The Flimsy Evidence for Flossing

While hundreds of flossing studies have been conducted, many are plagued by potential issues of researcher bias, as well as poor experimental design. In the past decade, three systematic reviews sought to navigate these muddy waters.

This article originally appeared in the Newton blog on RealClearScience. You can read the original here


While working as a research fellow at the Primate Research Institute at the University of Kyoto in Japan, French ethologist Jean-Baptiste Leca witnessed a sight that would prompt a toothy smile from any dentist. One day, a 14-year-old female Japanese macaque named Chonpe plucked a string of hair, stretched it taut between her hands, and ran the strand between her teeth. She was flossing.

To a dentist, the moment was undoubtedly as breathlessly epic as a certain scene from Stanley Kubrick's 2001: A Space Odyssey. You might remember it.



But macaques are not dentists. Though Chonpe continues to floss to this day, her fellow group members have not followed her example. But that's okay, because flossing isn't nearly as integral to oral health as your dentist would have you believe.

There are a few things that you can count on during a trip to the dentist. One, your gums will bescrubbed massacred and left in tattered, bloody shreds. Two, your jaw will be sore from holding it rigidly agape for 30 minutes. And three, you'll be relentlessly badgered to floss your teeth. Despite the nagging, only between 10 and 40 percent of Americans stick floss between their molars, canines, and incisors on a regular basis. But according to the gathered scientific evidence, it doesn't seem like abstainers are missing out.  

While hundreds of flossing studies have been conducted, many are plagued by potential issues of researcher bias, as well as poor experimental design. In the past decade, three systematic reviews sought to navigate these muddy waters.

The first, published in 2006, narrowed its focus to the effects of flossing in young children between the ages of four and thirteen. The reviewers found that children who had their teeth flossed by a professional hygienist five days per week for 1.7 years had a 40% decreased risk of cavities. However, those who were trained to floss and carried it out themselves did not enjoy any reduction.

Since daily flossings by trained hygienists aren't available to the general public, the researchers admitted that their findings were limited. Furthermore, the children in the study were found to have poor tooth-brushing habits and a low exposure to fluoride -- the anion of the element fluorine known to prevent cavities. Thus, the authors noted that the evidence was unable to answer the critical question of whether flossing provides tangible benefits if one is already brushing with a fluoride toothpaste.

Two years later, researchers at Inholland University for Applied Sciences in Amsterdam took up this query, reviewing the effects of flossing in addition to brushing on levels of plaque and gingivitis. What did they find? 

"A greater part of the studies did not show a benefit for floss on plaque and clinical parameters of gingivitis," they wrote. "Routine instruction to use floss is not supported by scientific evidence."

A third review, published in 2012, reexamined the issue. Scientists collected and analyzed randomized controlled trials that compared the effects of tooth brushing and flossing with tooth brushing alone. Twelve studies, encompassing nearly 1,100 subjects were deemed suitable. Flossing was found to yield statistically significant reductions in levels of gingivitis and plaque buildup, however the reductions were miniscule, almost to the point of being unnoticeable. And in regard to reducing plaque, the authors deemed the evidence to be "weak" and "very unreliable."

Overall, current scientific knowledge suggests that flossing simply isn't as vital as dentists make it out to be. Mouthwash may actually be a superior replacement

One plausible explanation for the dearth of supporting evidence for flossing is that we're all doing it incorrectly. 
Most flossers simply insert the string in between their teeth, yank out a few bits of food, and assume they're done. They're not. The American Dental Association recommendscurving the floss into a "C" shape against the side of every tooth and firmly, but carefully, motioning it up and down.

Somebody better tell Chonpe!

(Image: Woman Flossing via Shutterstock)

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Researchers hope the technology will further our understanding of the brain, but lawmakers may not be ready for the ethical challenges.

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Surprising Science
  • Researchers at the Yale School of Medicine successfully restored some functions to pig brains that had been dead for hours.
  • They hope the technology will advance our understanding of the brain, potentially developing new treatments for debilitating diseases and disorders.
  • The research raises many ethical questions and puts to the test our current understanding of death.

The image of an undead brain coming back to live again is the stuff of science fiction. Not just any science fiction, specifically B-grade sci fi. What instantly springs to mind is the black-and-white horrors of films like Fiend Without a Face. Bad acting. Plastic monstrosities. Visible strings. And a spinal cord that, for some reason, is also a tentacle?

