Original article
Association of long-duration breastfeeding and dental caries estimated with marginal structural models

https://doi.org/10.1016/j.annepidem.2014.01.013Get rights and content

Abstract

Purpose

To estimate the association between breastfeeding 24 months or beyond and severe early childhood caries (S-ECC).

Methods

Within a birth cohort (n = 715) from low-income families in Porto Alegre, Brazil, the age 38-month prevalence of S-ECC (≥4 affected tooth surfaces or ≥1 affected maxillary anterior teeth) was compared over breastfeeding duration categories using marginal structural models to account for time-dependent confounding by other feeding habits and child growth. Additional analyses assessed whether daily breastfeeding frequency modified the association of breastfeeding duration and S-ECC. Multiple imputation and censoring weights were used to address incomplete covariate information and missing outcomes, respectively. Confidence intervals (CIs) were estimated using bootstrap resampling.

Results

Breastfeeding 24 months or beyond was associated with the highest adjusted population-average S-ECC prevalence (0.45; 95% CI, 0.36 to 0.54) compared with breastfeeding less than 6 months (0.22; 95% CI, 0.15 to 0.28), 6–11 months (0.38; 95% CI, 0.25 to 0.53), or 12–23 months (0.39; 95% CI, 0.20 to 0.56). High-frequency breastfeeding enhanced the association between long-duration breastfeeding and caries (excess prevalence due to interaction: 0.13; 80% CI, −0.03 to 0.30).

Conclusions

In this population, breastfeeding 24 months or beyond, particularly if frequent, was associated with S-ECC. Dental health should be one consideration, among many, in evaluating health outcomes associated with breastfeeding 24 months or beyond.

Introduction

The World Health Organization (WHO) recommends continued breastfeeding up to age 2 years or beyond [1], and failure to breastfeed is associated with poor health consequences for both mother and child [2], [3]. However, the nature of the relationship between dental caries and the age to which children are breastfed remains uncertain. Caries is among the most common diseases worldwide and often goes untreated, particularly in low-resource settings [4], [5], [6], with negative quality of life implications [7]. Some laboratory models suggest that human milk can cause caries [8], [9], particularly in combination with added sugars [10], whereas some report no demineralization of tooth material by human milk alone [11]. The epidemiologic literature [12] includes studies that support a positive association between long-duration breastfeeding and early childhood caries (ECC) [13], [14], [15], [16] and others that do not [17], [18].

Breastfeeding timing relative to other feeding habits complicates study of breastfeeding duration and ECC. Early breastfeeding cessation might accelerate the introduction of particular foods [19], [20], and the foods consumed early in life likely influence caries development [21], [22], [23]. In turn, early-life food experiences might also influence the duration to which a breastfeeding child continues nursing [19]. Regression modeling is problematic in the presence of such time-dependent confounding, in which a variable (e.g., early-life food experiences) can be part of a causal pathway between an earlier aspect of exposure (e.g., early breastfeeding) and the outcome, whereas simultaneously operating as confounder with respect to a later aspect of exposure (e.g., continued breastfeeding). Marginal structural models (MSMs), in contrast, have been used to make causal inference from observational data in the presence of time-varying covariates [24], [25], [26], [27], [28]. Such techniques are particularly relevant for exposures, such as breastfeeding, that cannot be easily assigned as a randomized intervention.

We aimed to estimate the association between long-duration breastfeeding (≥24 months) and severe-ECC (S-ECC) in a birth cohort of urban, low-income Brazilian children. We hypothesized that long-duration breastfeeding is associated with greater caries occurrence. We secondarily hypothesized that the association between long-duration breastfeeding and S-ECC is stronger if daily breastfeeding episodes are more frequent.

Section snippets

Participants

We followed a birth cohort nested in a cluster-randomized trial in Porto Alegre, Brazil. The community water supply is optimally fluoridated [29], and 52 public health care centers provide primary medical services predominantly to low-income residents. A stratified random sample (n = 20 health centers) was selected from 31 eligible clinics for participation in the original trial of health care worker training [30], [31].

In 2008, 715 of 736 eligible pregnant women with appointments at

Results

Table 1 demonstrates selected characteristics of the study population. The fraction of mothers interviewed at the 6-month assessment to report initiating any breastfeeding was 0.99 (627/633); the fraction who breastfed to 12 months was 0.47 (282/598). Exclusive breastfeeding continued to mean age 2.1 months. Nearly half the children were introduced to commercially prepared infant formula by 6 months (0.49, 309/632), but few children used formula at 12 months (0.03, 18/539). Bottle use and soft

Discussion

In this population of low-income Brazilian families, we estimated an increase in S-ECC prevalence with breastfeeding 24 months or beyond. Although the overall health benefits of breastfeeding are considerable, this work adds evidence that, in some contexts, very extended and frequent breastfeeding might increase caries risk. In addition to exposing teeth to bacterially fermentable milk sugars, prolonged breastfeeding might enhance the fidelity with which caries-causing oral bacteria are

Acknowledgments

The authors thank Drs. Arthur Reingold and Barbara Abrams of the University of California Berkeley for comments on the manuscript, members of the Nutrition Research Group (NUPEN) at the Federal University of Health Sciences of Porto Alegre for participant recruitment, data collection, and data management, and Priscila Humbert Rodrigues of the Universidade Luterana do Brasil for assistance in data collection.

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