Longitudinal changes in juvenile and adolescent body mass indices before, during, and after the COVID-19 lockdown in New Zealand.

OBJECTIVE: This study uses longitudinal data from school children in Dunedin, New Zealand, to evaluate impacts of COVID-19 lockdown measures on changes in body mass (BMI, kg/m2 ). Impacts are assessed using two non-mutually exclusive hypotheses. The "structured days" hypothesis holds that children tend to alter sleep patterns, reduce activity and increase snacking when not in structured environments. The bidirectional hypothesis proposes that over-weight or obese children are predisposed to further gains in unstructured settings. METHODS: Juveniles and adolescents (n = 95, 60% female) were recruited from Dunedin schools. Repeated measures analyses assessed variation in intra-individual change in BMI during four periods: P1 (before summer break), P2 (during summer break), P3 (during the COVID-19 lockdown), and P4 (after the lockdown ended). The model also examined if these changes were influenced by participants' sex or body size early in the first period assessed using log-transformed BMI, log-transformed weight, height, or lower leg length. RESULTS: Repeated measures analyses of per month gains in BMI (kg/m2 ) during the four periods revealed consistent period (p ≤ .001), period by sex (p ≤ .010), and period by body size (p ≤ .001) interactions across all four body size proxies. Both sexes experienced the greatest gains during the lockdown (P3), but differed in response to their summer break (P2). CONCLUSION: Results are mostly consistent with the "structured days" hypothesis, but challenge the bidirectional hypothesis as defined. Further research better characterizing risks of gains in adiposity are needed.

Dental biorhythm is associated with adolescent weight gain.

Background: Evidence of a long-period biological rhythm present in mammalian hard tissue relates to species average body mass. Studies have just begun to investigate the role of this biorhythm in human physiology. Methods: The biorhythm is calculated from naturally exfoliated primary molars for 61 adolescents. We determine if the timing relates to longitudinal measures of their weight, height, lower leg length and body mass collected over 14 months between September 2019 to October 2020. We use univariate and multivariate statistical analyses to isolate and identify relationships with the biorhythm. Results: Participants with a faster biorhythm typically weigh less each month and gain significantly less weight and mass over 14-months, relative to those with a slower biorhythm. The biorhythm relates to sex differences in weight gain. Conclusions: We identify a previously unknown factor that associates with the rapid change in body size that accompanies human adolescence. Our findings provide a basis from which to explore novel relationships between the biorhythm and weight-related health risks.

Enamel daily secretion rates of deciduous molars from a global sample of children.

OBJECTIVE: To investigate and describe the variation in enamel daily secretion rates (DSRs) of naturally exfoliated deciduous molars (n = 345) from five modern-day populations (Aotearoa New Zealand, Britain, Canada, France, and Sweden). DESIGN: Each tooth was thin sectioned and examined using a high-powered Olympus BX51 microscope and DP25 digital microscope camera. Mean DSRs were recorded for the inner, mid, and outer regions of cuspal and lateral enamel, excluding enamel nearest the enamel-dentin junction and at the outermost crown surface. RESULTS: Mean DSRs did not vary significantly between populations, or by sex. Cuspal enamel grew slightly faster than lateral enamel (mean difference 0.16 µm per day; p < 0.001). The trajectory of DSRs remained relatively constant from inner to outer cuspal enamel and increased slightly in lateral enamel (p = 0.003). CONCLUSIONS: The DSRs of deciduous molars from modern-day children are remarkably consistent when compared among populations. While growth rates are faster in cuspal than lateral enamel, the trajectory of enamel formation changes only slightly from inner to outer regions. The trajectory of DSRs for deciduous molars differs to that of permanent molar enamel, which typically display a steep increase in matrix deposition from inner to outer enamel.

Technical note: Estimating original crown height in worn mandibular canines using aspects of dentin morphology.

