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Science in the News

Recent Research Illustrates How Histological Analysis of Deciduous Teeth Elucidates Our Relationship with the Environment

June 19, 2017 Deciduous or primary teeth are remarkably important research subjects, not only due to their resistance to diagenetic modification (i.e., post-mortem degradation and the fossilization process), but because the process of mineralization acts as a daily, unmodifiable record of life changes throughout childhood.1-3 As the crown develops, components of hydroxyapatite crystals are subject to replacement by elements to which they are exposed during mineralization; this daily hardening of the crown results in striations that remain after being shed, and can be analyzed histologically to determine presence of trace elements or isotopes, or stressful periods at each stage. The resilient nature of dental hard tissue means that this record can be ”retrieved” for potentially millions of years2 and can reconstruct dietary lifestyle changes, geographic movements, stressful environmental periods, or exposure to toxicity in an individual’s life.2, 3

A 2016 ADA Science in the News item reported on the use of strontium-90 (Sr-90) to evaluate radiation exposure, both in a cohort of children raised in the U.S. during Cold War nuclear testing and from another cohort of children who were in utero at the time of the 2011 Fukushima Daiichi nuclear accident in Japan.4 Recently, two studies5, 6 (discussed in the following text) utilized laser ablation-inductively coupled plasma-mass spectrometry to determine elemental ratios to reconstruct early life exposures.

This month, researchers from the Icahn School of Medicine at Mount Sinai in New York and the Karolinska Institutet in Sweden published an article that used shed primary teeth from cohorts of twins with and without autism spectrum disorder (ASD) to test the hypothesis that perinatal exposure (i.e., in utero or after birth) to neurotoxic metals (e.g., lead) or deficiency in essential elements (e.g., zinc and manganese) during critical developmental windows are associated with development of ASD.5, 7 Levels of these elements at each incremental stage were examined, and the researchers noted significantly higher levels of lead during the prenatal period and 5 months after birth, as well as lower zinc levels during the third trimester, and consistently low levels of manganese in the twins with ASD. While the authors stated that “caution should be exercised when generalizing our findings, and additional studies are needed,”5 they did suggest that their study corroborates the idea of genetic and environmental interaction in the etiology of ASD.5, 7

In a paper published in May, 2017, researchers investigated changes in barium/calcium (Ba/Ca) ratios of dental enamel from wild orangutans6, 8 hoping to elucidate patterns of variation of weaning times among primates. Humans are thought to have an unusually short nursing period for primates, but until recently, there has been a lack of research into nonhuman primate nursing practices. Usually arboreal (tree-living) and often feeding at night or hidden deep within a forest canopy, it has been difficult for fieldworkers to observe nursing in wild monkeys and apes.6 But using shed primary teeth from museum collections, researchers were able to investigate weaning practices among orangutans. Barium is highly concentrated in mother’s milk and, like lead, follows calcium pathways; nursing infants absorb barium from mother’s milk much more easily than from other dietary sources.1, 6 The researchers found that after an initial drop in barium levels between 16 and 18 months, the Ba/Ca ratio varied according to annual cycles that correlated with seasonal limitations on availability of fruits and other foods. This suggests that during lean times, the growing orangutans supplement their diet with mother’s milk, extending the weaning period well into childhood, sometimes as long as into the 9th year.6 This finding corresponds to other research on primates in marginal environments, and sheds light on the causes of the shortened weaning time of modern and archaic humans. A 2013 study of a juvenile Neanderthal tooth showed a drop in barium after 7.5 months, falling to prenatal levels at 1.2 years; this is consistent with modern humans, who can wean after 1 year “without serious health effects”1 although the World Health Organization recommends continuing until at least the 2nd year for optimal development.9


  1. Austin C, Smith TM, Bradman A, et al. Barium distributions in teeth reveal early-life dietary transitions in primates. Nature 2013;498(7453):216-19.
  2. Tsutaya T, Yoneda M. Reconstruction of breastfeeding and weaning practices using stable isotope and trace element analyses: A review. Am J Phys Anthropol 2015;156 Suppl 59:2-21.
  3. Humphrey LT, Dean MC, Jeffries TE, Penn M. Unlocking evidence of early diet from tooth enamel. Proc Natl Acad Sci U S A 2008;105(19):6834-9.
  4. ADA Accumulation of a Radioactive Isotope in Children’s Shed Deciduous Teeth Used to Estimate Radiation Exposure from Nuclear Testing and Accidents, Then and Now. American Dental Association 2016.  Accessed June 9, 2017.
  5. Arora M, Reichenberg A, Willfors C, et al. Fetal and postnatal metal dysregulation in autism. Nat Commun 2017;8:15493.
  6. Smith TM, Austin C, Hinde K, Vogel ER, Arora M. Cyclical nursing patterns in wild orangutans. Sci Adv 2017;3(5):e1601517.
  7. ScienceDaily. Exposure to specific toxins and nutrients during late pregnancy and early life correlate with autism risk. 2017.  Accessed June 09, 2017.
  8. Yin S. Nearly a Decade Nursing? Study Pierces Orangutans’ Mother-Child Bond. New York, NY:  2017.  Accessed June 16, 2017/
  9. WHO 10 Facts on Breastfeeding. World Health Organization.  Accessed June 16, 2017.

Prepared by: Center for Scientific Information, ADA Science Institute

About Science in the News

Science in the News is a service by the American Dental Association (ADA) to its members to present current information about science topics in the news. The ADA is a professional association of dentists committed to the public's oral health, ethics, science and professional advancement; leading a unified profession through initiatives in advocacy, education, research and the development of standards. As a science-based organization, the ADA's evaluation of the scientific evidence may change as more information becomes available. Your thoughts would be greatly appreciated.

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