'A hidden blood pump': How chewing more boosts your brain

While it's widely known that chewing more improves digestion, research suggests it can also boost our brains and even help fend off Alzheimer's.

For once chewing a shallot 722 times before swallowing it, Horace Fletcher was dubbed "The Great Masticator". The American self-taught nutritionist believed food should be chewed "until it is completely liquefied" and it "practically swallows itself". Fletcher even estimated that vigorous chewing could have saved the US economy of the early 20th Century more than half a million dollars a day (roughly $19.5m in today's money), because the average person would have ingested half a pound (227g) less food daily.

Fletcher's doctrine may have been a little extreme, "but in some aspects, he was actually right", says Mats Trulsson, professor in the department of dental health at the Karolinska Institutet in Sweden.

Chewing more can provide a wide range of health benefits, from improving digestion and helping people consume fewer calories to alleviating stressand anxiety and improving cognition by solidifying memory skills and boosting attention span. As there is a correlation between tooth health and Alzheimer's disease and dementia, some experts argue that improving patients' dental health could even help reverse mental ageing.

Like most animals, humans have "had teeth and jaws for millions of years," says evolutionary and ecological biochemist Adam van Casteren at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. But they've gone through many changes throughout prehistory.

The earliest hominins, who lived roughly six to seven million years ago, had teeth similar to those of apes today; especially helpful for eating "lots of large, fleshy fruit" abundant in the forest habitats that our early ancestors lived in, says Van Casteren. But as rainforests gave way to more woodlands, open habitats and even savannah-like ecologies, hominins had to contend with "more mechanically challenging foods", says Van Casteren, such as seeds, nuts, and tubers. So they evolved to favour an increase in molar size, with bigger jaws and faces to house all those teeth, alongside the larger muscles needed to power them.

With the development of tools, food processing, and agriculture, as well as fire to cook food, we also stopped needing such lengthy bouts of mastication, explains Van Casteren. Today, humans spend roughly 35 minutes chewing every day, compared to 4.5 hours for our closest ape relatives, chimpanzees and bonobos and 6.6 hours for gorillas and orangutans.

Despite these evolutionary changes, the purpose of chewing remains the same. "We mammals are such complicated chewers because we want to get as much energy out of our food to power our warm-blooded metabolisms," says Van Casteren.

At the most basic level, chewing breaks food down into small particles and moistens them with saliva so that they can be easily swallowed. "It's the first phase of digestion," says Andries van der Bilt, a pioneer in the field of oral physiology and chewing, who worked as a researcher at the University Medical Center Utrecht in the Netherlands for over three decades.

Not only does chewing increase saliva production and the amount of digestive enzymes like amylase that help break down food, it also triggers the gut and pancreas to secrete juices that will help process food further, too. "If you don't chew, the gut is not prepared to handle food," says Trulsson.

The act of breaking food particles into smaller pieces also increases their surface area, which means digestive juices can act on them more efficiently, says orofacial neuroscientist Abhishek Kumar, who works with Trulsson at the Karolinska Institutet. This is important for gut health. Bigger particles tend to linger in the gut longer, giving microorganisms more time to ferment them. This causes "feelings of bloatedness, fullness, constipation, and other symptoms", Kumar says.

The act of chewing helps release nutrients in food, allowing our bodies to absorb them more effectively. In a 2009 study, for instance, 13 healthy adults were asked to chew a small handful of almonds 10, 25, or 40 times. When researchers collected samples of participants' poo, they discovered that the more people chewed, the less fat they excreted, suggesting that the absorption of energy from the nuts was up to a third higher. (In the early 1900s, in fact, Fletcher believed chewing more helps produce poo of superior quality – "quite dry" and smelling of a "hot biscuit".)

What's more, chewing 40 times left participants feeling fuller for longer. A separate 2013 study echoed this satiety link: when 21 participants chewed a chicken-nugget sized slice of pizza either 15 or 40 times before swallowing, those in the latter group experienced a significant reduction in hunger. They also had higher levels of CCK and GIP, two hormones that coordinate digestion in the gut, alongside suppressed levels of the "hunger hormone" ghrelin.

Chewing more means you're also likely to consume less food, according to two separatemeta-analyses that reviewed nearly 50 studies.

That's because it takes around 20 minutes for the body to adjust its production of hunger-related hormones and send signals to the brain that you're full – and chewing buys you more time. It's one reason why so many dieticians and doctors advocate slow and mindful eating over wolfing down a meal, especially if you're trying to shed some pounds. A survey of 92 children in Brazil found that those who were obese "performed fewer mastication sequences and ate faster" compared to children of normal weight. 

One good way of slowing down your eating rate, in fact, is to eat more textured foods. Manystudies recommend choosing solids over liquids (think oranges rather than orange juice), and high-viscosity foods over low-viscosity ones (oatmeal and flaxseeds instead of white rice or pasta).

