Cesarean birth and the growth of Yucatec Maya and Toba/Qom children.

OBJECTIVE: Cesarean delivery is often epidemiologically associated with childhood obesity. However, little attention is paid to post-birth modulatory environments, and most studies are conducted in settings where obesity arises for a number of reasons in addition to birth mode. We therefore assess population differences in the relationship between birth mode and childhood growth using data from rural and peri-urban Latin American indigenous populations, and test predictions developed using life history theory. METHODS: Child height and weight were measured monthly in 80 Yucatec Maya and 58 Toba/Qom children aged 1-48 months (2007-2014, 3812 observations). Random-effects linear mixed models were used to compare children's growth by population, sex, and birth mode, accounting for potential confounders. RESULTS: Cesarean delivery rates were 47% (Toba/Qom) and 20% (Yucatec Maya). Childhood obesity and overweight rates were low in both populations. Cesarean-delivered children had significantly greater weight gain (but similar height grain) compared to vaginally-delivered children. By age 4, cesarean delivered Yucatec Maya girls and boys, and Toba/Qom boys (not girls), had significantly higher weight-for-age compared to vaginally-delivered children from their own sex and population. CONCLUSIONS: This provides one of the first attempts to document differences in children's growth patterns according to mode of birth in modernizing indigenous populations. Cesarean delivery is associated with young children's growth patterns, even in the absence of many obesity-inducing factors. There are also population, age, and sex differences in the relationship between birth mode and childhood weight trajectories that warrant future investigation.

Birth mode, breastfeeding and childhood infectious morbidity in the Yucatec Maya.

OBJECTIVES: Cesarean delivery is linked to breastfeeding complications and child morbidity. These outcomes may disproportionately affect Latin American indigenous populations that are experiencing rising cesarean delivery rates, but often inhabit environments that exacerbate postnatal morbidity risks. We therefore assess relationships between birth mode, infant feeding practices, and childhood infectious morbidity in a modernizing Yucatec Maya community, where prolonged breastfeeding is the norm. We predicted that under these conditions, cesarean delivery would increase risk of childhood infectious morbidity, but prolonged breastfeeding postcesarean would mitigate morbidity risk. METHODS: Using a longitudinal child health dataset (n = 88 children aged 0-60 months, 24% cesarean-delivered, 2290 observations total), we compare gastrointestinal infectious (GI) and respiratory infectious (RI) morbidity rates by birth mode. We model associations between cesarean delivery and breastfeeding duration, formula feeding and child nutritional status, then model GI and RI as a function of birth mode, child age, and feeding practices. RESULTS: Cesarean delivery was associated with longer breastfeeding durations and higher child weight-for-age, but not with formula feeding, GI, or RI. Adolescent motherhood and RI were risk factors for GI; formula feeding and GI were risk factors for RI. Regional housing materials protected against GI; breastfeeding protected against RI and mitigated the effect of formula feeding. CONCLUSIONS: We find no direct link between birth mode and child infectious morbidity. Yucatec Maya mothers practice prolonged breastfeeding, especially postcesarean, and in conjunction with formula feeding. This practice protects against childhood RI, but not GI, perhaps because GI is more susceptible to maternal and household factors.

Birth mode and infectious morbidity risks in Qom children of Argentina.

OBJECTIVES: Cesarean delivery may increase childhood infectious morbidity risks via altered birth exposures and subsequent immune, microbial, and epigenetic development. Many Latin American indigenous populations experience dual burdens of infectious and chronic diseases, and are particularly vulnerable to rising rates of cesarean delivery and associated adverse outcomes. The Qom/Toba are an indigenous population in Argentina experiencing rapid lifestyle transitions. We hypothesized that cesarean delivery would be associated with increased risk of infectious symptoms in Qom children after adjusting for gestational and nutritional factors. METHODS: We conducted a secondary analysis of birth records and monthly anthropometric and illness data collected previously from 90 Qom children (aged 1-55 months). We tested for additive effects of birth mode on risk of gastrointestinal (GI) and respiratory illness (RI) in mixed-effects logistic regression models adjusting for child weight-for-age (WAZ), weaning, and gestational and maternal age. RESULTS: Cesarean deliveries accounted for 46% of births and were associated with maternal age < 20 and ≥ 30 years, gestational age < 39 weeks, and prenatal complications. GI and RI risks were reduced in association with cesarean delivery, greater WAZ, weaning, maternal age ≥ 30 years, and gestational age < 39 weeks. CONCLUSIONS: The relationship between cesarean delivery and reduced infectious risks may reflect statistical confounding with relatively rapid postnatal growth and greater adiposity. Postnatal growth trajectories may be important mediators of long-term morbidity risks associated with cesarean delivery. The frequency of cesarean deliveries among the Qom remains concerning given traditionally high rates of fertility and adolescent pregnancy.

