Longer exclusive breastfeeding was associated with slightly lower BMI z-score

Design

Longitudinal cohort study, Toronto

Population

5,379 children followed from birth, average 3 years

Primary outcome

BMI z-score; overweight or obesity as secondary

Effect

Per month of exclusive breastfeeding: z-score -0.01 (95% CI -0.02 to 0.00), OR 0.96 (0.94 to 0.98) for overweight/obesity

This Toronto longitudinal cohort followed 5,379 children from birth, on average to 3 years and up to 10. The primary exposure was exclusive breastfeeding; secondary exposures were any breastfeeding and formula feeding. The primary outcome was BMI z-score, with overweight and obesity (z above 1) secondary.

Each extra month of exclusive breastfeeding was associated with a BMI z-score 0.01 lower (95% CI -0.02 to 0.00) and odds of overweight or obesity of 0.96 (0.94 to 0.98). Any breastfeeding gave similar results. Each month of formula feeding was associated with 0.01 higher z-score and odds of 1.02.

Formula feeding for 24 months compared with exclusive breastfeeding to 6 months plus any breastfeeding to 24 months was associated with a z-score 0.27 higher (0.14 to 0.41) and odds of overweight or obesity of 1.72 (1.22 to 2.44).

This is observational. Feeding choice is linked to maternal education, income and weight, and residual confounding is likely. Per-month effects are small.

• Support breastfeeding as a way to promote healthy growth, but present effects as modest associations.
• The 24-month formula versus breastfeeding contrast was larger (z-score 0.27) than per-month effects.
• The authors note that intensive, in-person postnatal support by trained professionals is the type of breastfeeding intervention that works.
• Do not use these data to criticise mothers who formula feed; confounding is likely.

Why it matters

It puts a size on the relationship between infant feeding and later weight.

Don't overread it

An observational cohort cannot separate feeding from family circumstances that also affect growth.

The statistics, in plain English

A z-score difference of 0.01 per month is tiny on its own; it matters only when cumulated over many months (0.27 at 24 months). Odds ratios above 1 for overweight at 1.02 per month compound similarly.

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