Infants will be fed a mixed meal test (meal challenge test) of human milk from his/her human milk donor (e.g., mother). Since all infants enrolled in the study consume human milk as a primary food source, this meal challenge test represents their standard meal, at a larger size. The meal test is provided inside a metabolic chamber to measure energy expenditure (VCO2 and VO2) before and after the meal. The amount of human milk that the infant is fed is estimated using the age and sex specific equations from the 2023 DRI for infants aged 0-2 years. Infants will be provided 20% of their predicted energy needs, which is equivalent to between 4-6 ounces of food for most infants of this age. The caloric content for human milk is be estimated at 20 kcals/ounce.

Infants will be fed a mixed meal test (meal challenge test) of ready-to-feed infant formula. Since all infants enrolled in the study consume human milk as a primary food source, this meal challenge test represents a meal different from their habitual food intake and is a standard meal that all participants receive. The meal test is provided inside a metabolic chamber to measure energy expenditure (VCO2 and VO2) before and after the meal. The amount of infant formula that the infant is fed is estimated using the age and sex specific equations from the 2023 DRI for infants aged 0-2 years. Infants will be provided 20% of their predicted energy needs, which is equivalent to between 4-6 ounces of food for most infants of this age. The caloric content for the ready-to-feed infant formula is 20 kcals/ounce.

Percent difference from baseline to postprandial energy expenditure

Absolute difference between baseline and postprandial energy expenditure

Number of infants with a thrifty and spendthrift phenotype

Difference in respiratory exchange ratio from baseline to postprandial