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Poisonous potato no small fry


Baked potatoes

(Source: iStockphoto)

The potato is a very popular food. Indeed, one billion people worldwide regularly eat potatoes. In the USA, the average consumption is about 60 kilograms per year, just over a kilogram per week.

The potato tastes good whether boiled, mashed, roasted, chipped, fried, in salads or as a packet of crisps (the salt helps). It's also a lovely comfort food.

So it's perfectly reasonable to think of the potato as a nutritious and harmless and essential part of your diet. But what most people don't know is that the humble spud can kill.

In the medical literature, potatoes have killed some 30 people (at a bare minimum) and have made over 2000 very sick indeed.

Plants don't have legs, so they can't run away from bad guys. Instead, they have evolved whole galaxies of exotic chemicals to fight off attackers, which include insects, predators and various infectious diseases.

So while the average person on the street might think potatoes love to be eaten, the botanists are not at all surprised that potatoes can fight back and, sometimes, even kill us.

Potatoes belong to the botanical family Solanaceae. The 2400 members of this botanical family include tobacco, tomato, capsicum, eggplant, jimson weed, mandrake and deadly nightshade.

In the middle ages, some of the more deadly members of this family were thought to "give witches the power to fly".

It seems that the family name, Solanaceae comes from the Latin solamen, which means 'quieting'. The quieting part of the name comes from the fact that some of Solanaceae family could make you very quiet indeed.

Mandrake could knock you out temporarily, while deadly nightshade (another member of the family) could knock you out permanently.

Deadly nightshade was also known as belladonna, meaning beautiful woman in Italian.

This was because it contained a chemical called atropine, which in very small doses, made a woman's pupils bigger, supposedly making her more beautiful. (It turns out that we humans are wired up in out brains to think that big eyes and small noses are beautiful. Think of kids' plastic dolls.)

So it's not a surprise that potato, being a member of this deadly family, has its own potentially poisonous chemical. And it does, and it's called 'solanine'.

Solanine was discovered in berries by a French chemist, back in 1820, and found in potatoes in 1826. Solanine protects the plant against pests and diseases, including the Colorado beetle and the potato leafhopper.

Originally, solanine was thought to be one single chemical. But we now know that solanine is a mixture of many related chemicals, all going under the fancy name of 'potato glycoalkaloids'.

But the really nasty ones are two very closely related chemicals: a-solanine {C45H73NO14}; and a-chaconine {C45H73NO15}.

Solanine is produced in all parts of the potato plant.

Firstly, solanine is made in the leaves, which makes sense in terms of fighting off attackers.

Secondly, solanine is also produced in the unripe fruits and flowers. In a potato, the vast majority of the solanine is in the outer three millimetres of the flesh.

There are many factors which affect how much solanine there is in the potato. These include the variety of potato, its maturity (higher in younger tubers), the intensity and the colour of the light, stress (bruising, cutting and slicing), the temperature (warmer is worse), the growing conditions (including the soil) and, of course, exposure to attackers.

Living creatures are complicated, so it's not surprising that solanine can have both good and bad effects. It has been used as a feedstock in the pharmaceutical industry, to make steroids.

It was also used to treat bronchitis, asthma and epilepsy. Laboratory tests have shown it partly effective against cervical cancer, liver cancer, stomach cancer and lymphoma.

It has anti-allergy, anti-fever and anti-inflammatory effects, it works against some bacteria and viruses, and it can help in vaginal candidiasis.

Without solanine, potatoes taste very bland. But too much can kill.

Solanine has two separate toxic effects.

First, it attacks the central nervous system by interfering with how nerves talk to each other. This particular pathway causes sweating, diarrhoea, bronchospasm, paralysis, heart failure, blindness, restlessness, confusion, weakness, heart failure and coma.

Second, solanine disrupts the movements of calcium (Ca++) and sodium (Na+) across cell membranes, and interferes with generalised membrane absorption.

This pathway leads to bursting of red blood cells, excess fluid in body cavities and, especially, problems in the gut (diarrhoea, vomiting, severe abdominal pain, etc).

But how can a potato make solanine, after it's been dug out of the ground? Well, I'll talk about that, and the killer potato, next time.

Tags: diseases-and-disorders, pharmaceuticals, botany, food-safety

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Published 21 April 2010

© 2014 Karl S. Kruszelnicki Pty Ltd