From Potatoes to Tuna: 10 Everyday Foods That Hide Natural Toxins

Humans are the only animals that knowingly put poison on the dinner plate and call it a delicacy. The pufferfish is the peak of this human boldness: in Japan, only specially licensed chefs may prepare fugu, because a single mistake can be deadly. No other species turns danger into a culinary attraction. And if we are willing to play with extremely toxic foods, it is no surprise that we pay little attention to the milder ones. It is almost as if we believe that only the risk of death is exciting enough to be interesting.

The less glamorous truth is that milder natural toxins are all around us, in the kitchen, in the pantry, and in the fruit bowl. They will not harm us after one meal, but it is worth knowing where they hide and how easily we can avoid them.

Even famous people have discovered that ordinary kitchen ingredients can have surprising effects. In The Autobiography of Malcolm X, the civil rights leader recalled his years in prison in the 1940s, when inmates stirred large amounts of nutmeg into water because of its effect on the mind. It is a striking reminder that a spice we sprinkle on eggnog and mashed potatoes is harmless in a pinch, but not in a handful. A reader shared a similar lesson from her own kitchen: after peeling a bag of green, sprouting potatoes for a family meal, she noticed a bitter taste and a stomach ache the next morning. "I never knew the green parts mattered," she told us. "Now I peel generously and keep potatoes in a dark cupboard."

As a nutritionist, I want to stress one thing before we start: all ten foods below can be part of a healthy diet. The point is not fear, but knowledge. If you enjoy lists like this, you may also like our articles Foods We Use Incorrectly and 5 Foods That Shorten Your Lifespan.

1. Cashew Nuts

Why are some people allergic to nuts? Nut allergies are an immune reaction to certain proteins, and they affect only a minority of people. Cashews, however, have an additional story. The shell around the raw cashew contains urushiol, the same irritating oil found in poison ivy, which can cause painful skin rashes in many people who touch it. That is why cashews are never sold truly raw in the shell: the "raw" cashews in shops have already been steamed or roasted and shelled, which removes the irritating oil. The cashews you buy are safe to eat, and those with a diagnosed nut allergy should simply follow their doctor's advice.

2. Potatoes

Potatoes that start to sprout, and green, unripe parts of potatoes, contain solanine and related compounds called glycoalkaloids. The plant produces them as a natural defence against insects and fungi. A review published in the Journal of the Royal College of Physicians of Edinburgh notes that many people do not realise that potatoes turning green or sprouting can build up these compounds. In larger amounts they may cause nausea, diarrhoea and vomiting.

But do not be too worried. Normal, well-stored potatoes contain very small amounts, and problems usually appear only when someone eats a large quantity of green, bitter or heavily sprouted potatoes. The European Food Safety Authority (EFSA) reviewed potato glycoalkaloids in 2020 and recommended simple habits: peel potatoes, cut away green spots and sprouts, and throw away any potato that tastes bitter. Store them in a cool, dark place. Potatoes remain a valuable food, and they are even a better source of potassium than bananas, as we explained in Bananas and Potassium: A Myth That Needs to Be Broken Down.

3. Tomatoes

Green and unripe tomatoes, like potatoes, belong to the nightshade family and contain glycoalkaloids. In tomatoes, the main compound is tomatine, and the highest amounts are found in the leaves and stems rather than in the fruit. According to a review in the Journal of Experimental and Basic Medical Sciences, these compounds can occur in the leaves, fruit and tubers of plants from this family, in unequal amounts. In very large quantities they may cause stomach discomfort, headaches and vomiting.

The good news: tomatine is considered much less toxic than potato solanine, and its level falls as the tomato ripens. That is why dishes such as fried green tomatoes or green tomato chutney are traditional in many cuisines and are eaten without problems in normal portions. Simply avoid eating tomato leaves and stems.

4. Farmed Salmon

Salmon has made headlines because of environmental pollutants. A well-known study published in the journal Science in 2004 found higher levels of organic pollutants, including polychlorinated biphenyls (PCBs) and dioxins, in farmed salmon than in wild salmon. In large, long-term exposure, such compounds are linked to effects on the immune system, liver and nervous system, which is why children and pregnant women are advised to choose fish wisely.

Good to know: since that study, fish farms have changed their feed, and monitoring has shown lower levels in many products. Wild salmon generally contains less of these substances. Nutrition experts at the Harvard T.H. Chan School of Public Health point out that for most people, the benefits of eating fish, such as omega-3 fatty acids, outweigh the possible risks. Eating salmon once or twice a week, and alternating between different types of fish, is a sensible approach.

