What's That White Liquid That Comes out of Lettuce When You Cut It?

romaine lettuce on board
romaine lettuce on board

Getty Images / Aniko Hobel

Have you ever noticed a milky white liquid appear after you cut through the core of a head of lettuce or slice through the thicker part of lettuce leaves? If you have, were you curious about what it is and whether the lettuce is still safe to consume? Maybe the white sap seeping out from your lettuce didn't catch your eye, but you may have seen those videos on social media claiming that drinking steeped lettuce water may help combat insomnia. Read on to find out what the white fluid is and what steeped lettuce water is all about.

What's the white liquid that comes out of lettuce?

Lactucarium is the milky white substance secreted by certain types of lettuce, such as romaine and wild lettuce, the latter of which is used for medicinal purposes rather than consumed as a cultivated vegetable. The root of the word lactucarium stems from the Latin lactus, meaning milk in English, but it doesn't taste like milk at all. It's slightly bitter and earthy, but fresh-tasting, like lettuce.

Next time you slice romaine, look for the white sap, but don't be alarmed. It's harmless. You may also be surprised to learn that lactucarium is a phytonutrient with a chemical structure similar to opium. So, it is also known as "lettuce opium," potentially inducing sleep and relaxation. Ancient peoples, such as the Egyptians and Romans, offered lettuce at the end of a meal to encourage sleep and calmness. However, lactucarium levels in today's cultivated lettuce are not high enough to induce such effects.

What are the effects of lactucarium?

To investigate the relationship between the sedative effects of lactucarium in romaine lettuce and sleep, a research study was conducted on rodents. Another study, published in the Biological and Pharmaceutical Bulletin, also found that rodents who consumed romaine lettuce extract had improved sleep. In both studies, the mice were given a large dose of romaine lettuce extract in a much higher concentration than what you would typically eat in a salad or or any reasonable portion of lettuce.

While there may be a possible link between this sap and sleep in rodent studies, there is very limited research to support the claim that lactucarium promotes sleep in people—since there haven't been research studies that involved humans—or that the amount consumed in a typical salad would have any effect. The mice involved in the clinical studies were also medicated while given the lettuce extract. In other words, there is little to no evidence indicating that lactucarium in romaine lettuce can help reduce insomnia.

The good news is that there are other natural and effective ways to improve your sleep quality, such as:

  • Avoiding caffeine and alcohol before sleep

  • Eating a balanced diet, including foods that may promote better sleep—bananas, oats, milk, yogurt, cheese and more

  • Limiting your exposure to bright lights, such as those from smartphones, tablets and monitors

  • Keeping your bedroom dark

Is it safe to eat lettuce that contains lactucarium?

You have nothing to worry about when you see this white sap seeping out from your romaine lettuce leaves and stems—it is perfectly safe to consume. The amount you eat in a few leaves is also very minimal—unless you ingest heads and heads of lettuce. You may then possibly notice lactucarium's sedative effects!

And the same goes for steeped lettuce water—there's likely a tiny amount of lactucarium from steeping a few lettuce leaves and stems. So, it's unlikely that drinking steeped lettuce water will actually induce sleepiness and relaxation.

Although science has not proven the benefits of drinking steeped lettuce water, you could still drink it, if it's something you enjoy. However, if you take certain medications, such as blood thinners, you should seek advice from your doctor on whether this beverage is safe for you.

Another consideration is that lettuce could potentially be contaminated with E. coli and Salmonella. The Centers for Disease Control and Prevention recommends washing and rinsing lettuce thoroughly before consumption to reduce food poisoning risk. While you do not necessarily need to wash lettuce that's packaged as "ready-to-eat," "triple washed" or "no washing necessary," eating pre-washed greens could still lead to food poisoning. The risk is especially high for adults older than 65, children younger than 5, pregnant people and those with a health condition or who take medications that impact their immunity. Eating lettuce cooked is likely the safest bet for these groups.

How to select and properly store lettuce

Look for lettuce that has bright-looking leaves that are crisp, not wilted, and free of damage or discoloration. Browned and yellowed leaves and dried leaves are signs of aging. Keep in mind that some spotting and holes are common in lettuce greens that were not treated with pesticides in the growing process and generally don't compromise safety. But lettuce with major damage or a slimy appearance indicates spoilage.

To keep your lettuce fresh, store it in its original packaging, in a plastic bag or an airtight container in the refrigerator. You can also stow it in the fridge's crisper drawer, where humidity is regulated. Remember, lettuce is perishable with a short shelf life. It will usually keep in the refrigerator for up to seven days.

Bottom line

There is nothing to worry about when you see that milky white liquid seeping from the cut end of romaine lettuce—lactucarium is safe to consume. The amount of lactucarium you ingest will be too minimal to aid sleep and relaxation. Still, lettuce leaves can be a part of a balanced and nutritious eating pattern—enjoy them as lettuce wraps, or in salad with grapefruit and shrimp and more!

Related: Is Romaine Lettuce Healthy? Here's What a Dietitian Has to Say