The Natural Toxins Already in Your Food

TL;DR. Ordinary food contains genuine poisons. Green potatoes contain solanine, kidney beans contain a lectin that causes violent vomiting if undercooked, cassava releases cyanide and has caused permanent paralysis in famine conditions, and apple pips and apricot kernels contain the same cyanide-releasing compound. Almost all of this is managed by ordinary cooking and ordinary quantities, and traditional food preparation techniques exist largely because of it. The separate category of antinutrients (phytate, oxalate, tannins) does not poison you; it blocks the absorption of minerals, which matters at the margins and is greatly reduced by soaking, fermenting, sprouting, and cooking.

Key takeaways

• "Natural" is not a safety category. The most toxic substances known are natural products.
• Raw or undercooked kidney beans cause severe vomiting and diarrhoea within hours, from phytohaemagglutinin. As few as four or five raw beans can do it. Slow cookers can make it worse by holding beans at a temperature that activates rather than destroys the lectin.
• Green or sprouted potatoes should be discarded, not just peeled, once green is extensive. Solanine poisoning is real and has killed.
• Antinutrients are a nuisance, not a threat, in mixed diets, and traditional preparation methods reduce them substantially.
• Nutmeg, liquorice, star fruit, and elderberries are the surprising domestic hazards.

The framing: dose, preparation, and quantity

Every point in this chapter is an application of one principle, covered properly in Chapter 87: toxicity is a property of dose, not of origin. Botulinum toxin, ricin, aflatoxin, tetrodotoxin, and amatoxin are all natural. Water is toxic at a high enough dose.

The second principle is that traditional food preparation is mostly toxicology. Soaking beans, fermenting cassava, nixtamalising maize, leaching acorns, curing olives, and boiling rather than steaming certain greens are all techniques that exist because the alternative made people ill. When cultures adopt a food without its processing, the results are historically grim: maize without nixtamalisation gave the American South pellagra; cassava without proper processing gives konzo.

Glycoalkaloids: green potatoes and tomato leaves

In short: Solanine and chaconine in potatoes are genuine neurotoxins that concentrate in green skin, sprouts, and damaged tissue.

Potatoes belong to the nightshade family and defend themselves with glycoalkaloids, principally solanine and alpha-chaconine. These inhibit acetylcholinesterase, the same enzyme organophosphate insecticides hit, and disrupt cell membranes.

Where they concentrate: the skin and the layer immediately beneath it (containing the large majority of the total), the sprouts and eyes, and any green tissue. Green colour is chlorophyll and is itself harmless; it is a marker for light exposure, and light exposure also drives glycoalkaloid synthesis. So green is a warning sign rather than the problem.

Levels: a normal potato contains roughly 10 to 100 mg/kg, and the accepted safety limit for commercial varieties is 200 mg/kg. Greened or sprouted potatoes can reach 500 to 1,000 mg/kg or more in the affected tissue.

Symptoms begin at roughly 1 to 3 mg per kg of body weight: nausea, vomiting, abdominal pain, and diarrhoea, progressing at higher doses to headache, confusion, hallucinations, fever, and, in severe cases, death. A famous 1979 incident at a London school hospitalised 78 boys after a meal of stored potatoes, several with serious neurological symptoms. Deaths are rare and documented.

Practical rules: store potatoes dark and cool but not refrigerated; discard sprouts and cut generously around them; peel and cut away any greening; and if a potato is extensively green, bitter, or heavily sprouted, throw it away. Cooking does not help much, because glycoalkaloids are heat-stable to about 170 °C. A bitter taste is the practical warning signal and should be respected.

Green tomatoes contain tomatine, a related glycoalkaloid that is considerably less toxic and is bound in the gut, which is why fried green tomatoes are a dish and not a hazard. Tomato leaves and stems are higher and are not eaten. Aubergine and peppers contain small amounts. The claim that "nightshades cause inflammation" in the general population is not supported by evidence.

Lectins: the kidney bean case

In short: Raw and undercooked red kidney beans cause violent gastrointestinal illness, and this is the one lectin story that is unambiguously real.

