You Are Eating Poisonous Organic Fruits & Vegetables?

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Robert Pavlis

Fruits and vegetables are supposed to be good for you – how can they be poisonous? You are probably thinking that I am talking about vegetables sprayed with pesticides but I’m not. Or maybe this refers to vegetables contaminated with pathogens – that’s not it either. I’ll briefly discuss both those problems, but this post is about non-GMO poisonous fruits and vegetables with no synthetic spray or pathogen contamination.

Some fruits and vegetables are poisonous, but not all of the claims are correct.

You Are Eating Poisonous Fruits & Vegetables Even If They're Organic!
You Are Eating Poisonous Fruits & Vegetables Even If They’re Organic!, photo by Harvard

Poisonous Fruits & Vegetables, and Pesticides

The general public are very concerned about ingesting synthetic pesticides, largely because of inaccurate information provided by groups in favor of organic food. One food blog said, “Would you believe that a single grape tested positive for 15 pesticides?” That should not be a surprise. Our testing equipment is so sensitive that it can find almost any chemical, anywhere. What the blog failed to point out is that the levels are so low, they don’t mean anything.

They also missed the most important point, that 99.99% of the pesticides we eat are natural pesticides that the plant produces to keep bugs from eating it. I have written about this in detail in, Natural Pesticides.

What this means is that all fruits and vegetables contain poisons but in most cases they are in very small amounts and our bodies have no problem dealing with them.

Pathogens on Vegetables

There are recalls on vegetables contaminated with a bacteria, like listeria, salmonella and E. coli, several times a year in North America. Eating these could kill you from the toxins produced by the pathogens, but that is not the topic of this post.

By the way, home grown vegetables can also have these problems, but they are never tested for pathogens so you don’t know if they are a problem.

Poisonous Fruits & Vegetables For Animals

This post is focused only on humans, but the topic also applies to animals including pets like cats and dogs. Each animal can be poisoned by a different set of plants. What harms a dog may not harm a cat, what harms a cat may not harm us. Saying something is poisonous means very little unless the animal is identified.

Check out this FREE 10 part Vegetable Growing Master Class.

Why are Some Fruits and Vegetables Poisonous?

All plants produce toxic chemicals to keep bugs from eating them and to keep pathogenic bacteria and fungi from growing on them. This is the natural way plants defend themselves.

If a plant produces higher levels of one of these poisons and humans are particularly sensitive to it, it can harm us.

How does this happen? Plants naturally produce these chemicals; they always have. But breeding can change things. It can reduce the level of a toxin, or it might increase the level. Cross breeding different species can even cause the offspring to produce a toxin that neither parent produced.

Microbe Science for Gardeners Book, by Robert Pavlis

I’m not talking about GMO breeding here, which is extremely safe and less likely to produce a toxic plant. I’m talking about natural “open pollination” and manual cross-pollination done by commercial breeding programs. You might even do this in your own garden. If you have several kinds of beans in the garden, and they cross pollinate, the seed you collect can be more toxic than either parent, especially if you collected the seed from plants that had less pest damage.

There is some testing for toxins in new commercial cultivars, but this is very limited and there is no testing done for seeds collected by home gardeners, or small seed producers. Increases in toxins are usually not found until people start getting sick.

This sounds very scary, but don’t be alarmed. Our bodies have developed very good ways to deal with toxins, after all we have been eating plants for a very long time.

Breeding Can Increase Toxicity

Here are a couple of examples of new cultivars that were too toxic and had to be removed from sale.

Celery naturally contains a toxic chemical called psoralens. The amount can vary significantly depending on cultivar and growing conditions. Breeding for reduced pest damage and aesthetic appeal resulted in cultivars that had psoralens levels so high they caused severe photodermatitis among farm workers and grocery staff. They had to be removed from the marketplace.

The Lenape potato, a heritage, developed in the nineteenth century was selected for improved disease resistance. After being commercialized it was found to contain very high levels of solanine, a chemical toxic to humans.

Variety is the spice of life or death!

Some Poisonous Fruits & Vegetables

I thought I would look at some specific poisonous fruits and vegetables and see if claims about their toxicity are valid.

As you read through these, think about the stupid advice that you should not eat anything with a chemical name you could not pronounce.

Poisonous Rhubarb

Don’t eat rhubarb leaves because they contain too much oxalic acid, or so they say. The truth is that rhubarb leaves contain about the same amount as the stalks and other commonly eaten vegetables, so they are not very toxic.

  • Carrot – 0.5 g/100 g
  • Chives – 1.48 g/100 g
  • Parsley – 1.70 g/100 g
  • Radish – 0.5 g/100 g
  • Rhubarb leaves – 0.5 g/100 g
  • Rhubarb stalks – 0.5 g/100 g
  • Spinach – 0.97 g/100 g

For more on the toxicity of rhubarb see, Will Oxalic Acid in Rhubarb Leaves Harm You?

