- Design
- Longitudinal birth cohort study (observational)
- Population
- 5,379 children in Toronto followed to a mean of about 3 years
- Primary outcome
- BMI z-score and overweight/obesity by feeding pattern
- Effect
- 24 months formula vs sustained breastfeeding: zBMI +0.27 (0.14–0.41); overweight OR 1.72 (1.22–2.44)
A Toronto cohort of 5,379 children, followed to about three years on average, examined infant feeding and growth. Each additional month of exclusive or any breastfeeding was associated with a slightly lower BMI z-score and lower odds of overweight, while each month of formula feeding went the other way.
The per-month effects are tiny, but they accumulate: 24 months of formula feeding was associated with a 0.27 higher BMI z-score and 72% higher odds of overweight or obesity (odds ratio 1.72) compared with exclusive breastfeeding to six months and any breastfeeding to 24 months.
This is observational and cannot prove breastfeeding causes leaner growth — families who breastfeed longer differ in many ways. But it adds weight to protecting and supporting breastfeeding, and the authors note that what works is intensive, in-person postnatal support, not leaflets. In India, that argues for strengthening lactation support rather than assuming promotion alone suffices.
- Longer breastfeeding was associated with slightly lower BMI z-score and less overweight.
- Longer formula feeding was associated with the opposite.
- Two years of formula feeding carried 72% higher odds of overweight versus sustained breastfeeding.
- Effective support is intensive and in-person, not information alone.
Why it matters
It links a modifiable infant exposure to later overweight and points to the kind of support that actually changes feeding.
Don't overread it
Observational and with tiny per-month effects — breastfeeding families differ in many ways, so this cannot show breastfeeding itself causes the lower weight.
The statistics, in plain English
A per-month BMI z-score change of 0.01 is negligible alone; it is the cumulative contrast over two years (z-score +0.27, odds ratio 1.72) that is meaningful, and even that is an association open to confounding.
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