Lectins are proteins that bind carbohydrates, present in many plants and concentrated in legumes and grains. Most are destroyed by cooking and are of no consequence.

Phytohaemagglutinin in red kidney beans is the exception, and it is serious.

• Raw red kidney beans contain 20,000 to 70,000 haemagglutinating units; properly cooked beans contain 200 to 400.
• As few as four or five raw beans can cause illness.
• Onset is fast: nausea and vomiting within one to three hours, followed by diarrhoea and abdominal pain. Recovery is usually within a day, and it is thoroughly unpleasant.

The dangerous method is the slow cooker. Heating beans to 80 °C actually increases toxicity to around five times the raw level, and a slow cooker on low may never exceed that. Documented outbreaks trace to exactly this.

The correct method, per the US FDA and UK food standards guidance:

1. Soak dried beans for at least 5 hours (or overnight).
2. Discard the soaking water.
3. Boil hard in fresh water for at least 10 minutes, which destroys the lectin.
4. Then simmer until tender, or transfer to a slow cooker after the hard boil.

Canned kidney beans have been through this process industrially and are safe as sold.

White kidney beans (cannellini) contain roughly a third as much, and broad beans about 5 percent. Other pulses are much lower.

On the broader "lectin-free" diet trend, promoted in popular books: the claim that dietary lectins from cooked legumes, grains, and nightshades cause widespread chronic disease is not supported. Legume consumption is consistently associated with lower disease risk in cohort studies, and pulses are a staple in most of the world's longest-lived populations. The kidney bean hazard is real, specific, and solved by boiling.

Cyanogenic glycosides: cassava, kernels, and pips

In short: Several foods contain compounds that release hydrogen cyanide when the tissue is damaged and the compound meets an enzyme.

The mechanism mirrors the brassica one: a stable glycoside stored in one compartment and an enzyme in another. Crush the tissue and they meet, releasing hydrogen cyanide, which blocks the final step of cellular respiration.

Cassava is the serious one. It is a drought-tolerant, high-yielding root crop that feeds several hundred million people, and bitter varieties contain enough linamarin to be lethal without processing. Traditional processing (soaking, grating, fermenting for several days, pressing, and roasting or drying) reduces cyanide by 95 percent or more, and every step matters. Under famine conditions, when processing is shortened, the result is konzo: a sudden, permanent, symmetrical spastic paralysis of the legs, mainly affecting children and women of childbearing age, documented in outbreaks across Central Africa. It is entirely preventable and it is a direct consequence of shortcutting food processing under duress. A simple "wetting method" developed for African communities reduces residual cyanide further and has been shown to prevent konzo in field trials.

Apricot kernels are the modern rich-country version of the problem. They are sold as a health food and as an alternative cancer treatment (as "laetrile" or "vitamin B17," which is neither a vitamin nor effective; controlled trials found no benefit and real cyanide toxicity). EFSA has assessed that as few as three small kernels can exceed the safe level for an adult, and one for a toddler. Poisonings and deaths are documented. This is one of the clearest cases in the book of "natural" marketing causing direct harm.

Apple pips are the question everyone asks. An apple contains perhaps 5 to 8 pips with roughly 1 to 4 mg of amygdalin each, releasing a fraction of a milligram of cyanide, and only if crushed. A lethal dose would require deliberately chewing the pips of dozens to hundreds of apples in a sitting. Swallowing a few whole does nothing at all, as the intact seed coat passes through. Do not worry about it; do not make a habit of chewing them.

Oxalates

In short: They bind calcium and, in susceptible people, form kidney stones. Rhubarb leaves are genuinely dangerous; rhubarb stalks are not.

Oxalic acid binds calcium, magnesium, and iron into insoluble crystals. Two consequences: the mineral becomes unavailable, and in the urinary tract the crystals can form calcium oxalate kidney stones, which account for roughly 80 percent of stones.

Rhubarb leaves are the domestic hazard. Poisonings occurred during the First World War when the leaves were promoted as a vegetable in Britain. The stalks are fine.

