Natural toxins in food


Food contains natural chemicals, including carbohydrates, sugars, proteins and vitamins. But some foods contain potentially harmful natural toxins. Sometimes a toxin is present as a naturally occurring pesticide to ward off insect attack or to protect the plant from spoilage when damaged by weather, handling, UV light or microbes.

Your own sensitivity to a natural toxin, the concentration (level) of the toxin in the food, and the amount of the food consumed will determine whether you have an adverse reaction and the severity of symptoms you experience.

If you have symptoms or feel unwell contact the Poisons Information Centre on 13 11 26 (24 hours)

The commonly eaten foods listed here may contain natural toxins and consumers are protected by maximum limits for them in the Australia New Zealand Food Standards Code You can also limit your exposure to natural toxins by following the simple practices outlined below.

Alcohol

The National Health and Medical Research Council’s Australian Guidelines to Reduce Health Risks from Drinking Alcohol advise both men and women to drink no more than two standard drinks per day to reduce their health risks over a lifetime. Young people (up to 18 years of age) are advised not to drink alcohol at all. Women who are pregnant, planning a pregnancy or breast feeding are advised not to drink.

Cassava and bamboo shoots

Naturally occurring cyanogenic glycosides in raw or unprocessed cassava and bamboo shoots can lead to exposure to the toxin hydrogen cyanide.

Cassava is also known as yuca, tapioca (a processed product of cassava), gaplek or manioc. Bamboo shoots, a traditional ingredient of Asian cuisine, are sourced from the underground stems of the bamboo plant.

To avoid exposure to these toxins, sweet cassava should be properly prepared before eating. Peel and slice the cassava and then cook it thoroughly, either by baking, boiling or roasting. Frozen cassava, and frozen peeled cassava should also be prepared in this way.

Fresh bamboo shoots should be sliced in half lengthwise, the outer leaves peeled away and any fibrous tissue at the base trimmed off. The remaining fresh shoots should then be thinly sliced into strips and boiled in lightly salted water for eight to ten minutes.

Fish: Escolar and oil fish

Escolar (Lepidocybium flavobrunneum) and oilfish (Ruvettus pretiosus) have been responsible for a number of food poisoning outbreaks involving a type of oily diarrhoea, called keriorrhoea. For example, investigation of some Australian outbreaks of oily diarrhoea suggest between 45 and 67 per cent of people may become ill after eating these fish species. There is probably a significant under-reporting of illness associated with the consumption of these fish as the symptoms can be mild and short-lived.

The oily diarrhoea is caused by indigestible oil contained in these fish, which accumulates in the rectum before being expelled. Symptoms range from an oily orange or yellow discharge, to severe diarrhoea with nausea and vomiting. Other symptoms may include stomach cramps, loose bowel movements and headache. The symptoms can occur without warning, usually within 2 ½ hours of consumption, but range from 1 to 90 hours later. Unlike other forms of diarrhoea, the oily diarrhoea caused by these fish does not cause significant loss of body fluid and is not life-threatening. Symptoms may last for one or two days.

If you are pregnant, have a bowel problem or malabsorption, you are advised not to consume these fish. If you are eating these fish for the first time, consume only a small portion. If you experience gastrointestinal symptoms after eating this fish, do not consume this fish in future and seek medical advice if symptoms persist.

Fish: naturally occurring mercury

Fish is great for your health and everyone should eat two to three serves of fish a week for good health.

However, some of the larger species of fish, such as shark, marlin and swordfish, have levels of naturally occurring mercury. These species can build up levels of mercury because they are predatory and eat smaller fish and they also live a long time absorbing mercury from the ocean.

The effects in babies exposed to high levels of mercury in the womb can include lower scores on tests that measure attention, learning and memory in their early years.

Pregnant women, women planning pregnancy and young children shouldn’t eat shark, broadbill, marlin and swordfish more than once a fortnight, with no other fish eaten that fortnight. They also shouldn’t eat orange roughy (also sold as sea perch) and catfish, more than once a week, with no other fish eaten during that week.

The general population should eat only one serve per week of Shark (Flake) or Billfish (Swordfish / Broadbill and Marlin) and no other fish that week.

Check the mercury in fish advice from Food Standards Australia New Zealand.

There are plenty of other types of fish to choose for your two to three serves a week. Check with your fish retailer about the type of fish you are buying or check the ingredient list on a package or can.

Fish: Ciguatera food poisoning

Ciguatera is an unusual form of food poisoning most typically caused by larger and older fish that live in warm ocean waters. However smaller warm ocean water finfish may also be implicated. The poisoning is caused by people eating fish containing the ciguatera toxin. The contaminated fish have eaten smaller fish that eats the algae that shelters a tiny organism responsible for producing a toxin that causes ciguatera.

Within 24 hours symptoms emerge including numbness around the fingers, toes and mouth, a burning sensation when in contact with cold, joint pain, nausea, itchiness and for people with high sensitivity, breathing difficulties.

Problems may be encountered with eating coral trout, spanish mackerel, reef cod, barracuda, emperor, groper, sturgeon, trevally and kingfish. Any warm water predatory fish over 6 kg should be treated with suspicion.

Under no circumstances should the head, roe or liver be eaten, and it is wise to eat a small portion of any large fish to test for reactions 24 hours before serving. If you develop symptoms seek medical advice.

Fish: Scombroid (histamine) fish poisoning

Scombroid or histamine poisoning is the result of inadequate temperature control of fish. It can occur when fish that has not been chilled to 4°C or below is eaten. Usually not of major concern,  deaths overseas have focused attention on Scombroid poisoning.

