Though most people find the thought of saliva rather disgusting, spit plays a vital role in our lives. It allows us to comfortably chew, swallow, and digest. It fights off bacteria in our mouths and elsewhere, and leads the mouth’s bold fight against cavities. Here are 11 facts that might have you reconsidering that unsung hero of bodily fluids: spit.
Saliva consists of about 99 percent water. The other 1 percent is made up of electrolytes and organic substances, including digestive enzymes and small quantities of uric acid, cholesterol, and mucins (the proteins that form mucus).
Healthy individuals accumulate between 2 and 6 cups of spit a day. That’s without stimulation from activities like eating or chewing gum, which open the spit floodgates [PDF].
Your body typically produces the most saliva in the late afternoon, and the least at night. Salivation is controlled by the autonomic nervous system (much like your heartbeat), meaning it’s an unconscious process.
Salivation has five distinct phases, most triggered by the passage of food through the body. Not all of them are a good thing. The first type of salivation is cephalic, the kind that occurs when you see or smell something delicious. The buccal phase is the body’s reflexive response to the actual presence of food in the mouth (which aids in swallowing). The esophageal involves the stimulation of the salivary glands as food moves through the esophagus. The gastric phase happens when something irritates your stomach—like when you’re just about to puke. The intestinal phase is triggered by a food that doesn’t agree with you passing through the upper intestine.
There’s a reason the phrase “lick your wounds” came about. Spit is full of infection-battling white blood cells. And, according to a 2015 study in the journal Blood, neutrophils—a type of white blood cell—are more effective at killing bacteria if they come from saliva than from anywhere else in the body. So adding saliva to a wound gives the body a powerful backup as it fights off infection.
The calcium, fluoride, and phosphate in saliva strengthen your teeth. Spit also fights cavity-causing bacteria, washes away bits of food, and neutralizes plaque acids, reducing tooth decay and cavities. That’s why chewing gum gets dentists’ stamp of approval—chewing increases the flow of saliva, thus protecting your oral health.
Saliva acts like a solvent for tastes, ferrying dissolved deliciousness to the sites of taste receptors. It also keeps those receptors healthy by preventing them from drying out and protecting them from bacterial infection. Many people who have dry mouth (or xerostomia) find their sense of taste affected by their oral cavity’s parched conditions. Because many medications have dry mouth as a side effect, scientists have developed artificial saliva sprays that mimic the lubrication of real spit.
A 10-second kiss involves the transfer of some 80 million bacteria, one study found.
Babies don’t start drooling until they’re 2 to 4 months old. Unfortunately, they also don’t really know what to do with their spit. They don’t have full control of the muscles of their mouth until they’re around 2 years old, so they can’t really swallow it effectively. Which is why we invented bibs.
The body’s fight-or-flight response is designed to give you the energy and strength needed to overcome a near-death experience, like, say, running into a bear or giving a big presentation at work. Your blood pressure goes up, the heart beats faster, and the lungs take in more oxygen. This is not the time to sit around and digest a meal, so the digestion system slows down production, including that of saliva.
In some ancient societies, saliva was used as a basic lie detector. In ancient India, accused liars had to chew grains of rice. If they were telling the truth, they would have enough saliva to spit them back out again. If someone was lying, their mouth would go dry and the rice would stick in their throat.
In 2003, after 13 years of study, international researchers working on the groundbreaking Human Genome Project published their findings. For the very first time, the genetic building blocks that make up humans were mapped out, allowing researchers “to begin to understand the blueprint for building a person,” according to the project's website. Humans are now known to have between 20,000 and 25,000 genes, but researchers still have much to learn about these small segments of DNA. Below, we’ve listed a few facts about gene expression, genetic diseases, and the ways genes make us who we are.
Although “father of genetics” Gregor Mendel conducted his pea plant experiments in the mid-1800s, it wasn’t until 1909 that Danish botanist Wilhelm Johannsen became the first person to describe Mendel's individual units of heredity. He called them genes—derived from pangenesis, the word Charles Darwin used for his now-disproven theory of heredity (among other ideas, Darwin suggested that acquired characteristics could be inherited).
Humans have a lot more in common than we might be inclined to believe. In fact, more than 99 percent of our genes are exactly the same from one person to the next. In other words, the diversity we see within the human population—including traits like eye color, height, and blood type—is due to genetic differences that account for less than 1 percent. More specifically, variations of the same gene, called alleles, are responsible for these differences.
Thanks to a combination of genes, most mammals are able to biologically produce their own Vitamin C in-house, so to speak. But some point throughout the course of human history, we lost the ability to make Vitamin C when one of those genes stopped functioning in humans long ago. “You can see it in our genome. We are missing half the gene,” Dr. Michael Jensen-Seaman, a genetics researcher and associate professor of biological sciences at Duquesne University in Pittsburgh, tells Mental Floss. “Generally speaking, when a species loses a gene during evolution, it’s usually because they don’t need it—and if you don’t use it, you lose it. All our ancestors probably ate so much fruit that there was never any need to make your own Vitamin C.” Jensen-Seaman says humans also lost hundreds of odorant receptors (proteins produced by genes that detect specific smells) because we rely mostly on vision. This explains why our sense of smell is worse than many other species.
A mutation of the aptly named FOXC2 gene gave Hollywood icon Elizabeth Taylor two rows of eyelashes. The technical term for this rare disorder is distichiasis, and while it may seem like a desirable problem to have, there can be complications. According to the American Academy of Ophthalmology, this extra set of lashes is sometimes “fine and well tolerated,” but in other cases they should be removed to prevent eye damage.
