Why Do Onions Make Us Cry?

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August 26, 2017 | 17,764 views

Story at-a-glance

  • Chopping onions was once thought to be “cut loose,” so to speak, by the enzyme alliinase, which breaks down into flavor molecules that give onions their characteristic bold flavor and tear-inducing fragrance
  • The chemical propanethial S-oxide is responsible for bringing tears when you chop onions; only three other molecules having such tear-inducing effects have been identified, and all are produced by plants
  • Cut or chopped, allium and allyl disulphide in onions changes to allicin, which fights several diseases, including diabetes and cancer, coronary and vascular diseases and stroke, and shows antibacterial, antiviral and antifungal activity

By Dr. Mercola

If you plan on making a salad, some kind of soup or a savory stir-fry, onions will most likely be part of the equation. You might also slice them into rings to place on sandwiches, or sauté them gently in coconut oil or grass fed butter until they're soft, translucent and absolutely delicious. But something else may be likely as well: When you're cutting or chopping them, before you get very far into the first onion, you're very likely to find your nose twitching and tears making your eyes smart.

Medical News Today suggests onions make your eyes spring a leak when you cut into them (or, to a lesser degree, other potent-smelling members of the allium family, such as garlic, chives, scallions and leeks) to discourage greedy predators from eating them. Whatever the reason, you can blame — or thank — specific volatile chemicals for giving onions their characteristic bold flavor and fragrance.

A perennial vegetable, onions form a bulb the first year, which happens to be quite flavorful and functions as the plant's energy store. The next year, they produce flowers and seeds, which keep the fragrance and cause the onion itself to keep reproducing.

Researchers have recently determined how it actually works. The eye-irritating factor released by chopping onions was once thought to be "cut loose," so to speak, by the enzyme alliinase, which breaks down into flavor molecules, and in the process, some are converted to lachrymatory factor (LF) by LF synthase.1 The journal Nature published the study, in which the researchers explained:

"Here we show that this factor is not formed as a by-product of this reaction, but that it is specifically synthesized by a previously undiscovered enzyme, lachrymatory-factor synthase. It may be possible to develop a non-lachrymatory onion that still retains its characteristic flavour and high nutritional value by downregulating the activity of this synthase enzyme."2

The Science Behind the Smell

Marcin Golczak, an assistant professor in the department of pharmacology at Case Western Reserve University in Cleveland, headed up the team that tackled the exact structure of the enzyme, known as lachrymatory factor synthase, or LF synthase, which shows that the enzyme responsible for the crying phenomenon is simply doing its job to create the heady fragrance and intensify the onion's flavor.

When the eau de parfum of onion reaches your nose and eyes (specifically your cornea), the nerve endings there are alerted that an irritant is in the vicinity. Your brain activates your tear ducts to send the sniffles and tears, which help wash away the offending irritation. Blinking helps, too. How LF synthase enzymes work is somewhat complex, which is why, since being identified in 2002, scientists haven't actually demonstrated how it works.

In order to do that, Golczak and company turned solutions of the onion-derived enzyme into microscopic crystals, after which they were detectable under X-ray, which revealed the enzyme's 3-D structure, as well as the tiny pocket where the LF conversion occurs. Medical News Today notes: "Detailed knowledge of this chemical process fills a fundamental gap in knowledge and gives scientists a better understanding of the biochemical potential of onions."

Onions Offer a Unique Set of Nutrients for Health

Some of the most potent vitamins and minerals contained in onions include vitamin C, pantothenic acid and vitamin B6 (pyridoxine), which helps augment levels of the neurotransmitter GABA (gamma-aminobutyric acid) in the brain, assure a good night's sleep and combat depression and anxiety. They also provide quite a lot of fiber.

Iron, folate, thiamin and potassium are additional minerals. Along with the vitamin C, manganese helps fight colds and flu because it also strongly discourages inflammation. But several valuable antioxidant compounds in onions, including flavonoids, polyphenols and quercetin, help enhance your health.