The result is a high level of histamine building up in the fish flesh. Often misdiagnosed as an allergic reaction, symptoms vary but include:

  • burning and tingling of the lips and mouth
  • dizziness
  • flushing of the face
  • an itchy rash, often on the face, neck, chest and upper back
  • sweating
  • vomiting and diarrhoea
  • headaches
  • blurred vision
  • palpitations

In severe cases, and particularly in those suffering from asthma, bronchial difficulties can occur.

Fruit seeds and pits

Apple and pear seeds and the inner stony pit (kernel) of apricots and peaches contain a naturally occurring substance called amygdalin which is a cyanogenic glycoside. Amygdalin can release hydrogen cyanide in the stomach causing discomfort or illness. It can sometimes be fatal.

There are different types of apricot kernels, some of which contain high levels of the toxin that can release cyanide into the body when eaten. Food Standards Australia New Zealand has issued advice that adults don’t eat more than three apricot kernels a day and children should not eat any.

Accidental swallowing of an occasional seed or pip is not a concern. However, do not make a habit of eating the seeds from these fruits. For a young child, swallowing only a few seeds or pits may cause illness and in rare cases can be fatal.

Kumara

Kumara, a member of the sweet potato family, can produce toxins in response to injury, insect attack and other stress. The most common toxin, ipomeamarone, can make the kumara taste bitter. There have been reports of cattle deaths after they have eaten mouldy kumara. The toxin levels are usually highest near the area of damage. It is recommended that any damaged parts on kumara are removed before cooking. Do not eat it if it tastes bitter after cooking.

Wild Mushrooms

Commercially grown and sold mushrooms are safe.

However, the Death Cap Mushroom is a deadly, poisonous introduced fungus that is responsible for 90% of all deaths related to mushroom consumption. It is commonly found in South Eastern Australia near established oak trees and possibly some other trees, usually during later summer to early winter after good rain or heavy irrigation.

Its cap is 40-160mm wide, may be white, but usually pale green to yellow in colour, or fawn if the mushroom is older, or located in full sun. The cap can be slippery or sticky to touch, and shiny when dry. Its gills are white, crowded and not attached to stalk. Its stalk is normally white in colour, but may be pale green and up to 15cm long with a papery cup shaped volva at the base (often buried in the ground). Normally a skirt-like ring is present high on the stem.

One Mushroom contains enough poison to kill an adult. Cooking or peeling does not inactivate the toxin, and all parts are poisonous. Onset of symptoms occurs 6-24 hours or more after ingestion of mushrooms. Symptoms include violent stomach pains, nausea, vomiting and diarrhoea. Symptoms may subside for 1 to 2 days giving a false impression of recovery. However, by this stage the toxin will have already caused serious liver damage. Death from liver failure can occur many days after ingestion.

A person who suspects that they may have eaten poisonous mushrooms should seek immediate medical attention, and where possible take a whole mushroom sample for identification. The sooner the treatment begins the better the chances of survival. Medical assistance should be sought at hospital emergency departments or by calling your local Poisons Information Centre on 13 11 26 (24 hours).

For absolute safety avoid any wild mushrooms, unless definitely identified as non-poisonous.

Parsnip

Parsnips commonly contain a group of natural toxins known as furocoumarins. These are probably produced as a way of protecting the plant when it has been stressed. The concentration of the toxin is usually highest in the peel or surface layer of the plant or around any damaged areas.

One of the furocoumarin toxins can cause stomach ache and may also cause a painful skin reaction when contact with the parsnip plant is combined with UV rays from sunlight.

It is important to peel the parsnip before cooking and remove any damaged parts. The levels of toxin drop when the parsnip is cooked by baking, microwaving or boiling. Discard any cooking water.

Potatoes

All potatoes contain natural toxins called glycoalkaloids. The levels are usually low but higher levels are found in potato sprouts, and the peel of potatoes that taste bitter. The toxins are produced by the plant in response to stress such as from micro-organisms and UV light, and damage such as bruising. The amount of toxin depends on the type of potato and the growing conditions.

Severe stomach ache and even death from glycoalkaloid poisoning has been reported overseas, but is very unusual. Glycoalkaloids are not destroyed by cooking, so it is important to avoid eating the sprouts and to remove any green or damaged parts before cooking. Do not eat cooked potatoes that still taste bitter. If you come across a green potato crisp, it’s probably best not to eat it. Remember to store potatoes in a dark, cool and dry place.

Kidney beans

Many types of beans contain toxins called lectins. The highest concentrations are found in kidney beans, especially red kidney beans. As few as four or five raw beans can cause severe stomach ache, vomiting and diarrhoea.

To destroy the toxins, soak the beans for at least five hours and then boil them briskly in fresh water for at least 10 minutes. Do not cook beans at a low temperature, for example in a slow cooker, as it may not destroy the toxin. Improperly cooked beans can be more toxic than raw ones. Tinned beans can be used without further cooking.

Rhubarb

Rhubarb contains naturally occurring oxalic acid. The amount depends on the age of the plant, the season, the climate and the type of soil. Highest concentrations are in the leaves and these should not be eaten.

Oxalic acid poisoning can cause muscle twitching, cramps, decreased breathing and heart action, vomiting, pain, headache, convulsions and coma.

Zucchini (courgette)

Zucchini may occasionally contain a group of natural toxins known as cucurbitacins. These toxins give zucchini a bitter taste. Bitterness in wild zucchinis has been known for a long time but is rarely found in commercially grown zucchinis.

Eating bitter zucchinis have caused people to experience vomiting, stomach cramps, diarrhoea and collapse. Do not eat zucchini that have a strong unpleasant smell or taste bitter.

Acknowledgements

This information has been prepared with assistance from ACT Health, Food Standards Australia New Zealand, New Zealand Ministry of Primary Industry and Queensland Health.