Infant age at egg introduction and malnutrition-related child growth in the United States.

To evaluate the relationship between infant age of egg introduction and malnutrition-related growth outcomes in the United States, we analysed secondary data of 1716 mother-child dyads in the Infant Feeding Practices Study II and its Year 6 Follow-Up Study. Malnutrition-related growth outcomes included body mass index z-score (BMIZ), obesity (weight-for-height z-score [WHZ] ≥3 or BMIZ ≥ 2), WHZ, wasting (WHZ < -2), height-for-age z-score (HAZ), and stunting (HAZ < -2). Infant age at egg introduction was analysed as a continuous variable. We used generalised estimating equations to estimate the mean difference in continuous outcomes and relative risk [RR]) for binary outcomes, adjusting for related maternal and child confounders. We also explored interactions with child sex, maternal race/ethnicity, maternal educational level, ever breastfeeding, and formula feeding. In the total sample, a later infant age at egg introduction was associated with a lower mean difference in HAZ (confounder-adjusted mean difference = -0.08, 95% confidence interval [CI]: -0.12 to -0.03 per month) and a higher risk of stunting (confounder-adjusted RR = 1.17, 95% CI: 1.03-1.33 per month) at 6 years. The associations between infant age at egg introduction and 12-month growth outcomes differed by child sex. Among females but not among males, later introduction of eggs was associated with a lower mean WHZ (-0.06 [-0.12 to 0.00] per month) at 12 months. Later egg introduction during infancy was associated with a lower mean HAZ and a higher risk of stunting in 6-year-old children. Besides this, it was associated with a lower WHZ among females at 12 months.