Measurement of Energy Metabolism in Infants

Inclusion Criteria: * Healthy, full-term infant * Aged 1 month (28 days) to 3 months (12 weeks, 6 days) at the first visit * Exclusively fed breast milk and/or infant formula * Willing to accept doubly-labeled water Exclusion Criteria: * Unable to complete 3 clinic visits within about 14 days at Pennington Biomedical Research Center * Born preterm (\< 37 weeks gestation) * Congenital abnormality or disability * Gastric reflux * Acute illness within 7 days of the study (fever, diarrhea) * Use of medications to treat a chronic condition (does not include use of vitamin supplements or PRN medication for flatulence or reflux such as Pepcid)