Infant allocare in traditional societies.

Across human societies infants receive care from both their mothers and others. Reproductive cooperation raises two important questions: how does allocare benefit mothers and infants, and why do caretakers help mothers when they could spend their time in other, perhaps more valuable ways? We use behavioral and biological data from three small-scale societies to evaluate 1) how allocare affects a nursing mother's time, 2) whether a mother's birth interval length, surviving fertility and infant weight vary as a function of the childcare help that she receives, and 3) the opportunity cost for helpers to spend time caring for children. Across our hunter-gatherer and agricultural samples we find that on average mothers provide 57% of the direct care that an infant receives and allocaretakers 43% (±20%). Model results show that for every 10% increase in allocare the probability that a mother engages in direct care diminishes by 25%, a potential savings of an estimated 165 kcals per day. While allocare has a significant immediate impact on mother's time, no detectable effect on delayed fitness measures (birth interval and surviving fertility) or on infant weight status was evident. Cross culturally we find that other than mothers, siblings spend the most time caretaking infants, and they do so without compromising the time that they might otherwise spend in play, economic activities or education. The low opportunity cost for children to help offers an alternative explanation why juveniles are common caretakers in many societies, even in the absence of delayed indirect fitness benefits. While we expect specific patterns to vary cross culturally, these results point to the importance of infant allocare and its immediate time benefits for mothers to maintain flexibility in balancing the competing demands to support both older and younger children.

Session 1 discussion: Time allocation across subsistence economies.

This article details the discussion following the first session, entitled "Time Allocation Across Subsistence Economies", of the 7th Biennial Ingestive Behavior Research Center Conference - The Pace of Life and Feeding: Health Implications.

Hunter-gatherer diets and human behavioral evolution.

Human behavior and physiology evolved under conditions vastly different from those which most humans inhabit today. This paper summarizes long-term dietary studies conducted on contemporary hunter-gatherer populations (sometimes referred to as foragers). Selected studies for the most part that use evolutionary theoretical perspectives and data collection methods derived from the academic field of human behavioral ecology, which derives relatively recently from the fields of evolutionary biology, ethology, population biology and ecological anthropology. I demonstrate how this body of research illuminates ancestral patterns of food production, consumption and sharing, infant feeding, and juvenile subsistence contributions in hunter-gatherer economies. Insights from hunter-gatherer studies are then briefly discussed within the context of better-studied human populations that are Westernized, Educated, Industrialized, Rich, and Democratic (WEIRD).

Childhood body mass is positively associated with cesarean birth in Yucatec Maya subsistence farmers.

OBJECTIVE: The epidemiologic link between cesarean birth and childhood obesity is unresolved, partly because most studies come from industrialized settings where many post-birth factors affect the risk for obesity. We take advantage of an unusual ethnographic situation where hospital and cesarean birth modes have recently been introduced among Yucatec Maya subsistence farmers, but young children have had minimal exposure to the nutritional transition. While we expect to find very low rates of childhood obesity, we predict that cesarean-born children will be larger and heavier than vaginally born children. METHODS: Weight and height were collected monthly on 108 children aged 0-5 (3576 observations total). Birth mode and birthweight were collected by maternal interview. Data were analyzed using linear mixed models that compare child growth [Maya population-specific Z-scores for weight-for-age and body mass index-for-age (WAZ and BMIZ)] in cesarean and vaginally born children aged 0-5 years. RESULTS: The cesarean rate was 20%, no children were obese, and 5% were overweight. Cesarean birth was a significant predictor of child WAZ and BMIZ after accounting for maternal effects, child birthweight, and sex. Children who were born by cesarean to mothers with high BMI had the highest WAZ of all children by 5 years of age, and the highest BMIZ of all children at all ages. CONCLUSION: Cesarean-born Maya children had higher BMI than vaginally born children, even in the absence of many known confounding factors that contribute to childhood obesity. Child growth was most sensitive to birth mode when mothers had high BMI.

Sibling Competition & Growth Tradeoffs. Biological vs. Statistical Significance.

Early childhood growth has many downstream effects on future health and reproduction and is an important measure of offspring quality. While a tradeoff between family size and child growth outcomes is theoretically predicted in high-fertility societies, empirical evidence is mixed. This is often attributed to phenotypic variation in parental condition. However, inconsistent study results may also arise because family size confounds the potentially differential effects that older and younger siblings can have on young children's growth. Additionally, inconsistent results might reflect that the biological significance associated with different growth trajectories is poorly understood. This paper addresses these concerns by tracking children's monthly gains in height and weight from weaning to age five in a high fertility Maya community. We predict that: 1) as an aggregate measure family size will not have a major impact on child growth during the post weaning period; 2) competition from young siblings will negatively impact child growth during the post weaning period; 3) however because of their economic value, older siblings will have a negligible effect on young children's growth. Accounting for parental condition, we use linear mixed models to evaluate the effects that family size, younger and older siblings have on children's growth. Congruent with our expectations, it is younger siblings who have the most detrimental effect on children's growth. While we find statistical evidence of a quantity/quality tradeoff effect, the biological significance of these results is negligible in early childhood. Our findings help to resolve why quantity/quality studies have had inconsistent results by showing that sibling competition varies with sibling age composition, not just family size, and that biological significance is distinct from statistical significance.