For most people, oxalate is not worth managing. For recurrent calcium oxalate stone formers, it is, and the advice is counterintuitive:

• Drink much more water. By far the most effective intervention, aiming for 2.5 litres of urine a day.
• Eat calcium with oxalate-rich meals. Calcium in the gut binds oxalate and prevents its absorption. Restricting dietary calcium increases stone risk, which is the single most common mistake.
• Reduce sodium, which increases urinary calcium.
• Cooking helps: boiling spinach and discarding the water removes 30 to 87 percent of its oxalate. Steaming removes less.
• Do not take high-dose vitamin C, which is metabolised partly to oxalate.

Green smoothies deserve a specific warning. Blending large quantities of raw spinach, beet greens, or chard daily delivers far more oxalate than anyone would eat as a salad, and cases of oxalate nephropathy (acute kidney injury from oxalate crystals) have been reported. Rotate the greens.

Phytate

In short: The main storage form of phosphorus in seeds; it binds iron, zinc, and calcium and reduces their absorption. Soaking, sprouting, and fermenting reduce it substantially.

Phytic acid is present in all whole grains, pulses, nuts, and seeds. It binds positively charged minerals tightly enough to prevent absorption.

The effect is genuine and quantitatively significant: phytate can reduce iron absorption by 50 to 80 percent and zinc by 30 to 50 percent from a given meal. In populations depending on unleavened wholegrain bread with little animal food, this contributes to widespread iron and zinc deficiency. This is a real global nutrition problem.

In a mixed diet, it is a minor issue, and phytate has demonstrated benefits of its own: it is an antioxidant, it may reduce colorectal cancer risk, and it reduces urinary crystal formation.

How to reduce it, all of which are traditional techniques:

Genuine sourdough is the standout: the long acidic fermentation activates phytase and degrades most of the phytate, which is one concrete reason traditional sourdough delivers more available minerals than fast-fermented commercial bread made from the same flour.

Goitrogens

In short: Compounds that interfere with thyroid iodine uptake, mainly in brassicas, soy, and cassava. Relevant if iodine intake is low, otherwise not.

Brassica glucosinolates break down partly into thiocyanates, which compete with iodine for uptake into the thyroid. Soy isoflavones inhibit an enzyme in thyroid hormone synthesis. Cassava's cyanide is detoxified to thiocyanate, with the same effect.

The practical position: in people with adequate iodine, normal consumption of these foods does not cause thyroid problems, and studies in soy-consuming populations find no increase in thyroid disease. Cooking reduces goitrogenic activity in brassicas substantially. The combination that matters is low iodine plus high goitrogen intake, which is exactly the historical picture in cassava-dependent, iodine-deficient regions.

People on levothyroxine should note a different issue: soy and high-fibre foods reduce absorption of the drug, so take it on an empty stomach, four hours apart from soy or calcium or iron (Chapter 79).

The surprising domestic hazards

In short: A short list of ordinary foods that cause genuine problems in ordinary kitchens.

Nutmeg. Contains myristicin. Doses of roughly 5 to 15 grams (one to three whole nutmegs) cause a genuine and thoroughly unpleasant intoxication: hallucinations, agitation, dry mouth, tachycardia, nausea, and a hangover lasting days. Poisoning cases and rare deaths are documented. Culinary quantities are irrelevant.

Liquorice. Real liquorice contains glycyrrhizin, which inhibits the enzyme that inactivates cortisol in the kidney, producing an effect like excess aldosterone: sodium retention, potassium loss, high blood pressure, and in severe cases dangerous hypokalaemia and cardiac arrhythmia. Cases of hospitalisation and at least one death from heavy liquorice consumption are documented, and the EFSA suggests no more than 100 mg of glycyrrhizin a day, roughly 50 g of liquorice sweets. Most "liquorice" confectionery in the US and UK is flavoured with anise and contains none; the traditional black liquorice in Northern Europe does.

Star fruit (carambola). Contains caramboxin, a neurotoxin normally cleared by the kidneys. In people with chronic kidney disease it accumulates and causes hiccups, vomiting, confusion, seizures, and death. Fatalities are well documented. Anyone with significant kidney impairment should avoid star fruit entirely, and it is not widely known.