"The texture of food can affect how full we feel, and therefore potentially help those struggling with obesity to lose weight by reducing their food intake," says Kumar.

Nutrition and digestion aside, researchers are increasingly uncovering that chewing plays an important role in other aspects of our wellbeing – especially brain health – as we get older. "There is growing interest in the 'bite–brain axis,' which proposes that mastication is directly linked to brain health," says Kumar. Tooth loss, for instance, has also been linked with a higher risk of Alzheimer's disease and dementia. 

Memory is affected, too. In a survey involving more than 28,500 people older than 50 across 14 European countries, participants with good chewing ability or those without dentures performed better on a battery of cognitive tests. They demonstrated significantly better word recall, verbal fluency, and numeracy skills than those with chewing problems. In one study of 273 healthy people between the ages of 55 and 80, scientists found that those who retained a greater number of their natural teeth had better semantic memory (linked to knowledge and facts of the world) and long-term memory. 

But why does chomping ability have anything to do with memory? Some researchers point to the multiple neural circuits connecting our chewing apparatus to the hippocampus – the region of the brain responsible for spatial learning and making new memories, which is one of the first to be damaged byAlzheimer's. Others suggest that chewing, especially moderately hard substances, may increase blood flow to the brain, as Japanese researchers demonstrated in experiments with gum chewers. "The theory is that chewing works like a pump, pumping blood to the brain," explains Trulsson. This keeps the brain sharp and working well, he says. 

To determine whether poor chewing ability can actually cause cognitive decline, and whether rehabilitation is possible, Trulsson's team is currently running an experiment replacing patients' missing teeth with implants and then studying their brain function before and up to a year after the procedure. MRI brain scans will also be used to examine if white matter lesions, a marker of poor brain vascular health, shrink with treatment.

"Wouldn't it be really cool if you can rehabilitate the brain by rehabilitating the dentition?" says Trulsson, who has recruited over 80 patients for his trial so far.

In some instances, chewing has also been found to improve concentration in the general population. One meta-analysis, comprising 21 studies, detected a weak but statistically significant improvement in attention levels of gum-chewing participants compared to non-chewers during some cognitively demanding tasks. (This research was funded by gum manufacturer Mars Wrigley, suggesting a potential conflict of interest.)

In an unrelated study of 80 participants, chewing improved alertness levels by 10% during a series of cognitive tasks. Gum-chewers also performed better on an intelligence test.

Scientists "don't really know exactly how it works," but the link between chewing and heightened attention is fairly strong, says Trulsson. There's a caveat, though: "The effect will probably not last for more than 15 to 20 minutes", although researchers aren't sure why.

Another experiment – in young adults who were asked to perform four computerised tasks simultaneously – also found significantly better alertness (nearly 20% higher) in gum chewers. Interestingly, this was accompanied by reductions in self-reported anxiety, stress and salivary cortisol levels (a common biomarker of stress).

Outside the lab, chewing is a good stress-reliever too. When a group of Turkish researchers studied 100 nursing students preparing for mid-term exams, they found that students who chewed gum for at least 30 minutes daily experienced lower levels of stress, anxiety and depression. This was regardless of whether they began chewing gum 15 days or two days before their exams.

For two separate groups of women undergoing elective gynaecological surgery in Korea, chewing on gum helped alleviate their pre-operative anxiety. It also had this effect on 73 Turkish children who were having an intravenous cannula inserted.

Chewing seems to be a natural reflex in stressful times, says Jianshe Chen, an oral processing researcher at Singapore's Agency for Science, Technology and Research. "When some people are stressed, they start to chew unconsciously." Teeth grinding or bruxism, which uses the same jaw muscles as chewing and affects roughly one in 10 adults, is commonly triggered by stress and anxiety.

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But the scientific data here is more contentious. Chen, for instance, says the evidence linking chewing to a calmer state of mind is "scattered". We're "still short of systematic studies" that confidently point to a strong association, he says. Another study, led by the same Korean researcher above, for instance, found that gum-chewing did little to alleviate the anxiety pregnant women felt as they were wheeled into the operating theatre for an elective caesarean section. It also failed to dent the stress levels of those working on an insolvable word puzzle.

One thing's for sure, though – eating often elevates our mood. And chewing, a crucial part of this process, releases flavours in food, and combined with texture and aroma, makes "your eating experience much richer and more pleasant," says Chen, who also studies the sensory perception of food. So, according to this logic, chewing your food better could boost your mental health, too. But rather than choosing sugary gum, you might want to consider chewing a healthy, textured snack before a stressful task.

Don't overdo it, though. Unlike Fletcher, most experts don't think there is a magic number for chewing. "Chew in a normal way until you feel it's okay to swallow, which will be different for different people," says Van der Bilt. "Just enjoy your food."

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