Food contains natural chemicals that are essential for growth and health, including carbohydrates, sugars, proteins and vitamins. But some foods contain potentially harmful natural toxins.
The reason for the presence of natural toxins is not always known. In some foods, a toxin is present as a naturally occurring pesticide to ward off insect attack. Or a toxin may be formed to protect the plant from spoilage when damaged by weather, handling, UV light or microbes.
Your own sensitivity to a natural toxin, as well as the concentration (strength) of the toxin present, will determine whether you have an adverse reaction and the strength of symptoms you experience.
The commonly eaten foods listed below may contain natural toxins. You can limit your exposure to natural toxins by following the simple practices outlined below.
Apple and pear seeds and the inner stony pit (kernel) of apricots and peaches contain a naturally occurring substance called amygdalin.
Amygdalin can release hydrogen cyanide in the gut causing discomfort or illness. It can be fatal if too much is consumed in a short period of time.
Apricot kernels are sold as health foods or dietary supplements. As amygdalin in the kernel is naturally produced by the fruit, the amounts in any particular kernel can be hard to predict or control. Levels may vary depending on growing conditions and variety of apricot.
The Australia New Zealand Food Standard Code states that naturally occurring toxins, including cyanide, should be kept as low as possible. To be safe, we recommend people don't eat more than four kernels a day. This is in line with advice from other regulatory agencies.
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, 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.
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.
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 micro-organisms, 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.
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 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 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.
A naturally occurring toxin called cyanogenic glycosides in raw or unprocessed cassava and bamboo shoots can lead to exposure to hydrogen cyanide and its related toxicity.
Cassava is also known as yucca, 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 this toxin, sweet cassava should be 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.
Toxic honey is produced as a result of bees feeding on tutu (Coriaria arborea) bushes. Tutu is a widely distributed native species found throughout New Zealand, particularly along stream banks and in regenerating native bush. The poison comes from bees gathering honeydew produced by a sap sucking vine hopper insect (Scolypopa sp) feeding on the tutu plants. Rare events of poisoning have occurred in people who have consumed toxic honey from Coromandel Peninsula, Eastern Bay of Plenty (EBOP) and the Marlborough Sounds, although it is possible it could occur anywhere.
It is important you include a wide variety of fresh fruit and vegetables in your diet. Eating fresh fruit and vegetables lowers the risk of more serious health problems such as heart disease, cancer, diabetes and obesity. The risk of these serious health problems is greater than the risk of health problems associated with natural plant toxins.
The Ministry of Health recommends 5+ servings of fruit and vegetables per day.