We present a novel method to estimate original crown height (OCH) for worn human mandibular canines using a cubic regression equation based on ratios of worn crown height and exposed dentin. This method may help alleviate issues frequently presented by worn teeth in dental analyses, including those in bioarchaeology. Mandibular canines (n = 28) from modern day New Zealand and English populations were selected. Crown height and dentin thickness were measured on dental thin sections (n = 19) and the resulting (log10) ratios were fitted to a cubic regression curve allowing OCH in worn crowns to be predicted. Variation in the dentin apex position was recorded and effects of angled wear slopes investigated allowing adjusted values to be generated. Our method is trialed for use on intact and sectioned teeth (n = 17). A cubic regression curve best describes the relationship between (log10) ratios and crown height deciles (R2  = 0.996, df1  = 3, df2  = 336, p < 0.001). No significant differences were detected between OCH estimates using our method and digitally recreated cusp outlines of the same crowns (t = 1.024, df = 16, p > 0.05), with a mean absolute error of 0.171 mm and an adjusted coefficient of determination of 0.923. Our approach offers a quantitative method to estimate the percentage of OCH remaining on worn mandibular canines, and by extension, the OCH. Our estimates are comparable to digitally recreated cusps but less subjective and not limited to crowns with minimal wear.

An inconstant biorhythm: The changing pace of Retzius periodicity in human permanent teeth.

OBJECTIVES: Human tooth enamel retains evidence of growth in the form of Retzius lines. The number of daily growth increments between the regularly occurring lines defines their repeat interval, or periodicity. Retzius periodicity is often incorporated into enamel formation times, age-at-death reconstructions, or used to provide a basis from which to explore an underlying biorhythm. Biological anthropologists typically assume that RP remains constant within an individual and does not vary along the tooth-row. Here, we test that assumption. MATERIALS AND METHODS: RP was calculated from n = 223 thin sections of human permanent teeth from individuals of British and southern African origin. Forty individuals provided multiple teeth (n = 102 teeth) and a further 121 individuals each provided a single tooth. RESULTS: We report first evidence that RP of permanent teeth does not always remain constant within an individual. Of those individuals that provided multiple teeth, 42% (n = 17/40) demonstrated a decrease in RP along the tooth row, with most shifting by two or more days (n = 11). Across the entire sample, mean RP of anterior teeth was significantly higher than molars. Mean premolar RP tended to be intermediate between anterior teeth and molars. DISCUSSION: Our data do not support the assumption that RP invariably remains constant within the permanent teeth of an individual. Transferring RP from molars to incisors within an individual can result in a miscalculation of formation time and age-at-death by up to 1 year. Implications for biological anthropologists and the source of the underlying long period biorhythm are discussed.

Social group dynamics predict stress variability among children in a New Zealand classroom.

Previous research proposes stress as a mechanism for linking social environments and biological bodies. In particular, non-human primate studies investigate relationships between cortisol as a measure of stress response and social hierarchies. Because human social structures often include hierarchies of dominance and social status, humans may exhibit similar patterns. Studies of non-human primates, however, have not reached consistent conclusions with respect to relationships between social position and levels of cortisol. While human studies report associations between cortisol and various aspects of social environments, studies that consider social status as a predictor of stress response also report mixed results. Others have argued that perceptions of social status may have different implications for stress response depending upon social context. We propose here that characteristics of children's social networks may be a better predictor of central tendencies and variability of stress response than their perceptions of social status. This is evaluated among 24 children from 9.4 to 11.3 years of age in one upper middle-class New Zealand primary school classroom, assessed through observation within the classroom, self-reports during semi-structured interviews and 221 serial saliva samples provided daily over 10 consecutive school days. A synthetic assessment of the children's networks and peer-relationships was developed prior to saliva-cortisol analysis. We found that greater stability of peer-relationships within groups significantly predicts lower within-group variation in mid-morning cortisol over the two-week period, but not overall within-group differences in mean cortisol.

How much impact do gains in height have on shoulder breadths within Taiwanese families?

