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Potato Glycoalkaloid Toxicity: Solanine

by Andrew Montario. Cornell University.

Each year the world produces approximately 350 million tons of potatoes. The U.S. per capita consumption of potatoes is about 61 kgs per year. As you can see potatoes (Solanum tubersum) serve as a major food source as well as an inexpensive source of energy and good quality protein. Potatoes are grown mainly for human consumption but they are also widely used as food for livestock.

It is a less commonly known fact that potatoes produce compounds called glycoalkaloids that have been shown to be toxic to both man and to animals. At high enough levels the glycoalkaloid found in potatoes known as solanine can cause irritation of the gastrointestinal tract, impairment of the nervous system, and it is believed that they can cause teratogenic or birth defects. Neurological signs can include ataxia, convulsions, coma, muscle weakness, and involuntary urination.

Potatoes produce several different glycoalkaloids. The most well known one of these is Solanine; it is believed to be responsible for food poisoning. Solanine is a glycoalkaloid containing a steroid alkaloid nucleus with a side chain of three sugars. Solanine is the most active cholinesterase inhibitor found in food due to its location in edible portions of plants.

It has the ability to prevent the breakdown of acetylcholine -a very active neurotransmitter- in the human body. The accumulation of acteylcholine in neuromuscular junctions impairs the function of the nervous system and its effect on organs. Studies have linked birth defects such as spinal bifida in humans as well as breeding problems in animals to potato glycoalkaloids.

Alpha-Chaconine is another glycoalkaloid that is found in potatoes.

Solanine Levels and Burning Sensation

Steroidal glycoalkaloids (SGA's) of potatoes may be detrimental to the health of humans and animals, but they are beneficial to the plants. The SGA's such as solanine are thought to be a component of the certain varieties resistance to insects. The insects they provide protection from the potato beetle, potato leafhopper, and wireworms.

It is also believed that SGA's may assist some crops in resistance to disease. At a low pH level some SGA's have shown to have antifungal effects. Synergistic fungi inhibitory effects were reported when alpha-chaconine and alpha-solanine where present simultaneously.

In addition low levels of glycoalkaloids are required in order to produce a desired flavor. More bitter varieties contain excess solanine and other SGA's and are more toxic as well.

Solanine levels above 14mg/100g are bitter in taste. Cultivar with greater than 20mg/100g cause a burning sensation in the throat and mouth. The second most abundant steroidal glycoalkaloid in potatoes is alpha-chaconine.

In ruminant animals, potato glycoalkaloids are hydrolyzed to solanidine. Solanidine is then further metabolized into 5,6 dihydro analog.

The Solanine Content of Potatoes

The solanine content of tubers varies depending on numerous factors. Most commercial varieties contain less than 12 mg/100g and are normally between 2 and 13 mg/100g. Of the many factors affecting solanine content in potatoes: variety, greening, and maturity seem to cause the largest variation. The conditions which vary the solanine content differ depending on what particular tuber is being discussed.

A large level of variability in solanine content of potatoes can be attributed to differences in variety of the tuber. A certain variety may increase or decrease in solanine content with maturation, and increase or decrease or remain unaffected by fertilization. Other varieties may be affected greatly or slightly by storage conditions. Some varieties have shown increased solanine content upon storage .

Sprouting potatoes contain greater amounts of solanine, with higher concentrations found near the skin. Solanine amounts decrease toward the center of the tuber.

One variety of potato with significantly high solanine levels is Lenape. The level in these potatoes is approximately 30mg/100g.

Solanine can also be found in other poisonous solanumae including nightshade (Solanum nigrum and S. americanum), eggplant (Solanum melongena), and green peppers (Capsicum anum). The common nightshade can contain as much as 7.6 to 8.2 mg/100g. Common peppers from 7.7 to 9.2mg/100g of solanine. Eggplant solanine content varies from 6.1 to 11.33mg/100g.

Potato vines also contain solanine but they are a valuable feed for livestock. One might ask how the vines can be ingested when they too contain a high level of solanine. When the vines are harvested, it is done before the vines dry out and die. At the point of harvest the vines are non-toxic and serve as a nutritious food source for ruminants. The practice of harvesting the vines also minimizes the transfer of viruses from the vines to the tubers. The main problem with this feeding practice is the cost of the machinery needed to do the harvesting.

The Mechanism of Action

Solanum alkaloids are cholinesterase inhibitors which result in neural function impairment. A cholinesterase inhibitor is a chemical compound that inhibits acetylcholinesterase from removing acetylcholine from neuromuscular junctions. Acetylcholine is the neurotransmitter of the parasympathetic nervous system.

It is released from the terminal bouton of the preganglionic nerve fibers when action potential frequencies reach a sufficient level. When an inhibitor of acetylcholinesterase such as solanine is present in the cleft acetylcholine accumulates. The presence of acetylcholine in nerve tissue or organs is responsible for the neurological signs associated with solanine poisoning.

Other Glycoalkaloids in Potato Tissues

Other glycoalkaloids that are known to accumulate in tissues of the body and tend to concentrate most significantly in the liver. High concentrations have also been found in other major organs such as the kidney, heart, lungs, and in the brain.

Toxic Concentrations in Potatoes

Contents as high as 100mg/100g have been measured in the skin along with lethal amounts in the sprouts. The ability to have such high levels has led to several cases of potato poisoning. Since they contain not just solanine but also other glycoalkaloids, it is likely that the symptoms of potato poisoning are due to a combination of alkaloids.

In humans the oral dose of 28mg/kg body mass may cause both neurological impairment in the form of hyperesthesia, dyspnea, itchy neck, and drowsiness. These symptoms can be accompanied by gastrointestinal effects such as diarrhea and vomiting.

Regulations have been planned to limit the legally allowable level of solanine in commercial potatoes. New varieties are screened and glycoalkaloid levels must be less than 20mg/100g.

Human Symptoms of Glycoalkaloid Toxicity

In the human alpha-solanine and alpha-chaconine(another glycoalkaloid in potatoes) toxicity begins with gastrointestinal disturbances, vomiting, diarrhea, abdominal pain, then followed by neurological disorders at higher doses, low blood pressure, fever, rapid weak pulse. The symptoms listed are associated with sublethal doses

Little information is available on the chronic toxicity of glycoalkaloids. There are indications that alpha-solanine and other potato glycoalkaloids can accumulate in tissues.

Solanine "poisoning" has a variety of symptoms. These symptoms may include: nausea, diarrhea, abdominal cramps, fever, headache, weak pulse, rapid breathing, hallucinations, delirium, and induce a coma.

Reasons for Low Oral Toxicity

Despite the effects this chemical has, it's oral lethal dose is relatively high in animals; it has a low oral toxicity. This low oral toxicity is a result of how the compound of how the compound is dealt with by the body.

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.