Certain types of foods contain potentially harmful natural toxins, which are classified as Naturally Occurring Toxic Substances (NOTS). Sometimes a toxin is present in food plants as a naturally occurring insecticide that functions to ward off insect attack. Other toxins protect plants from spoilage by microbes or when damaged by weather, handling, or by UV rays from sunlight.
The various factors that determine whether a person will have an adverse reaction to a toxin as well as the severity of the symptoms upon exposure to the toxin include the following:
- The person’s sensitivity to the natural toxin
- The concentration of the toxin in the food
- The amount of food consumed
Foods that contain natural toxins are discussed below. These foods may be of fungal, plant or marine origin.
Mushroom toxin: While commercially grown and sold mushrooms are safe, the Death Cap Mushroom is deadly, and is responsible for 90% of all deaths related to mushroom consumption. It is commonly found during autumn after a good rain. One mushroom contains enough poison to kill an adult. Cooking or peeling does not inactivate the toxin, and all parts are poisonous. The onset of symptoms occurs 6-24 hours or more after ingestion of mushrooms. Symptoms include violent stomach pains, nausea, vomiting, and diarrhea. Symptoms may subside for 1-2 days giving a false impression of recovery. However, by this stage, the toxin will have already caused serious liver damage. Death due to liver failure can occur many days after ingestion.
Hydrogen cyanide: This is a NOTS that can be found in raw or unprocessed cassava and bamboo shoots. Consumption of these shoots can lead to exposure to the toxin. Hydrogen cyanide (HCN), sometimes called prussic acid, is a colorless organic compound that is extremely poisonous. Its solution in water is called hydrocyanic acid and has the same molecular formula. The so-called “bitter” roots of the cassava plant may contain up to 1 g of HCN per kg. HCN is also found in fruits that have a pit, such as cherries, apricots, apples, and bitter almonds, from which almond oil and flavoring are made. Many of these pits contain small amounts of cyanohydrins such as mandelonitrile and amygdalin, which slowly release HCN. One hundred grams of crushed apple seeds can yield about 70 mg of HCN.
Furocoumarins: Natural toxins known as furocoumarins are commonly found in Parsnips. These are produced to protect the plant against stress. 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. Any water used for cooking should be discarded.
Glycoalkaloids: 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 microbes and UV light and damage such as bruising. The amount of toxin depends on the type of potato and the growing conditions. 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.
Lectins: Many types of beans contain toxins called lectins. The highest concentrations are found in Kidney Beans, especially Red Kidney Beans. Raw beans can cause severe stomach ache, vomiting, and diarrhea. To destroy the toxins, the beans should be soaked for at least five hours and then boiled briskly in fresh water for at least 10 minutes.
Oxalic acid: Oxalic acid naturally occurs in Rhubarb. 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.
Cucurbitacins: A group of natural toxins known as cucurbitacins can sometimes occur in Zucchini (Courgette) or Summer Squash. These toxins give zucchini a bitter taste, which is present in the wild varieties, but not in the commercially grown ones. Eating bitter zucchinis have caused people to experience vomiting, stomach cramps, diarrhea, and collapse.
Mercury: Certain large species of fish, such as shark, marlin, broadbill, 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. Babies exposed to high levels of mercury in utero can lead to lower IQ scores in their early years. Therefore, pregnant women, women planning pregnancy and young children should avoid shark, broadbill, marlin, and swordfish in their diet.
Ciguatera toxin: Ciguatera is an unusual form of food poisoning most typically caused by larger and older fish that live in warm ocean waters. The poisoning is caused by consumption of fish containing the ciguatera toxin. The contaminated fish have eaten smaller fish that eats the algae that shelter a microscopic organism responsible for producing the ciguatera toxin. Symptoms include numbness around the fingers, toes, and mouth, a burning sensation upon exposure to cold, joint pain, nausea, and itchiness. Some of the fishes harboring the ciguatera toxin include coral trout, Spanish mackerel, reef cod, barracuda, kingfish, among others. In fact, any warm water predatory fish over 6 kg should be treated with suspicion.
Scombroid poisoning: This occurs as a result of inadequate temperature control of fish. It can occur when fish that has not been chilled to 4°C or below is eaten. The poisoning is due to histamine, which builds up to high levels 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, urticaria, sweating, vomiting & diarrhea, headaches, blurred vision, and palpitations. In severe cases, bronchial difficulties can occur, especially in asthmatics.
Oily diarrhea: Fishes such as Escolar (Lepidocybium flavobrunneum) and Oil-fish (Ruvettus pretiosus) have been responsible for a number of food poisoning outbreaks involving a type of oily diarrhea, called keriorrhea. Oily diarrhea 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 stool to severe diarrhea with nausea and vomiting. Other symptoms may include stomach cramps, loose motions, and headache. The symptoms usually appear within 2½ hours of consumption, but range from 1 to 90 hours later. Unlike other forms of diarrhea, oily diarrhea 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.
Safety limits for some NOTS as per FSSAI guidelines
The Food Safety and Standards Authority of India (FSSAI) has recommended the safe upper limit of a number of NOTS, which are presented in Table 1.