Overweight in childhood has been characterized by a distinct profile of fecal short chain fatty acids (SCFA), derived from the gut microbial fermentation of dietary substrates. Overweight children are also less likely to be breastfed as infants. The study objective was to explore associations between fecal SCFA and infant diet at 3–4 months of age, as well as associations between fecal metabolites at 3–4 months of age and overweight status at age 3 years. Metabolic profiling (using nuclear magnetic resonance (NMR)) of fecal samples taken at 3–4 months from a subset of 118 infants enrolled in the Canadian Healthy Infant Longitudinal Study (CHILD) general population cohort was conducted. Infant and maternal characteristics, including breastfeeding status at 3 months of age (none, partial breastfeeding, exclusive breastfeeding), were collected using standardized questionnaires. Anthropometric measurements taken at 3 years of age were used to classify children as normal weight (<85 th centile), at‐risk of overweight (≥85 th centile) or overweight/obese (≥97 th centile) according to BMI‐for‐age z‐scores. Seventy seven (65.3%) infants were classified as normal weight, 31 (26.3%) as at‐risk of overweight and 10 (8.5%) as overweight or obese at 3 years of age. 28.2% were exclusively breastfed, 45.3% were partially breastfed and 26.5% were not breastfed at 3–4 months of age. Exclusively breastfed infants had significantly lower fecal concentrations of total SCFA, acetate, butyrate, propionate, isobutyrate, isovalerate, and valerate compared to non‐breastfed infants and partially breastfed infants (p≤0.002; Mann Whitney U test adjusted for multiple comparisons). Exclusively breastfed infants had significantly higher acetate compared to non‐breastfed and partially breastfed infants when measured as a proportion of total SCFA concentration (p<0.001; Mann Whitney U test). There was a weak positive correlation between total SCFA concentration at 3–4 months of age and BMIz score at age 3 years (Spearman's rho 0.24, p=0.01). No correlation between SCFA and BMIz score was seen following stratification by breastfeeding status. Total SCFA concentrations were significantly higher in children classified as at risk of overweight, overweight or obese at 3 years compared to normal weight children. No significant differences in relative proportions of SCFA according to overweight status were observed. Odds of being at risk, overweight or obese at 3 years were over 3 times higher in infants with total SCFA or acetate concentrations in the 4 th quartile compared to 1 st quartile (OR 3.58 95% CI 1.13 to 11.37 and 3.23 95% CI 1.08 to 9.66 respectively). After adjustment for breastfeeding the effect was still evident but diminished (aOR 3.08 95% CI 0.85 to 11.12 and 2.83 95% CI 0.83 to 9.63 respectively). In preliminary analysis, significant differences in fecal SCFA concentrations were observed according to breastfeeding status at 3–4 months; concentrations of all SCFA were lower in exclusively breastfed infants compared to partially or non‐breastfed infants. Fecal SCFA in early life were predictive of overweight status at 3 years of age, but may be explained to an extent by breastfeeding. Nonetheless, fecal SCFA concentrations during infancy may provide a novel biomarker for childhood overweight risk. Support or Funding Information Canadian Institutes of Health Research (Grant #227312); Women and Children's Health Research Institute
