- Design
- Prospective primary care cohort
- Population
- 5,379 children in Toronto followed from birth (mean to age 3)
- Primary outcome
- BMI z-score and overweight or obesity
- Effect
- Per month exclusive breastfeeding: OR 0.96 (95% CI 0.94–0.98); 24 months formula: OR 1.72 (1.22–2.44)
A Toronto primary care cohort followed 5,379 children from birth, on average to age three, recording feeding practices and BMI.
Each additional month of exclusive breastfeeding was associated with a BMI z-score 0.01 lower and 4% lower odds of overweight or obesity. Each month of formula feeding had the opposite association. Formula feeding for 24 months, compared with exclusive breastfeeding to six months and any breastfeeding to two years, was associated with 72% higher odds of overweight.
The per-month effect is small, and the cohort cannot separate feeding from the many family factors that go with it, such as income, maternal BMI and later diet. It adds to consistent observational evidence rather than proving causation. The counselling message is unchanged and positive: support breastfeeding without making formula-feeding parents feel they have caused future obesity.
- Support exclusive breastfeeding to six months, with in-person help in the early weeks.
- Continue to encourage any breastfeeding alongside complementary food into the second year.
- Advise on responsive feeding and appropriate volumes for formula-fed infants.
- Avoid blaming parents who formula feed; frame advice around what can still be done.
Why it matters
Early feeding is one of the few modifiable influences on childhood weight that every clinician touches.
Don't overread it
An observational cohort cannot show that breastfeeding itself prevents overweight.
The statistics, in plain English
A change of 0.01 in BMI z-score per month is tiny for an individual child. The 72% higher odds of overweight compares extremes of feeding and is the more noticeable figure, but residual confounding could explain part of it.
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