There’s a Scientific Explanation for Why Adults Are More Likely to Tolerate Leafy Greens

It’s no surprise that most kids abhor the taste of leafy greens. To some, kale’s ruffled edges, vein-like texture and earthy flavor lend it roughly the same appeal as a dirt-drenched clump of grass plucked straight from the backyard; brussels sprouts’ odious rotten egg smell and seemingly endless layers of bitter leaves are similarly unenticing.

Still, new research presented at this week’s 256th National Meeting & Exposition of the American Chemical Society suggests the best way to overcome an aversion to bitter greens is to simply grin and bear it—eventually, you’ll develop a taste for the foods you once despised.

The New York Times’ Livia Albeck-Ripka reports a team of researchers led by Cordelia A. Running, a food scientist and nutritionist at Indiana’s Purdue University, has shown that repeatedly exposing individuals to bitter tastes enables proteins in their saliva to render the flavors less offensive over time.

Saliva is made up of water and thousands of proteins released by salivary glands. These proteins are sensitive to different sensations—including bitterness, sweetness and astringency, or dryness—and can bind to flavor compounds or taste centers found in the mouth.

“If we can change the expression of these proteins, maybe we can make the 'bad' flavors ... weaker,” Running explains in a statement.

The scientists recruited 64 volunteers to participate in a six-week study, James Gaines writes for Inside Science. One week, subjects were asked to refrain from eating bitter foods. The next week, they were instructed to drink three glasses of chocolate milk per day, rating each drinks’ bitterness and astringency, or pucker-inducing, dryness (think biting into a green unripe banana). Chocolate milk typically doesn’t qualify as a bitter drink, Albeck-Ripka explains, but the samples used in the study contained less added sugar than your average cocoa fix. Researchers collected volunteers’ spit in order to assess protein changes and repeated the two-week cycle three times.

After analyzing participants’ flavor ratings and saliva samples, scientists realized that decreased perceptions of tang and texture were accompanied by an increase in proteins capable of binding these unpalatable compounds.

“We think the body adapts to reduce the negative sensation of these bitter compounds," Running says.

Although salivary protein adaptations helped participants overcome their initial aversion to the bitter chocolate drink, Running notes that these benefits would only continue if subjects maintained a diet filled with bitter foods. Otherwise, tolerance would fall once again.

According to Albeck-Ripka, humans have a natural disinclination toward bitter foods, as such flavors often serve as signs of toxicity. In fact, some bitter delicacies—including leafy greens—can be harmful when consumed in excessively high quantities. As Running notes, “[These vegetables] seem to stimulate systems in the body that help us respond to threats because they are themselves—in really high doses—threats.”

The researchers suspect that proteins bind to bitter compounds not only to improve flavor, but to stop the body from absorbing potentially harmful foods. It’s unclear whether this is a protective measure or simply an unhelpful suppressant of the foods’ nutritional value.

Moving forward, Running hopes to study specific combinations of food compounds and salivary proteins, as well as the amount of time needed for proteins to adapt to bitter tastes. Eventually, she hopes to assess the possibility of adding a substitute for salivary protein to food in order to enhance its flavor.

“Saliva modifies flavor, which in turn modifies dietary choices," Running summarizes. "Those choices then influence exposure to flavors, which over time may stimulate altered expression of saliva proteins, and the circle begins anew. Maybe this knowledge will help someone stick to a healthier diet long enough to adapt to like it."

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's senior associate digital editor, history.