Throughout much of the natural world, a class of genes called sperm competition genes are becoming better and better at fertilizing eggs. This is true for various species, including some primates and marine invertebrates. Consider promiscuous primates, like chimpanzees, whose females mate with multiple males in a short period of time. As a result, the males are competing at the genetic level—via their sperm—to father offspring. “What’s happening, we think, is there’s sort of an arms race among genes that are involved in either sperm production or any aspect of male reproduction,” Jensen-Seaman says. Essentially, the proteins in these genes are changing to help males rise to the occasion.
In a 2018 study published in Cell Reports, researchers from the University of Chicago found that a copy of a cancer-suppressing gene that was previously “dead” (or non-functioning) in elephants turned back on at some point. They don’t know why or how it happened, but this reanimated “zombie gene” might explain why elephants have such low rates of cancer—just 5 percent die from the disease, compared to 11 to 25 percent of humans. Some have suggested that a drug could theoretically be created to mimic the function of this gene in order to treat cancer in humans.
Cephalopods like squids, cuttlefish, and octopuses are incredibly intelligent and wily creatures—so much so that they can rewrite the genetic information in their neurons. Instead of one gene coding for one protein, which is normally the case, a process called recoding lets one octopus gene produce multiple proteins. Scientists discovered that this process helps some Antarctic species “keep their nerves firing in frigid waters,” The Washington Post notes.
After a botched experiment in The Fly, Jeff Goldblum morphs into a fly-like creature. Surprisingly, that premise might, uh, fly—at least on some genetic level. Although different researchers come up with different estimates, humans share about 52 percent of the same genes with fruit flies, and scientists figure that the number is roughly the same for house flies.
So, could Jeff Goldblum theoretically turn into a human-fly hybrid if his genes got mixed up with the insect's in a futuristic teleportation device? Not exactly, but there are some scientific parallels. “With genetic engineering, we can select genes and insert them into other organisms’ genomes,” DNA researcher Erica Zahnle tells the Chicago Tribune. “We do it all the time. Right now there’s a hybrid of a tomato that has a fish gene in it.”
Despite advances in medicine, there might be a biological cap on how long humans can stick around. Several studies have suggested that we’ve already peaked, with the maximum extent for human life falling between 115 and 125 years. According to this theory, cells can only replicate so many times, and they often become damaged with age. Even if we’re able to modify our genes via gene therapy, we probably can’t modify them fast enough to make much of a difference, Judith Campisi from the Buck Institute for Research on Aging tells The Atlantic.
“For such reasons, it is meaningless to claim that most human will live for 200–500 years in the near future, thanks to medical or scientific progress, or that ‘within 15 years, we'll be adding more than a year every year to our remaining life expectancy,’” the authors of a 2017 study write in Frontiers in Physiology, citing previous studies from 2003 and 2010, respectively. “Raising false hopes without taking into account that human beings are already extremely ‘optimized’ for lifespan seems inappropriate.”
Forget what you may have learned about earlobes and genetics in middle school. While your genes probably play some role in determining whether you have attached earlobes (a supposedly dominant trait) or unattached earlobes, the idea that this trait is controlled by a single gene is simply untrue. On top of that, earlobes don’t even fall into two distinct categories. There’s also a third, which University of Delaware associate professor John H. McDonald calls intermediate earlobes. "It doesn't look to me as if there are just two categories; instead, there is continuous variation in the height of the attachment point," McDonald writes on his website. A better example of a trait controlled by a single gene is blood type. Whether you have an A, B, or O blood type is determined by three variations—or alleles—of one gene, according to Jensen-Seaman.
Every now and then, new studies will come out that seem to suggest a genetic source for various personality traits, preferences, or talents. In 2015, there was talk of a “wanderlust gene” that inspires certain people to travel, and several other reports have suggested musical aptitude is also inherited. However, like many things in science, the reality isn’t so simple. “Part of the problem is that when we’re in school, we learn examples of traits that are controlled by a single gene, like Mendel’s peas, and we start to think that all variation is determined by a single gene,” Jensen-Seaman says. “But other than a variety of rare genetic diseases, most of the interesting things in medicine, or in human behavior or human variation, are what we call complex traits.” These complex traits typically involve hundreds—if not thousands—of genes, as well as the environmental factors you’re exposed to throughout your life.
Much like your talents and personality, intelligence is also a complex trait that's difficult to measure because it’s influenced by many different genes. One 2017 study identified 52 genes associated with higher or lower intelligence, but the predictive power of those genes—or ability to tell how smart you are—is less than 5 percent. Another study from 2018 identified 538 genes associated with intelligence, which have a 7 percent predictive power. Put simply, no DNA testing kit can accurately predict whether you're a genius or dunce, even if the company claims it can. And, even if scientists make improvements in this field of study, DNA tests can't account for the environmental factors that also influence intelligence.
Do you recoil from the scent of your urine after eating asparagus? If so, you’re among the nearly 40 percent of people who are able to detect the smell of metabolized asparagus in pee, according to a study of nearly 7000 people of European-American descent that was published in The BMJ's 2016 Christmas issue. (The BMJ has an annual tradition of publishing strange and light-hearted studies around this time of year, and the asparagus pee study is no exception.) Again, there isn’t one gene in particular to pin the blame on, though. Multiple olfactory receptor genes—and 871 sequence variations on said genes—are involved in determining whether you have a talent for sniffing out asparagus pee.