Elderberries. Raw berries, and especially the leaves, stems, and unripe fruit, contain cyanogenic glycosides and cause nausea, vomiting, and diarrhoea. Cooking destroys them, which is why elderberry is always used cooked. An outbreak in California in 1983 hospitalised people who drank juice made from raw berries with leaves and stems included.

Raw or undercooked red kidney beans, as above.

Raw egg white in quantity binds biotin (avidin); more relevantly it carries a small salmonella risk. In the UK, eggs bearing the British Lion mark are considered safe to eat raw or lightly cooked by the FSA, including for vulnerable groups, following the vaccination programme in laying hens. Elsewhere the advice differs.

Green almonds, bitter almonds, and unripe ackee. Ackee, Jamaica's national fruit, contains hypoglycin A and must be harvested only when the pods open naturally. Unripe ackee causes "Jamaican vomiting sickness": severe hypoglycaemia, vomiting, seizures, and death.

Wild mushrooms. Outside the scope of this chapter and worth one sentence: several species that kill are hard to distinguish from edible ones, Amanita phalloides accounts for most fatal mushroom poisonings worldwide, and the characteristic pattern is symptoms that subside after a day, giving false reassurance while the liver fails. Do not forage on app identification.

Fish and shellfish toxins. Scombroid poisoning from histamine in poorly refrigerated tuna and mackerel; ciguatera from reef fish in tropical regions, which is heat-stable and can last months; and paralytic shellfish poisoning from algal blooms. None of these are destroyed by cooking, which is the important point.

The bottom line

• Real food contains real toxins. "Natural" describes origin, not safety.
• The genuine domestic dangers are green and sprouted potatoes, undercooked red kidney beans (especially in slow cookers), rhubarb leaves, apricot kernels sold as health food, and, for people with kidney disease, star fruit.
• Traditional preparation methods (soaking, boiling, fermenting, nixtamalising) exist because of this chemistry, and skipping them has caused mass illness repeatedly in history.
• Antinutrients (phytate, oxalate, tannins) block mineral absorption rather than poisoning you. They matter in monotonous plant-based diets, and much less in varied ones. Soaking, sprouting, and fermenting cut them substantially.
• For recurrent kidney stone formers, drink far more water and keep dietary calcium up, not down.

Sources and notes

Glycoalkaloid levels, symptoms, and the 200 mg/kg limit follow EFSA's 2020 opinion on glycoalkaloids in potatoes and the FAO/WHO evaluations; the 1979 London school incident is McMillan and Thompson, Quarterly Journal of Medicine, 1979. Phytohaemagglutinin figures and the slow cooker warning follow the US FDA Bad Bug Book and UK FSA guidance. Cassava processing, cyanide reduction, and konzo follow Banea-Mayambu's and Howard Bradbury's field work and WHO reporting. Apricot kernel risk follows EFSA's 2016 opinion on cyanogenic glycosides in apricot kernels; laetrile trial failure is Moertel et al., New England Journal of Medicine, 1982. Oxalate contents follow Holmes and Kennedy's analyses and the Harvard oxalate content list; stone prevention advice follows the American Urological Association guideline, including the counterintuitive dietary calcium finding from Curhan et al. Phytate reduction by soaking, sprouting, and fermentation follows Gibson, Perlas, and Hotz. Goitrogen relevance follows Bajaj et al. and the bok choy myxoedema case report, New England Journal of Medicine, 2010. Nutmeg, liquorice, star fruit (caramboxin, Garcia-Cairasco et al.), ackee, and elderberry hazards follow published case series and the EFSA glycyrrhizin opinion.

Open questions. Whether moderate oxalate intake matters for people who have never formed a stone is unclear, and green smoothie nephropathy is documented only in case reports. The threshold at which brassica goitrogens matter in iodine-replete people has not been established.

👉 Next: when to eat, where chronobiology has more to say than either side of the meal-timing argument usually admits.