological disorders. Symptoms include nausea, diarrhea, vomiting, stomach cramps, burning of the throat, heart arrhythmia, headache and dizziness. Hallucinations, loss of sensation, paralysis, fever, jaundice, dilated pupils and hypothermia have been reported in more severe cases.

In large quantities, solanine poisoning can cause death. One study suggests that doses of 2 to 5 mg per kilogram of body weight can cause toxic symptoms, and doses of 3 to 6 mg per kilogram of body weight can be fatal.

Symptoms usually occur 8 to 12 hours after ingestion, but may occur as rapidly as 30 minutes after eating high-solanine foods.

Correlation with birth defects
Some studies show a correlation between the consumption of potatoes suffering from late-blight (which increases solanine and other glycoalkaloid levels) and the incidence of congenital spina bifida in humans. However other studies have shown no correlation between potato consumption and the incidence of birth defects. 

Solanine in potatoes
Solanine occurs naturally in all nightshades, including tomatoes, capsicum, tobacco and eggplant, as well as plants from other species. However the most ingested solanine is from the consumption of potatoes.

Potatoes naturally produce solanine and chaconine, a related glycoalkaloid, as a defense mechanism against insects, disease, and predators. Potato leaves and stems and shoots are naturally high in glycoalkaloids.

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.

Some diseases, such as potato blight, can dramatically increase the levels of glycoalkaloids present in potatoes. Mechanically damaged potatoes also produce increased levels of glycoalkaloids. This is believed to be a natural reaction of the plant in response to disease and damage.

Commercial varieties of potatoes are screened for solanine levels, and most have a solanine content of less than 0.2mg/g. However potatoes that have been exposed to light and started to green can show concentrations of 1mg/g or more. In these situations a single unpeeled potato can result in a dangerous dose.

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.

Deep-frying potatoes at 170° C (306° F) is known to effectively lower glycoalkaloid levels, whereas microwaving is only somewhat effective and boiling has no effect.

Other uses of solanine
Solanine has fungicidal and pesticidal properties, and solanine hydrochloride (a salt of of solanine) has been used as a commercial pesticide, but never on a large scale.

Solanine has sedative and anticonvulsant properties, and has been used as a treatment for asthma, as well as for cough and cold medicines. However, its effectiveness for either use is questionable.