White liquid dripping out of lettuce?

Many plants produce their own specialized latexes. Latex is a type of sap, and, depending on the plant, can help us make products ranging from rubber to opium.

And while lettuce clearly does make latex, it’s unusual for consumers to spot it in modern, store-bought lettuce, according to Germán Sandoya, an assistant professor in breeding and genetics at the University of Florida. Sandoya, whose work includes adapting lettuce varieties to Florida’s climate, said current cultivars simply don’t make much of the stuff.

If you do see the creeping white fluid, it’s a sign of a very fresh head. Latex production stops and it dries up soon after harvest. So, congrats to Kristy Lamb, the reader who sent in this image – your romaine lettuce was extra fresh!

But while lettuce latex is rare in today’s salad bars, long ago the white liquid was such a dominant feature that the veggie’s scientific name Lactuca sativa starts with “lac,” as in, “milk,” in Latin. 

Are the rumors true…can lettuce latex help you sleep?

Probably not. At least not the store-bought stuff these days. But back in ancient times? Maybe. 

The Oxford Companion to Food describes the latex flowing through wild lettuce as having a “mild soporific effect.” Way back when, Romans reportedly ate lettuce at the end of a meal to calm their minds and promote sleep. Egyptian wall paintings feature lettuce, though they were more focused on perceived aphrodisiac effects from the leafy veg.

But, as the centuries went by, people selected for varieties of lettuce that were less bitter and therefore had less latex and less of a sleep-inducing effect (and presumably fewer other effects as well). 

“By the 1600s, the narcotic effect of lettuce had been much reduced. It seems that the only known cases of lettuce actually having sent people into a stupor are when, in times of shortage, they have been reduced to eating large amounts of the stems of lettuces which have bolted or gone to seed,” reports the Oxford Companion to Food. (When a plant like lettuce “bolts,” it prematurely starts producing seeds.)

Still, people continued to search out wild lettuce for its sleep-inducing effects. According to Rebeca Rupp, writing in How Carrots Won The Trojan War, balls of dried lettuce latex were used to induce sleep in Medieval England; a mild sedative, made from wild lettuce extracts and dubbed “lactucarium,” was used in hospitals through World War II. In the 1970s, entrepreneurs tried to market smokable lettuce opium, but, as Rupp writes, it never found a market likely because “nobody got a bang out of smoking lettuce because there wasn’t actually much of anything in it.” 

What’s in lettuce latex that might promote sleep?

The sleep-inducing qualities of certain types of lettuce latex may come from a compound called lactucin. In a study published in Food Science and Biotechnology in 2017, researchers found that extracts from romaine lettuce caused mice to fall asleep faster and stay asleep longer. That said, the study examined extracts and didn’t simply give the mice big, mice-sized bowls of Caesar salad. Its authors concluded, “further study for the sleep structure and sleep mechanisms induced by romaine lettuce extracts is required before they can be used as dietary complements or as natural food antioxidants.” I emailed one of the study’s authors to find out how the extracts given to mice might translate to a volume of intact lettuce leaves a human might consume. I have not yet heard back. 

What is lettuce latex made of?

Latex is essentially plant sap with a slurry of other compounds and particles mixed in (we’ve previously written about how mangoes emit a caustic latex at harvest, which eventually fades to a gentler clear sap). Lettuce latex includes “a complex mix of proteins, alkaloids, starches, sugars, tannins, resins, and gums that coagulate when exposed to air,” Rupp wrote for NationalGeographic.com. 

While plant latexes are technically types of sap, the “there are no chemical or functional correlations to the other main plant saps, such as those produced in the phloem or xylem,” according to Laticifers, Latex, and Their Role in Plant Defense, a 2019 paper published in Trends In Plant Sciences. 

Laticifers, which are cells or rows of cells that synthesize and store latex, are particularly abundant in leaves. So it’s no wonder our salad staple used to pack lots of the stuff.

Why do lettuce plants produce latex?

Lettuce latex is “believed to be a protectant against insects or microorganisms that may attack the crop,” Sandoya told me. “However, there is not clear evidence on this. It certainly helps to move nutrients, water and other elements throughout the plant which are needed for their normal growing.”

Plants typically store latex under pressure; it bursts forth once a plant’s tissues are broken. When a mango is first picked, its spray of latex can burn human skin. Another defensive latex trick is to literally gum up insects’ mouths with sticky goo. Researchers have also found, however, that some leaf-eating creatures cut into latex-exuding plants, wait for the latex to drain away, and then proceed with their herbaceous meal.

Another common component of plant latexes, peptidases, are involved in biological functions ranging from seed germination to protein turnover, but can also injure a would-be nibbling creature. According to the Trends In Plant Sciences article referenced above, large volumes of latex peptidases could damage animal tissues in ways similar to snake venom. 

Intrigued, and maybe a little worried, I reached out to one of the paper’s authors, Márcio Viana Ramos, who conducts research at the plant latex laboratory in the Department of Biochemistry and Molecular Biology at the Universidade Federal do Ceará in Brazil. He told me that the amount of latex we consume in foods is too small to cause us any discomfort. In fact, he pointed out, these peptidase plant enzymes could even help our digestion.

Researchers have identified thousands of small molecules in plant latexes and there’s much we still don’t know about them. Given plant latexes’ potentially defensive properties, and the workarounds some leaf-eaters have devised, researcher William F. Pickard concluded in New Phytologist in 2007, “there would seem to be a duel of lunge and parry between a laticiferous species and its enemies.”   

Can you be allergic to the latex in lettuce?

If you have a latex allergy, could you also be allergic to the white milky sap, sometimes also called latex, exuding from lettuce?

That’s unlikely, Eric Macy, a physician in the allergy department at Kaiser Permanente in San Diego, told me. 

“Allergy to lettuce is pretty rare,” he said, noting that what allergists deem a natural rubber latex protein allergy is triggered by an immune response to specific proteins that are in the sap produced by the rubber tree. Someone with such a “latex allergy” is not assumed to necessarily be also allergic to any type of milky sap from any plant. A plant that doesn’t make any kind of latex, but does manufacture a certain protein could trigger an allergy in a sensitized person. And, a plant that does make latex, but makes it without an allergy-triggering proteins, won’t cause a reaction. 

It’s possible that handling lots of fresh lettuce, which you might get from extended time in the garden or working on a farm, could eventually make your skin more sensitive to compounds found in lettuce sap, leading to contact dermatitis, eczema, or other irritant skin rashes, according a 2015 paper in Contact Dermatitis. But such reactions are rarely reported, the paper’s authors wrote.

A lettuce rubber band?

About 10 percent of flowering plants make latex, Rupp wrote for National Geographic. While plant latex is typically milky, researchers report that it can be yellow, orange, red, brown or colorless.

We turn latex from rubber trees into rubber, and latex from poppies into opium. Anyone whose childhood involved making dandelion crowns probably remembers the white sticky stuff–latex, naturally–that bled from ripped stems of the yellow-flowered weed. Rupp writes that, during World War II, Russia, having lost access to rubber drawn from the signature trees, made a rubber substitute from dandelion latex. “Lettuce latex,” she writes, “given a little chemical time and effort could conceivably yield, if not a cost-effective bicycle tire, at least an occasional rubber band.”