1 Breastfeeding and risk of overweight and obesity at 9 years of age Cathal McCrory Richard Layte Economic and Social Research Institute, Dublin

2 Introduction The belief that breastfeeding is protective against disease features prominently in the epidemiological literature. - Gastro-intestinal/diarrhoeal Infections - Respiratory illness and atopic manifestations Somewhat more contentious is whether breastfeeding is protective against pediatric overweight and obesity. This topic has been the subject of numerous reviews and meta-analyses. - Arenz, Ruckerl, Koletzko & Von Kries (2004) - Owen, Martin, Whincup, Davey Smith & Cook (2005) - Harder, Bergmann, Kallischnigg & Plagemann (2005)

3 Arenz and collaborators (2004) Calculated an OR of 0.78 (95% CI: 0.71 – 0.85) across 9 studies. However... Of the 7 studies that included a measure of Parental BMI 3 reported a protective effect of breastfeeding – Bergmann et al (2003) – Gillman et al (2001) – Toschke et al (2002) 4 did not. – Hediger et al (2001) – Li et al (2003) – Poulton & Williams (2001) – O’Callaghan et al (1997)

4 Owen and collaborators (2005) Pooled OR across 28 studies was markedly reduced when adjusted for socio-economic status, parental BMI and maternal smoking Decreasing from 0.86 (95% CI: ) to 0.93 (95% CI: 0.88 – 0.99) - but remaining significant.

5 Harder et al (2006) 1. Included only studies where 95% CI reported 2. Where duration of breastfeeding was measured 3. Included an exclusively breastfed reference group They noted a clear temporal ordering in the data – 4% reduction in risk for each month of breastfeeding. Of the 17 studies – only 5 included a measure of parental BMI. Notably four of these found no effect of breastfeeding after taking account of confounding variables.

6 Rationale Failure to adjust for Parental BMI is a major methodological shortcoming. Parental BMI is amongst the strongest determinants of childhood overweight reflecting the confluence of shared genes and shared environment. (e.g. Danielzik et al, 2002; Li et al, 2009) Women who are obese are less likely to breastfeed (Amir & Donath, 2007). Relationship may be the result of residual confounding.

7 Sample 8,568 nine-year old children participating in the Growing Up in Ireland Project. Selected through the school system using a Probability Proportionate to Size (PPS) sampling method with schools serving as the primary sampling units (PSU’s) schools from the national total of 3,200 primary schools were selected for inclusion. 82% response rate at the school level and 57% at the household level (i.e. eligible child selected within the school) The data were weighted to be nationally representative

8 Breastfeeding Measure Information on initiation and duration were obtained retrospectively at nine years of age via parental report. –Strong concurrence for initiation (85%) when parent report was compared with clinic reports 15 years later (Tienboon et al, 1994). –Less agreement for duration with only 37% of the sample recalling to within one month accuracy. 6-level ordered categorical variable representing variable durations of breastfeeding exposure.

9 Outcome Variable Body Mass Index (BMI) Has been shown to correlate strongly with measures of body fat obtained using direct physiological assessment (Lindsay et al, 2001). – Height was measured with a Leicester portable height stick – Weight was measured with a SECA 761 medically approved scales We use the International Obesity Task Force (IOTF) cut-offs for children aged 9.5 years of age.

10 Missing Cases Valid BMI measurements obtained for 8,091 children (94.4% of the sample) HH income missing for 626 cases Multiple imputation by chained equations (Royston) used Final case base = 7,798

11 Percentage of children who were overweight or obese by duration of breastfeeding

12 Mean BMI and the crude odds of being overweight or obese by duration of breastfeeding exposure DurationnBMI (SD) OverweightObesity Weighted % OR (95% CI)Weighted % OR (95% CI) Never breastfed (3.33) Breastfed 4 weeks or less (3.06) ( ) ( ) Breastfed 5-8 weeks (2.86) ( ) ( )* Breastfed 9-12 weeks (2.65) ( ) ( )** Breastfed weeks (2.73) ( ) ( )*** Breastfed 26 wks (2.77) ( ) ( )*** Reference category for the categorical outcome variable: Non-Overweight * significant at the 0.05 level; ** significant at the 0.01 level; *** significant at the level

13 Proportion of children overweight/obese and proportion breastfed by parental weight status

14 Confounding Variables 6 Category CSO Social Class Measure Net Household Income using Modified OECD equivalence scale – Income quintiles Mother’s highest educational level (four groups) Mother’s Primary Economic Status Maternal age (4 groups) and Nationality Prenatal smoking status Child sex, birth-weight, gestational age Television screen time, Frequency of hard exercise, Childhood dietary quality (FFQ) Parental BMI (Primary & Secondary Caregiver)

15 The odds that a child will be obese at 9 years of age by duration of breastfeeding in the crude and multivariate adjusted model Adjusted for: Child’s Gender, Birth-weight, Gestational age, Nationality, Prenatal Smoking, TV Screen Time, Freq. Hard Exercise, Dietary Quality, Household Social Class, Household Income, PCG Education, PCG Employment Status Parental BMI.

16 Discussion Being breastfed in excess of 3 months was associated with significantly reduced odds of being obese at 9 years of age. Dose-response relationship evident BF weeks (38% reduction in risk) BF 26 weeks+ (52% reduction in risk) What might explain the protective effect? Differences in composition of human breast-milk compared with infant formula 1. Energy Density % higher in infant formula (e.g. Heinig et al, 1993) 2. Protein Density – higher in infant formula compared with breast-milk 3. Bioactive Compounds – e.g. leptin or ghrelin (Lawrence, 2010) 4. Different behavioural patterns (Bartok & Ventura, 2009)

17 Conclusions This study adds to the body of evidence which finds that breastfeeding is protective against obesity in middle childhood. But...temporal stability? Strengths Ability to control for a large range of confounding variables (including parental BMI). The large and representative nature of the sample. Limitations Data was observational and cross-sectional in nature. No indication of timing of transition to solid foods.