Breast is Best, But for How Long Testing the association between breastfeeding duration and cognitiv

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Breast is Best, But for How Long? Testing the
association between breastfeeding duration and
cognitive abilityOrla Doyle Lori
TimminsUCD Geary Institute UCD Geary Institute
Prepared for Faculty
of Public Health Medicine Winter Scientific
Meeting, Dec 6th 07
UCD SPHPS
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OPTIMAL DURATION OF BREASTFEEDING (BF)

• WHO (2001) exclusive breastfeeding for 6 months
• AAP (2005) exclusive breastfeeding for 4-8
  months
• Benefits of BF
• Lower rates of disease infection
• e.g. diarrhoea, bacterial meningitis
• Reduced risk of asthma allergies
• Benefits for mother-an earlier return to
• pre-pregnant weight reduced risk of
• breast cancer

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EFFECT OF BREASTFEEDING ON COGNITIVE ABILITY

• BF linked to increases in IQ enhanced cognitive
  ability (Hoefer Hardy, 1929)
• BUT
• Is this a causal relationship?
• Methodological Problems
• Link lessened when control for confounding
  variables (Petryk et al. 2007 Jacobson et
  al. 1999)
• Lack of RCTs
• Duration of BF 1 week or 6 months?

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AIMS OF STUDY

• Questions
• Does duration of BF matter for IQ?
• Is more always better? - few studies test optimal
  duration
• Methods
• Model association between BF duration and
  cognitive ability at age 3 in the UK
• Examine difference between exclusive
  non-exclusive BF
• Estimate a semi-parametric model do not assume
  linear relationship between BF IQ
• Examine impact of confounding factors
• Objective
• Provide a test of the WHO recommendation in terms
  of cognitive ability

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UK Millennium Cohort Study (MCS)

• Longitudinal study of 18,819 children born in UK
  between 2000-2002
• 2 MCS waves to date (9 months 3 yrs old)
• Over-represented areas of high child poverty,
  high concentration of ethnic minorities smaller
  UK countries
• Aim Follow cohort members throughout their
  lives, designed as a multi-purpose,
  multi-disciplinary study
• Wealth of info on child development, parental
  resources and home environment

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COGNITIVE DEVELOPMENT AGE 3 Bracken School
Readiness Assessment

• Assesses basic concept development in children
  which predicts readiness for formal education
• Six of the 11 sub-tests of the Bracken Basic
  Concepts Scale- Revised (BBSC-R) School readiness
  Assessment
• 88 concepts of colors, numbers, letters, sizes,
  shapes were tested
• Scale validated shown to predict future
  educational attainment (Panter 2000)
• Use the normed standardised composite score
  (m106, sd15, min56 max148)

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DURATION OF BREASTFEEDING IN UK

• Never breastfed 32.26
• Ever breastfed 67.73
• Breastfed exclusively lt1 week 20.64
• Breastfed exclusively lt4 months 31.37
• Breastfed exclusively lt6 months 14.67
• Breastfed exclusively gt6 months 1.07

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COGNITIVE ABILITY DURATION OF BF
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STATISTICAL MODEL

• Model the relationship between cognitive ability
  and
• No. of weeks exclusive BF
• No. of weeks non-exclusive BF
• Controlling not controlling for cofounders
• ? What kinds of relationship exists?

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BEYOND LINEARITY

• Most studies assume relationship between BF
  cognitive ability is linear
• y increases/decreases with X, for all values of X
• BUT
• Relationship between y and X may not be the same
  for all values of X
• i.e. Is more BF always better?
• Test if the relationship is specific to different
  values of X
• WHO recommendations suggest the returns to BF may
  fall at some point
• ? Use a more flexible modelling method that does
  not impose linear relationship

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SOLUTION Generalised additive models (GAM)

• GAM allows us to fit X variables (e.g. BF
  duration) with nonparametric (non-linear)
  functional forms
• Does not assume a linear relationship
• Allows flexibility in relationship between BF and
  cognitive ability
• Do not obtain a set of regression coefficients
• Graphical methods use to interpret non-parametric
  components
• Estimate model where we allow BF to behave
  non-parametrically confounders to behave
  linearly (a semi-parametric model)

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COVARIATES

• Child characteristics
• Gender
• Age
• Ethnicity
• Birthweight
• No. of siblings
• Breastfeeding factors
• Duration of BF in weeks
• Duration of exclusive BF in weeks

• Maternal characteristics
• Age at birth
• Age at first birth
• Single at birth
• Work status
• Malaise/depression inventory
• Maternal attachment score
• Smoke when pregnant

• SES characteristics
• Low mom education (defined as having failed
  junior cycle or lt16 yrs)

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GAM (1) - Cognitive Ability Exclusive Breastfeed
ing
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GAM (2) - Cognitive Ability Non
exclusive Breastfeeding
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Summary

• Controlling for confounding factors ?
  impact of BF on ability
• Exclusive BF
• No cofounders 22 weeks 5 months optimal
• Cofounders Initial increase until 10 weeks,
  levels off b/w 10-18, then increases slightly
• Non-Exclusive BF
• No cofounders b/w 25-35 weeks 6-8 months
  optimal
• Cofounders Initial increase until 25 weeks
  (6mnts), levels off b/w 25-35 (6-8 mnths), then
  increases slightly
• Identify a non-linear relationship b/w BF
  cognitive ability

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Conclusion

• BF Cognitive ability a causal relationship?
• Breast milk contains long-chain polyunsaturated
  fatty acids known to be important for brain
  growth development
• BUT
• - May not capture all confounding factors
• ? Caution interpreting a causal relationship
• ? Other methods needed (IV, sib-diff models,
  RCTs)
• Duration of BF matters
• ? WHO guidelines hold for maximal cognitive
  ability