Exposure to secondhand cigarette smoke during the first days of life can cause changes in tooth enamel mineralization without causing visible changes to the teeth. This was demonstrated in an animal model study conducted by researchers at the Ribeirão Preto School of Dentistry at the University of São Paulo (FORP-USP) in Brazil. The results were published in May in the scientific journal Calcified Tissue International.
The research was conducted in the Department of Pediatric Dentistry and is part of a broader line of studies investigating how early-life adversities affect development. Supported by FAPESP , the study was conducted during Juliana de Lima Gonçalves's doctoral studies. According to Francisco Wanderley Garcia de Paula-Silva , the study advisor, the group has been conducting a series of studies for several years evaluating how cigarette smoke, alcohol, and toxic stress impact the growth, development, tooth formation, and behavior of these animals.
According to the professor, the study is part of the DOHaD (Developmental Origins of Health and Disease) theory. This theory posits that many health problems that arise in adulthood originate as early as the embryonic period or the first years of life. In this context, tooth enamel functions as a kind of biological record.
"Enamel is a very interesting structure because it can function as a biomarker. It forms at that specific point in life and doesn't remodel afterward, unlike bone, which continues to remodel throughout a person's entire life," says Paula-Silva. "So, detecting changes in enamel can provide important information about very early exposures that a person sometimes can't even recall," he explains.
He explains that enamel formation begins during pregnancy, especially for baby teeth. However, most of the mineralization of permanent teeth occurs during childhood. According to the researcher, although different "windows" of development exist depending on the dental group, childhood is a critical period for the formation and maturation of dental enamel.
How the study was conducted
To investigate the possible effects of secondhand smoke on enamel formation, the researchers divided baby rats into two groups. One group was exposed to cigarette smoke for five minutes twice a day from the second through the 28th day of life. The control group remained unexposed. The animals were placed inside an acrylic chamber where smoke was released in a controlled manner. "Rodents are a very interesting model because we can control their exposure. The animals are exposed only to cigarette smoke and nothing else, unlike humans, who are exposed to other environmental factors," he says.
Paula-Silva also notes that rodents' incisors grow continuously, which allows for more dynamic monitoring of enamel formation. "We were able to observe the cells forming the enamel and the enamel itself," he states.
The researchers monitored the animals' weight, body growth, and dental development. They found that the pups exposed to smoke gained less weight than the control group, which is consistent with previous research on the effects of secondhand smoke on weight gain. "Even minimal exposure to cigarette smoke impacted the growth of these animals," says Paula-Silva.
However, when the teeth were examined with the naked eye, no significant differences were apparent between the groups. For this reason, the team decided to conduct microscopic and chemical analyses of the teeth. As a result, the researchers identified an increase in the space between the enamel prisms, which are microscopic structures that organize the tooth's mineral composition. They also observed reduced levels of phosphorus in the enamel of the animals exposed to smoke.
According to the professor, phosphorus is a component of hydroxyapatite, the molecule that gives tooth enamel its hardness. Changes to this composition can affect a tooth's mechanical properties, such as its resistance to wear and fractures. The study also identified an increase in carbon content in the enamel of animals exposed to secondhand smoke. According to Paula-Silva, this finding is noteworthy because tooth enamel is essentially a mineral structure that contains virtually no organic matter.
"Enamel is the hardest structure in the human body precisely because it's highly mineralized. The increase in carbon suggests a higher presence of organic material in the enamel, which isn't considered normal," Paula-Silva explains, noting that this change, combined with the reduction in phosphorus observed in the study, could alter important enamel characteristics, such as strength and hardness.
Nevertheless, the tests revealed no reduction in the initial hardness of the enamel. One hypothesis the professor has to explain this result is that the animals were very young and hadn't used their teeth much since they were weaned at 21 days, and the experiment ended a week later. "They practically didn't use those teeth at all. What we need to understand now is how this enamel will behave with use over time. Will this change in chemical composition impact the tooth's strength in the future? That's still an open question," he comments.
To answer this question, the group has begun a new phase of the research. This time, the animals are being exposed to more cigarette smoke – for about an hour, twice a day. The goal is to determine whether a more intense exposure regimen can produce more pronounced changes in the enamel structure and create defects that more closely resemble those observed in humans. "Perhaps what we're seeing now is a milder manifestation of this problem. With greater exposure, we'll be able to better understand the full extent of these changes," says the researcher.
The scientists are also seeking to understand at what stage of enamel formation cigarette smoke has the greatest influence. Enamel undergoes several developmental stages: first, an organic matrix is deposited, followed by the incorporation of minerals that give the tissue its hardness. According to Paula-Silva, the results suggest that exposure to cigarette smoke primarily interferes with this maturation phase. "We didn't observe changes in enamel thickness, indicating that its formation occurred normally. What we found was a change in chemical composition, suggesting an effect on the mineralization and maturation stages," he explains.
Although the results cannot be directly applied to humans, Paula-Silva believes the study may help shed light on the possible causes of enamel formation defects observed in children. One such defect is molar-incisor hypomineralization, which affects about one in five children, leaving their enamel more fragile and susceptible to fractures and cavities. "Today, we still don't know exactly why these defects occur. When we observe that early exposure to smoke alters the composition and structure of enamel, it suggests that secondhand smoke may be one of the causes involved in this process."
However, the professor emphasizes that this is only a hypothesis because the study does not establish a direct link between secondhand smoke and dental hypomineralization in humans. Nevertheless, he says the findings reinforce concerns about the effects of cigarette smoke exposure during childhood. "Smoking is already recognized as a major risk factor for respiratory diseases and child development. What this study shows is that teeth may also bear the marks of such early exposures."
About São Paulo Research Foundation (FAPESP)
The São Paulo Research Foundation (FAPESP) is a public institution with the mission of supporting scientific research in all fields of knowledge by awarding scholarships, fellowships and grants to investigators linked with higher education and research institutions in the State of São Paulo, Brazil. FAPESP is aware that the very best research can only be done by working with the best researchers internationally. Therefore, it has established partnerships with funding agencies, higher education, private companies, and research organizations in other countries known for the quality of their research and has been encouraging scientists funded by its grants to further develop their international collaboration. You can learn more about FAPESP at www.fapesp.br/en and visit FAPESP news agency at www.agencia.fapesp.br/en to keep updated with the latest scientific breakthroughs FAPESP helps achieve through its many programs, awards and research centers. You may also subscribe to FAPESP news agency at http://agencia.fapesp.br/subscribe
