59 Age of introduction to cow milk and childhood growth

BACKGROUND: Considerable debate exists around what age children should start consuming cow milk. The Canadian Paediatric Society recommends that cow milk be introduced between 9 and 12 months of age while American, Australian and UK guidelines recommend waiting until 12 months. Children who consume cow milk tend to be taller than those who do not, so earlier introduction may positively impact child height. However, later introduction may decrease weight gain and lower the risk of child obesity. OBJECTIVES: The objectives of the study were to evaluate the association between the age of introduction to cow milk and child height (primary objective) and adiposity (secondary objective). DESIGN/METHODS: A prospective cohort study was conducted through a large practice-based research network. The primary exposure was child age at introduction to cow milk measured using a parent-completed standardized questionnaire. The primary outcome was height-for-age z-score and the secondary outcome was body mass index z-score (BMIz) measured between 3 and 5 years of age. Multiple linear regression was used to evaluate the associations. RESULTS: A total of 1864 children were included in the study (54% male). On average, cow milk was introduced at 11.8 months (SD = 2.2) and almost all children (94.2%) had been introduced to cow milk by 2 years of age. Mean age at follow-up was 46.9 months (SD = 7.5). In the primary analysis, earlier introduction to cow milk was associated with taller children (p< 0.001). Each month earlier that cow milk was introduced was associated with 0.04 higher height-for-age z-score (95% CI: 0.02–0.06). For example, the height difference between a 4-year-old child introduced to cow milk at 12 months relative to 18 months was 0.25 height-for-age z-score units (95% CI: 013-0.36) or 1.05 cm (95% CI: 0.55–1.54 cm). Exploration of non-linearity using linear splines revealed the association between earlier introduction to cow milk and child height was statistically significant after 10 months of age but not before. In the secondary analysis, there was no association between age of introduction to cow milk and BMIz (p= 0.27). CONCLUSION: Earlier introduction to cow milk was associated with taller children by 3 to 5 years of age without increasing child adiposity. Canadian Paediatric Society recommendations for cow milk introduction as early as 9 months of age appear to be appropriate for optimizing childhood height. Future research is needed to understand the causal relationship between starting cow milk earlier and taller height.