Early infant feeding and weight gain can impact the development of chronic diseases. 1,5 anhydroglucitol (1,5-AG) is a measure of hyperglycemic excursions (HEs) and decreases during HEs due to increased urinary excretion. We hypothesized that HEs as measured by 1,5-AG are associated with infant rapid weight gain (RWG). Infants from the Maternal and Infant Nutrition Trial, a randomized controlled feeding trial, participated in this exploratory secondary analysis. Saliva from 127 infants aged 5 mo was collected before the introduction of complementary foods. Saliva 1,5-AG concentration was measured using liquid chromatography-mass spectrometry and log transformed for statistical analyses. Demographic, anthropometric, and feeding mode [exclusively human milk-fed (EHMF), exclusively formula fed (EFF), and mixed fed (MF)] data were collected. RWG was classified as an increase in the weight-for-age z-score (WAZ) >0.67 from birth to 5 mo. Saliva 1,5-AG concentrations were negatively correlated with an increase of WAZ at 5 mo (r = -0.238, P = 0.01). EHMF and MF infants had significantly greater log transformed 1,5-AG μg/dl values (-0.252 ± 0.639 and 0.332 ± 0.912, respectively) than EFF infants (-1.754 ± 0.705, P = 0.001). Birth weight, but not sex, age, or maternal body mass index, was positively associated with WAZ changes. Saliva 1,5-AG concentrations remained inversely associated with the change in WAZ values in adjusted regression models when controlling for birth weight (β: -0.214; 95% confidence interval: -0.38, -0.05), but not when mode of feeding was included. The odds ratio for 1,5-AG (0.50; 95% confidence interval: 0.28, 0.90) was also significantly associated with RWG using logistic regression controlling for birth weight. These results suggest that human milk-fed infants have fewer HEs than EFF infants. Lower saliva 1,5-AG concentrations were associated with an increase in WAZ and RWG. Additional analyses are needed to evaluate the underlying drivers of HEs and their association with infant growth and health. This study was registered at clinicaltrials.gov as NCT05012930.