Birth and breastfeeding dynamics in a modernizing indigenous community.

BACKGROUND: Changes in health care access and birthing practices may pose barriers to optimal breastfeeding in modernizing rural populations. OBJECTIVES: We evaluated temporal and maternal age-related trends in birth and breastfeeding in a modernizing Maya agriculturalist community. We tested 2 hypotheses: (1) home births would be associated with better breastfeeding outcomes than hospital births, and (2) vaginal births would be associated with better breastfeeding outcomes than cesarean births. METHODS: We interviewed 58 Maya mothers (ages 21-85) regarding their births and breastfeeding practices. General linear models were used to evaluate trends in birthing practices and breastfeeding outcomes (timing of breastfeeding initiation, use of infant formula, age of introduction of complementary feeding, and breastfeeding duration). We then compared breastfeeding outcomes by location (home or hospital) and mode of birth (vaginal or cesarean). RESULTS: Timing of breastfeeding initiation and the rate of formula feeding both increased significantly over time. Younger mothers introduced complementary foods earlier, breastfed for shorter durations, and formula fed more than older mothers. Vaginal hospital births were associated with earlier breastfeeding initiation and longer breastfeeding durations than home births. Cesarean births were associated with later breastfeeding initiation, shorter breastfeeding durations, and more formula feeding than vaginal hospital births. CONCLUSION: We have observed temporal and maternal age-related trends toward suboptimal breastfeeding patterns in the Maya community. Contrary to our first hypothesis, hospital births per se were not associated with negative breastfeeding outcomes. In support of our second hypothesis, cesarean versus vaginal births were associated with negative breastfeeding outcomes.

Modernization is associated with intensive breastfeeding patterns in the Bolivian Amazon.

For many traditional, non-industrialized populations, intensive and prolonged breastfeeding buffers infant health against poverty, poor sanitation, and limited health care. Due to novel influences on local economies, values, and beliefs, the traditional and largely beneficial breastfeeding patterns of such populations may be changing to the detriment of infant health. To assess if and why such changes are occurring in a traditional breastfeeding population, we document breastfeeding patterns in the Bolivian Tsimane, a forager-horticulturalist population in the early stages of modernization. Three predictions are developed and tested to evaluate the general hypothesis that modernizing influences encourage less intensive breastfeeding in the Tsimane: 1) Tsimane mothers in regions of higher infant mortality will practice more intensive BF; 2) Tsimane mothers who are located closer to a local market town will practice more intensive BF; and 3) Older Tsimane mothers will practice more intensive BF. Predictions were tested using a series of maternal interviews (from 2003 to 2011, n = 215) and observations of mother-infant dyads (from 2002 to 2007, n = 133). Tsimane breastfeeding patterns were generally intensive: 72% of mothers reported initiating BF within a few hours of birth, mean (±SD) age of CF introduction was 4.1 ± 2.0 months, and mean (±SD) weaning age was 19.2 ± 7.3 months. There was, however, intra-population variation in several dimensions of breastfeeding (initiation, frequency, duration, and complementary feeding). Contrary to our predictions, breastfeeding was most intensive in the most modernized Tsimane villages, and maternal age was not a significant predictor of breastfeeding patterns. Regional differences accounted for variation in most dimensions of breastfeeding (initiation, frequency, and complementary feeding). Future research should therefore identify constraints on breastfeeding in the less modernized Tsimane regions, and examine the formation of maternal beliefs regarding infant feeding.

Infant growth and the thymus: data from two South American native societies.

The thymus plays an important role in the development of the immune system, yet little is known about the patterns and sources of variation in postnatal thymic development. The aim of this study is to contribute cross-cultural data on thymus size in infants from two South American native populations, the Tsimane of Bolivia and the Pumé of Venezuela. Thymic ultrasonography was performed and standard anthropometric measures collected from 86 Tsimane and Pumé infants. Patterns of infant growth and thymus size were compared between the two populations and the relationship between nutritional status and thymus size was assessed. Despite nearly identical anthropometric trajectories, Tsimane infants had larger thymuses than Pumé infants at all ages. Population, infant age, and infant mid-upper arm circumference were significant predictors of thymus area in the Tsimane and Pumé infants. This finding reveals a cross-cultural difference in thymus size that is not driven by nutritional status. We suggest that future studies focus on isolating prenatal and postnatal environmental factors underlying cross-cultural variation in thymic development.