But like any good science fiction, it's only a matter of time before some manner of it seeps into our reality. This week's Nature published the findings of researchers who managed to restore function to pigs' brains that were clinically dead. At least, what we once thought of as dead.

What's dead may never die, it seems

The researchers did not hail from House Greyjoy — "What is dead may never die" — but came largely from the Yale School of Medicine. They connected 32 pig brains to a system called BrainEx. BrainEx is an artificial perfusion system — that is, a system that takes over the functions normally regulated by the organ. The pigs had been killed four hours earlier at a U.S. Department of Agriculture slaughterhouse; their brains completely removed from the skulls.

BrainEx pumped an experiment solution into the brain that essentially mimic blood flow. It brought oxygen and nutrients to the tissues, giving brain cells the resources to begin many normal functions. The cells began consuming and metabolizing sugars. The brains' immune systems kicked in. Neuron samples could carry an electrical signal. Some brain cells even responded to drugs.

The researchers have managed to keep some brains alive for up to 36 hours, and currently do not know if BrainEx can have sustained the brains longer. "It is conceivable we are just preventing the inevitable, and the brain won't be able to recover," said Nenad Sestan, Yale neuroscientist and the lead researcher.

As a control, other brains received either a fake solution or no solution at all. None revived brain activity and deteriorated as normal.

The researchers hope the technology can enhance our ability to study the brain and its cellular functions. One of the main avenues of such studies would be brain disorders and diseases. This could point the way to developing new of treatments for the likes of brain injuries, Alzheimer's, Huntington's, and neurodegenerative conditions.

"This is an extraordinary and very promising breakthrough for neuroscience. It immediately offers a much better model for studying the human brain, which is extraordinarily important, given the vast amount of human suffering from diseases of the mind [and] brain," Nita Farahany, the bioethicists at the Duke University School of Law who wrote the study's commentary, told National Geographic.

An ethical gray matter

Before anyone gets an Island of Dr. Moreau vibe, it's worth noting that the brains did not approach neural activity anywhere near consciousness.

The BrainEx solution contained chemicals that prevented neurons from firing. To be extra cautious, the researchers also monitored the brains for any such activity and were prepared to administer an anesthetic should they have seen signs of consciousness.

Even so, the research signals a massive debate to come regarding medical ethics and our definition of death.

Most countries define death, clinically speaking, as the irreversible loss of brain or circulatory function. This definition was already at odds with some folk- and value-centric understandings, but where do we go if it becomes possible to reverse clinical death with artificial perfusion?

"This is wild," Jonathan Moreno, a bioethicist at the University of Pennsylvania, told the New York Times. "If ever there was an issue that merited big public deliberation on the ethics of science and medicine, this is one."

One possible consequence involves organ donations. Some European countries require emergency responders to use a process that preserves organs when they cannot resuscitate a person. They continue to pump blood throughout the body, but use a "thoracic aortic occlusion balloon" to prevent that blood from reaching the brain.

The system is already controversial because it raises concerns about what caused the patient's death. But what happens when brain death becomes readily reversible? Stuart Younger, a bioethicist at Case Western Reserve University, told Nature that if BrainEx were to become widely available, it could shrink the pool of eligible donors.

"There's a potential conflict here between the interests of potential donors — who might not even be donors — and people who are waiting for organs," he said.

It will be a while before such experiments go anywhere near human subjects. A more immediate ethical question relates to how such experiments harm animal subjects.

Ethical review boards evaluate research protocols and can reject any that causes undue pain, suffering, or distress. Since dead animals feel no pain, suffer no trauma, they are typically approved as subjects. But how do such boards make a judgement regarding the suffering of a "cellularly active" brain? The distress of a partially alive brain?

The dilemma is unprecedented.

Setting new boundaries

Another science fiction story that comes to mind when discussing this story is, of course, Frankenstein. As Farahany told National Geographic: "It is definitely has [sic] a good science-fiction element to it, and it is restoring cellular function where we previously thought impossible. But to have Frankenstein, you need some degree of consciousness, some 'there' there. [The researchers] did not recover any form of consciousness in this study, and it is still unclear if we ever could. But we are one step closer to that possibility."

She's right. The researchers undertook their research for the betterment of humanity, and we may one day reap some unimaginable medical benefits from it. The ethical questions, however, remain as unsettling as the stories they remind us of.

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