An underappreciated body fluid is emerging as a powerful tool for research, medical diagnosis and health. Spit, as it turns out, contains all sorts of juicy information.
According to researchers at Johns Hopkins University's Center for Interdisciplinary Salivary Bioscience Research — yes, a research center dedicated to spit — saliva holds a "treasure trove" of data that is easily collected and inexpensively analyzed. It has the potential to expose secrets of human biology and genetics, as well as helping combat disease. "There's lots of potential in exploring what's in saliva," said Doug Granger, the center's director.
But what can spit do for you?
One-third of heart attack victims drop dead without ever knowing they had high cholesterol, hypertension or the other factors that increased their risk of cardiac arrest. That's partly because the blood test currently used to diagnose a person's heart disease risk is quite the ordeal — it's painful, requires a clinic visit, and takes weeks to be processed — and so most people don't take it as often as they should.
Now, in a new study, Granger and his colleagues have proposed a saliva test to replace the standard blood test. According to the researchers, spit contains the same protein, called C-reactive protein, that indicates a risk of heart disease when found in blood at elevated levels, and spit can therefore give a rough proxy of a patients' heart health. Once a saliva test is available, "more people would be willing to have the test done. It could be done on a more regular basis, even in their homes," Granger said in a press release.
Your father's daughter?
Research shows that daughters who have warm relationships with their fathers begin puberty later, wait longer to start dating and having sex, and are more likely to be monogamous. But why? As detailed in a recent paper by Granger and his team, the answer may be swimming in spit.
The researchers found that when a girl's father-daughter relationship is characterized by rejection, chaos and coercion, her saliva exhibits lower-than-normal levels of the stress hormone cortisol in the morning, and elevated cortisol levels when she is discussing problems or anxieties with friends. These off-balance levels indicate emotional over-sensitivity to stressful situations, a trait that can negatively affect life choices and stress management. [Why Stress is Deadly]
Salivary signs of stress
Stress triggers the body's fight-or-flight response, causing, among other things, a rush of adrenaline, an increased heart rate and salivation. The salivary glands flood the mouth with an enzyme called salivary alpha-amylase (sAA), and this can serve as a marker of stress.
The stress or emotional trauma that a pregnant woman experiences can affect the health of her fetus. The Johns Hopkins team has developed a method of gauging the impact of a mother's stress on her unborn baby by monitoring sAA levels in her saliva. In a study published in February, they identified the way sAA levels naturally change over the course of pregnancy, and the pattern by which they vary throughout the day; this lays the groundwork for future research investigating the effect of unusually high stress levels on infants.
Pre-mastication — the act of pre-chewing adult food and feeding it to one's baby — was standard practice among our blender-lacking ancestors and remains common in many of the world's cultures. Now, it may be coming back in vogue in the West, too, thanks to research indicating a mother's saliva helps boost her infant's immune system.
By exposing infants to traces of disease pathogens present in a mother's spit, pre-mastication gears up their production of antibodies, teaching their immune systems how to deal with those same pathogens later in life. It could also reduce their risk of the autoimmune diseases, such as asthma, that are common in industrialized countries — epidemiology evidence indicates that these ailments result from underexposure to pathogens before age 2.
Fortunately, spit is also swimming with antibodies that reduce the infectiousness of the bacteria in their midst; this means exposing babies to spit gives them a taste of pathogens while usually preventing them from getting sick. [Is a Dog's Mouth Cleaner than a Human's?]
Your spit contains your entire genetic blueprint, and in a form that may be easier to work with than DNA extracted by other methods. "One-half of an eyedropper drop [of spit] is enough to get a reasonable sample of DNA," Granger said. "Samples can be frozen and thawed multiple times. They can be sent through the mail, and we're able to extract high-quality, high-quantity DNA."
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Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.