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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

Medical Myths Medical Myths

The Seeds of Worry

October 10, 2013

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

Just the other day, I was asked if it was true that swallowing an apple seed or a watermelon seed could cause appendicitis. As a kid, I recall warnings from a neighborhood friend: Never swallow seeds, because they are poisonous. I now know that he was not the most reliable source of medical information, but at the time, it seemed plausible. Of course, he also told me not to swallow seeds because a tree could start growing in my stomach; I was terrified. Is there any truth here, or are these better viewed as folklore, myth or useful notions only for their entertainment value? And does the type of seed or how many you eat matter?

Swallowing Seeds: The Worst That Could Happen

It is true that swallowing the seeds of commonly consumed fruits or vegetables can cause problems, but fortunately, these problems rarely occur and are rarely serious. They include diseases within the intestinal tract, lungs and the body as a whole.

Intestinal Problems

  • Direct injury, such as a tear or even perforation
  • Infection and/or inflammation — appendicitis, diverticulitis or abscess
  • Blockage of the intestine (obstruction)

Any poorly digestible, thin, sharp objects, including seeds, may rarely cause these problems in the intestine. The vast majority of people with appendicitis develop it for no apparent reason and no food or other ingested material can be identified, even in the removed appendix. Similarly, intestinal obstruction is more common because of tumors or scar tissue (a common problem related to past surgery) than to anything ingested.

When obstruction occurs as a result of something eaten, it is usually due to ingestion of a large amount of undigestible material; if it blocks the digestive tract, it is called a “bezoar.” Although common among cats, it is rare in humans. When it does occur, it is most common among young children or adults with major psychiatric illness, particularly an obsessive/compulsive disorder manifested by hair pulling and eating the hair.

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Lung Airway Problems

Another way seeds or pits may produce problems is by blocking an airway that leads to the lungs. Seeds and pits are rarely large enough to block a large airway, but if inhaled (or if they “go down the wrong way” and wind up in the lungs), they may lead to pneumonia or partial collapse of the lung. This is one reason to eat sitting up and not to let young children eat foods with seeds or pits.

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Seeds and Pits as Poison

Swallowed seeds may cause trouble not only by their mechanical effects on the intestinal tract but also by toxic (or poisonous) effects on the body. This may happen either because of the seed’s natural components or due to a contaminant (such as pesticide or bacteria). Fortunately, this type of illness is also rare. For example, I could find no reports in the medical literature linking ingestion of apple seeds with serious illness. Similarly, there is almost nothing published linking disease with the accidental ingestion of pits or seeds from other commonly eaten fruits and vegetables.

There is a "kernel" of truth, however, to the adage that apple seeds are poisonous. Perhaps you have heard of cyanide, a poisonous substance that can cause dangerously elevated acid levels in the body, seizures, coma, and death. Under certain unusual circumstances, cyanide poisoning can follow the ingestion of seeds or pits of several common fruits that are members of the Prunus species, including, apricots, cherries, almonds, and peaches, as well as apples. All of these seeds and pits contain amygdalin. You may have heard of amygdalin as a component of Laetrile, an "alternative" cancer treatment of no proven value that may be associated with cyanide poisoning. This harmless chemical lies inside the seed, but when the seed is moistened and crushed, it can be converted by bacteria in the intestinal tract or by an enzyme within the seed into cyanide. Because the amount of amygdalin in an apple seed is quite small, it is highly unlikely that you would become ill from swallowing one or two seeds, especially if they were not chewed. In fact, many other foods, including lima beans, also contain amygdalin in tiny amounts considered harmless.

Apricot pits and bitter almond, a completely separate food from commonly consumed sweet almonds, may cause significant illness or even death if chewed and swallowed, but these are not foods most people would find appetizing. It is tempting to believe that amygdalin resides in these seeds and pits to discourage animals (including humans) from eating them — if you do it once and become ill, you are unlikely to come back for more.

It is certainly possible to become ill from eating seeds or pits, but unless your diet is unusual, it is difficult to become sick from swallowing seeds or pits inadvertently. A 1998 report in the Annals of Emergency Medicine described cyanide poisoning in a woman who bought apricot kernels at a health food store, became severely ill within 20 minutes of ingestion and was close to death when she reached the emergency room. Fortunately, the cause of her illness was identified and she recovered, but it was the first report of its kind in nearly 20 years.

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Once Again, Your Parents Were Right

Did your parents ever remind you to chew slowly, carefully and thoroughly? On this one, they were right. One of the reasons that this is a good idea is that chewing well before swallowing helps sort out inedible from edible material. With chewing, an inedible object (such as a seed or pit) can be detected and spit out rather than being swallowed. The habit of eating an entire apple, seeds and all, cannot be recommended, although the risk of even this habit is probably small.

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The Bottom Line

It is true that swallowing the seeds and pits of commonly consumed fruits and vegetables may occasionally cause illness. However, such events are so rare that there is little reason to recommend a change in behavior based on this risk. Common sense should prevail: Don’t go out of your way to eat what seems to be inedible, such as the pits or large, hard seeds of fruits or vegetables. If you accidentally swallow an apple seed, do not panic. In the grand scheme of things, you are still better off eating an apple — even with the occasional seed — than eating junk food.

Urban legends and medical myths often describe dire (but potentially plausible) consequences of activities we do every day. Claims of deodorant use causing breast cancer and death from smelling free perfume samples (both untrue) are recent examples that have circulated through cyberspace. The dangers of swallowing seeds probably belong in this category. In the end, my neighborhood pal taught me something much more important than how dangerous apple seeds could be. He taught me not to believe everything I hear.

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Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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