Research shows that the same compounds that initiate the tears — allium and allyl disulphide — change to allicin when they're cut or chopped and have the ability to fight diabetes and cancer, specifically stomach and ovarian cancers.3 This is significant, as other serious diseases and disorders, most being related, are also positively impacted, according to Nutrition and You,4 including:

Being antidiabetic, anticarcinogenic and anti-inflammatory, onions and the compounds they hold:

Something about the compounds in onions that can bring about healing also brings about tears, but the tears are for protection, Medical News Today explains: "The chemical at the heart of our discomfort is called propanethial S-oxide, which is also known as lachrymatory factor (LF). The technical term for our tear glands is 'lacrimal glands,' and LF is a chemical that causes tears."5

Not All Tears Are the Same, Especially If You're Chopping Onions

In 2010, a photographer specializing in microscopic views, Rose-Lynn Fisher, found that depending on the emotion or cause of peoples' tears, when they've dried, they look different up close. Using powerful scanning electron microscopes, she was able to produce in pictures how different dried human tears appear. According to Smithsonian Magazine, when Fisher caught one of her own tears on a slide and examined it under a microscope, she noted:

"It was really interesting. It looked like an aerial view, almost as if I was looking down at a landscape from a plane … Eventually, I started wondering — would a tear of grief look any different than a tear of joy? And how would they compare to, say, an onion tear?"6

Her query spawned an interesting photographic undertaking, and a quite abstract one at that — literally, as each photograph has all the elements of a topographical map — hence the name of her project, "Topography of Tears." It entailed collecting, examining and photographing more than 100 tears, essentially crystalized salt, from several subjects, including a newborn baby.

Some appear similar to a wall treatment known as Spanish Lace — flat, pale and abstract in spots, and dark, rounded and almost geometric in others. Those associated with grief or regret seemed paler and less complex that those evoked by so-called "happy" or "laughing" tears, Fisher noted.

Basal tears are usually small and sparse for the purpose of lubrication of the cornea, while reflex tears are in response to an irritant, such as sawdust or tear gas. The third type is from crying due to an emotion such as stress or pain. The article noted:

"All tears contain a variety of biological substances (including oils, antibodies and enzymes) suspended in salt water, but as Fisher saw, tears from each of the different categories include distinct molecules as well. Emotional tears, for instance, have been found to contain protein-based hormones including the neurotransmitter leucine encephalin, a natural painkiller that is released when the body is under stress."7

Interestingly, the slides with tears collected from someone crushing or cutting onions were beautifully patterned and completely unique, almost like a series of snowflakes packed on a windowpane. However, Fisher concedes that the way the tear dries can make all the difference in its final appearance, including variables such as "the chemistry, the viscosity, the setting, the evaporation rate and the settings of the microscope."8

Another Reason Why Tears Are Useful

Believe it or not, if you find chopping onions to be so bothersome you'd rather just buy the chopped and dried variety, you might be interested in knowing that at least two groups of scientists have been trying to produce an onion devoid of the tearjerking response most people are affected by. One of the new "tearless" onion varieties inhibits platelet aggregation, which is linked to cardiovascular disease,9 but one has to wonder if, by silencing LF synthase, the nutritional value of the onion is compromised.

Tears are an interesting phenomenon, nonetheless, that have been studied or at least wondered about for thousands of years. Not so long ago, scientists found that tears can be used just as effectively to monitor glucose levels in diabetic patients. The research,10 conducted by scientists from the University of Michigan, would provide a painless way to accurately measure such levels, even in animals, using an electrochemical sensor device. According to the study:

"A strong correlation between tear and blood glucose levels was found, suggesting that measurement of tear glucose is a potential noninvasive substitute for blood glucose measurements, and the new sensor configuration could aid in conducting further research in this direction. It may be possible to measure tear glucose levels multiple times per day to monitor blood glucose changes without the potential pain from the repeated invasive blood drawing method."11

Another thing this research proved is that tears aren't always a bad thing, especially if they're produced by chopping onions. Buying organic onions is always best, and you may be interested in knowing that red onions have particularly beneficial properties for your health, although each variety has tremendous potential depending on your needs. In past articles, I've discussed different onion types and the best ways to cook and store them.

[+]Sources and References [-]Sources and References

  • 1 American Chemical Society July 14, 2017
  • 2 Nature 419, 685 (17 October 2002)
  • 3 Scientific Reports 2016; 6: 29588
  • 4 Nutrition and You 2009-2017
  • 5 Medical News Today August 7, 2017
  • 6, 7, 8 Smithsonian November 19, 2013
  • 9 J. Agric. Food Chem., 2013, 61 (44), pp 10574–10581
  • 10, 11 J Diabetes Sci Technol. 2011 January; 5(1): 166–172