OBJECTIVES: This study investigates allometric changes in shoulder breadths relative to changes in stature arising from rapidly changing developmental circumstances within 107 Taiwanese families. It speaks to broader issues related to the extent of phenotypic plasticity of body breadths humans are capable of in response to reductions in developmental stressors. METHODS: An examination of relationships between shoulder breadth and height within individuals in each generation was followed by evaluation of patterns of difference between same-sex parent-offspring pairs in height and shoulder breadth. RESULTS: Height was similarly positively correlated with shoulder breadth within fathers, mothers, sons, and daughters (P ≤ 0.002). Variance accounted for ranged from an adjusted R2 of 0.201 among fathers to 0.151 for sons, with mothers' and daughters' values being 0.187 and 0.181, respectively. Comparisons of differences within families indicate that parents who were shorter than their same-sex offspring also tended to have modestly narrower biacromial breadths (father-son pairs: adjusted R2 = 0.112; t = 2.82, P = .007; mother-daughter pairs: adjusted R2 = 0.135; t = 2.97, P = 0.005). CONCLUSIONS: Taken as a whole, results here support the view that secular changes in stature are not accompanied by similar changes in body breadths, perhaps so that responses to developmental environmental improvements do not alter thermoregulatory equilibria that reflect long-term evolutionary processes. These results indirectly constrain plausible hypotheses about how ancestors of Austronesian speakers altered their body size and shape as they voyaged to Fiji, Western Polynesia, and beyond.

The magnitude of changes in linear growth within Taiwanese families: intrinsic sex-associated biology, socially mediated behaviors, or both?

OBJECTIVE: This study evaluates competing hypotheses about implications of greater female growth canalization for differences in male and female growth as developmental settings improve. PARTICIPANTS AND METHODS: Intergenerational declines in gender bias and rapid economic growth in Taiwan permit assessment of the two hypotheses using anthropometric and background information from 107 Taiwanese parents and their young adult offspring. Differences in parents' values were first evaluated across grandfathers' occupational status categories reflecting good to relatively poor early circumstances in the parental generation. The extent of intergenerational change in heights or knee heights within 56 father-son pairs were then compared with those in 51 statistically independent mother-daughter pairs across the same occupational categories using repeated measures analyses. RESULTS: Change in mean heights and knee heights across grandfathers' occupational categories were noticeably greater for fathers than mothers. Overall, intergenerational gains within families in height and knee height were statistically significant (p < 0.0005) in father-son (3.89 cm; 1.85 cm) and mother-daughter pairs (4.14 cm; 1.67 cm). However, among families where grandfather's occupations were "privileged," father-son pairs are similar in average height (Δ = -0.36 cm) and knee height (Δ = 0.53 cm) while mothers were significantly shorter (Δ = 3.88 cm) with shorter knee heights (Δ = 1.74 cm) than their daughters. DISCUSSION: The hypothesis that females are inherently less capable of responding to growth promoting characteristics of early environments is not supported. Intergenerational declines in male preference combined with rapid but equitable economic growth better account for intergenerational changes reported.

Adoption Does Not Increase the Risk of Mortality among Taiwanese Girls in a Longitudinal Analysis.

Adopted children often experience health and well-being disadvantages compared to biological children remaining in their natal households. The degree of genetic relatedness is thought to mediate the level of parental investment in children, leading to poorer outcomes of biologically unrelated children. We explore whether mortality is related to adoption in a historical Taiwanese population where adoption rarely occurred among kin. Using Cox proportional hazards models in which adoption is included as a time-dependent covariate, we show that adoption of girls does not increase the risk of mortality, as previously suggested; in fact, it is either protective or neutral with respect to mortality. These results suggest that socio-structural variables may produce positive outcomes for adopted children, even compared to biological children who remain in the care of their parents.

Estimating striae of Retzius periodicity nondestructively using partial counts of perikymata.