5. Rhubarb Leaves

Not everything green is healthy. Rhubarb stalks are delicious in pies and compotes, but the leaves contain much higher amounts of oxalic acid, a compound that binds calcium and in large amounts can stress the kidneys. Eating the leaves may cause breathing difficulties, nausea, diarrhoea, eye pain, a burning feeling in the mouth and throat, and reddish urine. The National Capital Poison Center advises that only the stalks should be eaten. Simply cut the leaves off and put them in the compost.

6. Elderberry

Elderberry leaves, stems, bark and unripe berries contain compounds called cyanogenic glycosides, which can release small amounts of cyanide. The largest amounts are in the leaves and stems, and less in the seeds. Raw or undercooked berries, or products made with leaves and stems, may cause diarrhoea, headaches, dizziness, drowsiness and a drop in blood pressure. The U.S. Centers for Disease Control and Prevention (CDC) once described a group of people in California who became ill after drinking juice pressed from elderberries together with leaves and stems.

The solution is simple: use only ripe berries, remove all stems and leaves, and always cook the berries before eating them. Cooked elderberry syrups, jams and the traditional elderflower drinks are enjoyed safely across Europe. If you have concerns about your blood pressure or take medication, it is a good idea to ask your doctor before using elderberry products regularly.

7. Nutmeg

Nutmeg contains myristicin, a compound that can affect the nervous system when taken in large amounts, which is why nutmeg has a long history of misuse as a psychoactive substance. Poison centres report that taking several grams at once, roughly one to two teaspoons or more, may cause confusion, headache, nausea, dizziness, dry mouth, red eyes, a racing heart and unpleasant hallucinations lasting many hours. Severe cases are rare, but they do happen, mostly when people deliberately experiment with it.

In the kitchen, there is no reason for concern. The pinch of nutmeg in b{-15447}chamel sauce, pumpkin pie or mulled wine is far below any problematic amount. Just keep the spice jar out of reach of children and never use nutmeg as a "natural high".

8. Honey

Honey deserves a clarification. The substance known as grayanotoxin does not occur in ordinary honey, but in so-called "mad honey", which bees make from the nectar of certain rhododendron species, mainly in regions around the Black Sea in Turkey and in Nepal. According to the U.S. Food and Drug Administration's Bad Bug Book, it can cause sweating, weakness, dizziness, nausea and vomiting, and in rare, more severe cases a slow heart rate and low blood pressure. Symptoms usually pass within about a day.

There is another, much more important reason why doctors advise against honey, raw or pasteurised, for babies under 12 months: it can contain spores of the bacterium that causes infant botulism. For healthy adults and older children, a spoonful of good-quality honey a day is a perfectly reasonable part of a balanced diet. Honey can even be part of a clever breakfast, but some morning combinations are better avoided, as explained in Why is it dangerous to eat these foods for breakfast.

9. Tuna

Some types of fish and shellfish contain larger amounts of mercury, especially large predatory fish at the top of the food chain, such as bigeye tuna, swordfish and shark. The World Health Organization warns that methylmercury can affect the nervous system, kidneys and digestive system, and that the developing brain of an unborn child is the most sensitive. That is why pregnant women, breastfeeding mothers and young children need to be most careful.

This does not mean tuna has to disappear from your menu. The U.S. FDA and EPA advice on eating fish recommends that pregnant women and children choose fish from the "best choices" list, which includes canned light tuna, and limit albacore (white) tuna to one serving a week, while avoiding bigeye tuna altogether. For most adults, a varied fish menu of two to three servings a week is a healthy choice.

10. Popcorn

Microwave popcorn used to contain diacetyl, the compound that gives it a buttery smell. The main risk from diacetyl comes from breathing it in, not from eating it. Heavy exposure was linked to a serious lung condition among workers in popcorn factories, which led to the nickname "popcorn lung", and the U.S. National Institute for Occupational Safety and Health (NIOSH) investigated these cases. One widely reported case involved a consumer in the United States who ate several bags of microwave popcorn a day for years and regularly inhaled the steam from freshly opened bags. He developed lung problems, but survived.

After these reports, the largest popcorn manufacturers removed diacetyl from their products, although some flavourings were replaced with similar compounds. If you love popcorn, the simplest choice is to make it yourself in a pot with a little oil and salt, and to let the steam escape before you lean over a freshly opened bag. Young children in particular should not inhale the hot steam.

The Bottom Line

Almost every food contains some natural substance that could be harmful in extreme amounts. As the old saying goes, the dose makes the poison. Store potatoes in the dark, cut off green parts, eat only rhubarb stalks, cook elderberries, use nutmeg by the pinch, keep honey away from babies, and choose fish wisely. With these simple habits, all ten foods can stay on your table. And if you sometimes feel bloated after certain meals, take a look at Food that makes you fat, even if it does not fatten.