Poisonous Green Tomatoes

Tomatoes (Solanum lycopersicum) are part of the Solanaceae family which also includes deadly nightshade and datura, both of which are quite toxic. This family contains toxic alkaloids including solanine, atropine and nicotine. That’s right, when you eat a tomato you eat some nicotine.

Tomatoes are richest in another alkaloid called tomatine. Green tomatoes contain quite a bit (500 mg/kg fresh fruit weight) to keep bugs from eating the fruit containing unripe seeds but as the fruit matures the amount drops (5 mg/kg) because now the plant wants to encourage animals to distribute the seed. Ripe tomatoes have almost no tomatine.

So how toxic is tomatine? That is not the right question to ask. The right question is, is the level of tomatine in green tomatoes high enough to cause health issues?

The answer is, probably not. We still do not know much about how tomatine reacts in the body, but most of it seems to be passed through our digestive system without being absorbed. A review of clinical cases found little evidence of green tomato poisoning. In fact, lots people eat both the leaves and green tomatoes without harm.

There is some preliminary lab data that suggests tomatine may kill certain human cancer cells.

The story about tomatoes also applies to other solanaceae fruit including eggplants, peppers, and ground cherries.

Poisonous Apples, Plums and Peaches

The seeds of some stone fruit (apricots, cherries, peaches, pears, plums and prunes) contain a natural toxin called amygdalin, a cyanogenic glycoside, which is converted to hydrogen cyanide in the stomach’s acid. The lethal dose of cyanide ranges from 0.5 to 3.0 mg per kilogram of body weight, which makes it very toxic. Even chewing on a few seeds can be harmful.

These seeds are not normally consumed, but apricot pits are used to flavor food, which is not a great idea.

Apple seeds also contain high levels of amygdalin and have the same problem. If eaten and not chewed, they will just pass through, but chewing too many of them can cause cyanide poisoning.

Poisoning Fiddleheads

Fiddleheads for dinner

Fiddleheads are the young unfurled growth of ostrich ferns and are considered a delicacy. When eaten raw or improperly cooked they cause symptoms resembling food poisoning. The cause has not been researched but it is believed to be a natural toxin. When cooked thoroughly, for 10-15 minutes, until tender, they are safe to eat. Don’t reuse the water that was use to boil them.

Sprouts from other types of ferns should not be eaten. Some are toxic even if cooked and others may be carcinogenic.

Poisonous Kidney Beans

Red kidney beans contain phytohaemagglutinin, which causes diarrhea and vomiting in even small doses. They need to be soaked for several hours and boiled for a minimum of 10 minutes to reduce the toxin to a safe level.

Most raw beans contain the same toxin, but in lower doses. The dose makes the poison.

Elderberry Poison

“On August 26, 1983, eight people with acute gastrointestinal and neurologic symptoms were flown by helicopter to a Monterey, California, hospital. Earlier that day, they had attended a gathering for 25 persons of a religious/philosophic group. Within 15 minutes after drinking refreshments, 11 persons began to have nausea and vomiting. All recovered quickly, including the patient hospitalized overnight. The cause: elderberry poisoning probably from the cyanide-inducing glycosides found in Sambucus mexicana.

In this case, stems, leaves and the seeds were crushed to make the juice and it was not cooked.

How toxic are elderberries? The above story is from a CDC 1984 report that commented, “a review of the medical literature revealed no other reports of elderberry juice poisoning in the past 20 years, there are older, anecdotal reports of poisoning in children from the related elder, S. canadensis.”

The leaves, stems, seeds and especially the root of both red elderberry Sambucus racemosa and blue/black elderberry Sambucas nigra (canadensis) are toxic – don’t eat them. The berries are not toxic, although some sources still recommend cooking them before eating. Seeds that are swallowed whole will not cause harm.

Spinach, Beet Greens, and Swiss Chard Contain Oxalic Acid

The claim is that spinach, beet greens and Swiss chard all contain high levels of oxalic acid and eating too much causes tremors, low blood pressure, convulsions and kidney stones.

As explained above, the level of oxalic acid in these plants is not really very high and provided you don’t eat pounds of them at one time, they won’t cause immediate health issues. But what about kidney stones – ouch?

Oxalic acid can combine with minerals like calcium and form insoluble calcium oxalate that can accumulate and form kidney stones. However, it is always important to drill down and understand the details.

Only 80% of kidney stones are made from calcium oxalate. If kidney stones do form from oxalic acid, they only develop in a small percentage of the population and only when oxalate levels are high and urine levels are low. Most people don’t develop kidney stones from oxalic acid.