Accurate age estimations for enamel formation and the timing of enamel hypoplasia have traditionally only been available through histological analyses of dental thin sections, which is a difficult and destructive process. However, an association between striae of Retzius periodicity, crucial for accurate aging, and the total number of striae in imbricational enamel has been reported in the literature. This means periodicity can be estimated nondestructively but is reliant on all perikymata being visible along the crown surface. Therefore, crowns with worn or damaged surfaces may not be able to be assessed, potentially limiting sample sizes. We tested this relationship in a modern New Zealand sample and investigated whether reliable associations might be identified using only partial perikymata counts from the cervical half of the crown. Using mandibular canines (n = 11), the distribution of perikymata per decile was recorded using high definition replica surfaces. Thin sections of the same crowns were used to assess periodicity histologically along with striae of Retzius distributions. A strong correlation between total striae numbers and periodicity was also identified in our sample. Furthermore, we report strong correlations that allow periodicity to be estimated from perikymata counts using only 10% of crown height when certain deciles are used. Based on these findings, we propose a simple matrix that can be developed for nondestructively estimating periodicity based on the range of perikymata counts in the sixth to ninth deciles.

Enamel extension rate patterns in modern human teeth: two approaches designed to establish an integrated comparative context for fossil primates.

Enamel extension rates (EERs), the rates at which ameloblasts differentiate, determine how fast tooth crowns grow in height. Studies of fossil primate (including hominin) enamel microstructure usually focus on species differences in enamel formation time, but they have also begun to address species-level variation in enamel extension rates. To improve our ability to compare EERs among primate species, a better understanding how EERs vary within species is necessary. Using a large and diverse modern human histological sample, we find that initial EERs and patterns of EER change along the enamel-dentine junction (EDJ) vary in relation to EDJ length. We also find that enamel formation time varies in relation to EDJ length, but that it does so independently of initial EERs. These results suggest that EDJ length variation within a species sample can affect interspecific comparisons not only of EERs but also of enamel formation times. Additionally, these results lend within-species support to the hypothesis, based on comparisons among hominin species, that EERs and crown formation times can vary independently (Dean, 2009). In a second approach, we analyzed EER changes specifically in the lateral enamel of two modern human population samples as these changes relate to the distribution of perikymata. As surface manifestations of internal enamel growth increments, perikymata provide a valuable source of information about enamel growth in fossils. We find that EER declines in the lateral enamel are associated with an increase in perikymata density from first to last-formed lateral enamel. Moreover, variation in the extent of EER decline among individuals is associated with variation in the distribution of perikymata along their enamel surfaces. These latter findings suggest that the distribution of perikymata on the enamel surface provides information about rates of EER decline in lateral enamel, at least in modern humans.

Evidence of age-related responses to short-term environmental variation: time series analysis of cross-sectional data from Taiwan, 1969 to 1990.

Cross-sectional heights and weights collected by Taiwan's Ministry of Education since 1964, and reported separately for Taipei and rural townships from 1969 through 1990, were used to test the hypothesis that in Taiwan the degree to which increments of change in height and weight of urban or rural boys and girls tracked each other from year to year was a function of sex and age-related capacities to respond to common environmental forces. Five testable implications follow from this hypothesis. Results were evaluated using detrended time series derived correlations of yearly or near yearly change in sex-specific age group height or weight means within and across regions, cross-sectionally estimated ages of maximum change of height and weight, and evidence of response to two recessions. The hypothesis was largely supported by the available evidence. The similarities in secular change from year to year, with only a few exceptions, suggested that socioeconomic trends indirectly affected the pace of growth of children in both urban and rural Taiwan measured in the spring of the following year. The strength and age-related pattern of these correlations were consistent with sex and age-related capacities to respond to shared environmental factors. Evidence of lags between height and weight, found both in cross-sectionally estimated mean ages of maximum change in height and weight and in the pattern of within sex correlations also argued for the hypothesis. The evidence further suggested that when the economy faltered briefly in Taiwan following worldwide oil crises, children's growth was rapidly affected in an age-dependent manner. This study bolsters the view that secular changes in growth reflect environmental quality. It suggests as well that under some circumstances, environmental change may be rapidly reflected as mean changes in height or weight of certain age groups. The intensity of the response appears to be related to the degree of improvement or deprivation, the maturationally mediated pace of growth, and probably initial energy balance.