Diet or genetics? Why so many NZers have crooked teeth

Thousands of New Zealand children have had tooth extractions by age 12 - often under general anaesthetic and some 118,000 have not been checked by a dentist for as many as two years. Yet at the same time about a third of young adults have had expensive orthodontic treatment to perfect teeth that hardly have a filling. What’s going on? We know why some children today have appalling rates of tooth decay and gum disease: too much sugar, poor oral hygiene and lack of dental care. But why are teeth now so crowded and crooked, and why do so many wisdom teeth fail to come through?

Are these trends genetic, are they the result of our modern diets or is it a matter of complicated nature-nurture interactions? Are there major differences in jaw development and malocclusion - a catch-all term for crooked, crowded or congenitally missing teeth, and jaws that don’t meet as they should - among and within populations around the world? We are, with few exceptions, born with the potential to develop jaws spacious enough for all our teeth, say the authors of Jaws: The Story of a Hidden Epidemic, which explores the reasons behind the “hidden epidemic of oral-facial problems”.

The unusual collaboration between experienced US orthodontist Sandra Kahn and Stanford University population biologist Paul Ehrlich came about over dinner in California. Their book, illustrated with crooked smiles, long narrow faces and weak, disappearing chins, is endorsed by no less a scientist than Jared Diamond. Ehrlich became famous for his 1968 book, The Population Bomb, co- authored with his wife, Dr Anne Ehrlich. It predicted widespread famine and environmental disaster as a result of overpopulation and advocated measures to control the growing numbers, to be led by the US. Jaws is no less controversial, albeit in the smaller echo chamber of orthodontic circles.

The argument of Jaws is neatly summarised in a response written by Ehrlich after a review of the book by three professors of orthodontics, who called it “sensationalist” and said it lacks scientific evidence for some of its claims. “Over the past 10,000 years, there has been a huge, self-administered set of changes in the human environment,” Ehrlich wrote. “People began to reduce the amount of breastfeeding, move to a more liquid diet and move indoors where they are exposed to more allergens.” Over that period, there has also been a decline in human jaw size and a resultant increase in malocclusion and failure of the last molars [wisdom teeth] to erupt.

The evidence, documented in the hundreds of scientific references in Jaws, strongly suggests that reduced chewing pressures and mouth-breathing as a result of softer food and stuffy noses, respectively, disrupt normal jaw development.

In Jaws, Kahn and Ehrlich argue that none of the changes to our mandibles (lower jawbone), tooth alignment and facial shape is genetic; they all result from modern living. In the same online debate, Ehrlich wrote: “Even if there were strong selection favouring small jaws (which seems unlikely, to say the least), there haven’t been enough generations. The idea that migrations have led mobs of big-toothed men to inseminate small-jawed women hardly needs rebuttal. We were very clear in Jaws to state where our conclusions were based on research, clinical results or speculation.” He added, “Those who say we are wrong need to present a coherent argument for another explanation of the epidemic and, to the degree possible, cite the scientific literature to back it up.”

The second main argument in Jaws is that children now live in allergenic environments and are constantly exposed to viruses in the closed confines of child-care centres and schools. As a result, noses are often blocked and mouth-breathing becomes established, leading to receding chins, long, narrow faces and thin upper lips. If the mouth is not kept closed (when not eating or talking), the jaws and palate are not subject to the gentle pressure of the tongue, over time, to keep the oral cavity in shape. Cosmetic considerations aside, Kahn and Ehrlich warn that there are serious potential health problems from disturbed sleep as a result of the tongue blocking the airway.

Sleep apnoea is associated with many non-trivial health problems, including heart disease, high blood pressure and dementia. Road deaths increase and job or school performance drops off because of fatigue. Mouth-breathing also dries up saliva, which reduces protection against tooth decay. More of the food we eat, such as yoghurts and smoothies, bypasses our teeth altogether. Stewed fruits are sold in convenient pouches and squeezed into the mouths of babies; blenders, wands and bullets are doing a lot of the chewing for children and adults. Parents and childcare staff are also worried about giving raw carrot and apple to toddlers following such cases as the tragic choking incident at a Rotorua childcare centre in May 2016 that left an infant with brain damage.

But if we exercised our jaws more by following a paleo-style diet free of processed foods and rich in meat, fish, eggs, seeds, nuts, fruits and vegetables, would we all have strong jaws and perfect smiles? Paleofantasy: What Evolution Really Tells Us about Sex, Diet, and How We Live, by US evolutionary biologist Marlene Zuk, debunks the myths about Stone Age diets and lifestyles and pokes academic fun at the silliness of those who try to replicate them. She cites a study of the dental health of modern humans compared with that of either our fossilised ancestors or modern peoples eating more traditional diets, such as the Maya of Mexico. “People in industrialised societies not only have far more cavities than either the other two groups …. but their jaws are shaped differently, with malocclusion and overcrowding of the teeth,” she wrote.