The human body naturally produces oxalic acid. Even if you don’t get any from a food source, your body still has significant levels, and it is now believed it is these higher levels that cause the kidney stones. “Oxalate restriction in the diet is no longer recommended to every person with kidney stones. This is because most of the oxalate found in urine is produced by the body, rather than absorbed from food.”

The issue of causing kidney stones from oxalic acid in vegetable greens is a myth.

Brazil Nuts Cause Selenosis

Brazil nut show seed and seed capsule, photo by Britannica
Brazil nut show seed and seed capsule, photo by Britannica

Selenosis is due to an overdose of the heavy metal selenium and can cause damage to the nervous system, fatigue, irritability, as well as hair and nail loss.

A 55 year old lady read about the use of selenium to prevent cancer so she ordered some paradise nuts (Lecythis ollaria), and ate 10-15 nuts per day for 20 days. Paradise nuts are very similar to Brazil nuts (Bertholletia excelsa); both are in the lecythis family. She developed headaches, dizziness, vomiting, and hair loss before being admitted to hospital. Diagnosis; selenium poisoning (5 times the normal level).

Selenium is essential for proper health, but too much is harmful. An average Brazil nut contains 96 mcg (ug)  but some can have as much as 400 mcg, depending on the level in the soil. The recommended daily intake for adults is 55 mcg and the upper limit for safe consumption is 400 mcg.

Now you know why you don’t find many Brazil nuts in a nut mix! 🙂

Are Potatoes Toxic?

Potatoes contain two natural glycoalkaloids that are of concern; solanine and chaconine. The entire plant contains these but they’re found in higher doses in leaves, flowers, “eyes,” green skin, and sprouts. The white inside of the potato has low levels. These glycoalkaloids are not reduced by cooking.

The first concern is eating sprouted potatoes. These have growing eyes and if left long enough they also have sprouts. Once these grow, the amount of glycoalkaloids starts to increase inside the potato, but most is concentrated in the new growth. It is unclear when the levels of toxins become high enough to cause a problem. If the potato tastes bitter it is time to toss them. The new growths should certainly be removed. Provided you don’t eat huge amounts of such potatoes they are probably safe to eat, although the National Capital Poison Center recommends you don’t eat them.

The second concern deals with a green skin. When the tuber is exposed to light, the skin develops chlorophyll which causes the green skin, which in turn increases glycoalkaloids. This is why potatoes are hilled up in the garden. It is unclear how safe these are to eat and government groups recommend not eating them, but they are probably safe to eat, provided the green skin is removed.

The skin of the potato contains 30-80% of it’s solanine, so removing the skin will reduce any possible toxicity.

The safe level of all GA (glycoalkaloids) is 200 mg/Kg fresh weight. A tuber becomes bitter around 500 mg/Kg, and the flesh has a range of 12 – 100 mg/Kg.  The skin has a value of about 500 mg/Kg, which seems high, but remember a potato has very little skin. Eating levels in the order of 1 mg/Kg body weight should be safe. So a 100 Kg person can eat 500 gm or 1/2 pound of potato flesh without skins. Serious glycoalkaloid poisoning in humans is rare. Concerns over potato poisoning seem to be exaggerated.

Keep potatoes from sprouting by keeping them dark and cool. The common belief that nearby onions cause potatoes to sprout has no scientific basis.

Toxic Squash Syndrome

Cucurbits (squash, pumpkins, cucumbers, zucchini) naturally develop cucurbitacin in order to protect themselves against insects. Domestically grown cucurbits have been bred to have lower amounts because it tastes bitter, but wild populations can have high levels. Some cucurbits easily cross pollinate and the new hybrids can also have high levels. It is common for gardeners to collect seed and find weird looking gourds being produced. In other cases, open-pollinated fruit left in the garden results in volunteer seedlings. These new fruits can contain such high levels of cucurbitacin that they’re toxic, causing Toxic Squash Syndrome. They are unlikely to kill you, but they can make you quite sick. 

If you find a strange squash in the garden, taste it. If it is bitter, don’t eat it.

Coconuts Can Kill You

They can kill you if they fall on you.

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

15 thoughts on “You Are Eating Poisonous Organic Fruits & Vegetables?”

  1. I’ve often read that it’s not advisable to grow edibles over drainage fields or human wate compost. What is your view on pathogens being taken up by plants grown in composted human waste? I always imagined the plants merely absorb the nutrients they need and leave the rest. The only ingestion would be improperly washed produce?

    • I don’t think plants are able to absorb the pathogens. But the pathogens could live on the surface of the plants, especially root crops.

      My personal view is that this is quite a remote possibility.


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