To determine plasma lactate and beta-hydroxybutyrate (BHB) concentrations of healthy infants in the first 5 days and their relationships with glucose concentrations.

Prospective masked observational study in Hamilton, New Zealand. Term, appropriately grown singletons had heel-prick blood samples, 4 in the first 24 hours then twice daily.

In 67 infants, plasma lactate concentrations were higher in the first 12 hours (median, 20; range, 10-55 mg/dL [median, 2.2 mmol/L; range, 1.1-6.2 mmol/L]), decreasing to 12 mg/dL (range, 7-29 mg/dL [median, 1.4 mmol/L; range, 0.8-3.3 mmol/L]) after 48 hours. Plasma BHB concentrations were low in the first 12 hours (median, 0.9 mg/dL; range, 0.5-5.2 mg/dL [median, 0.1 mmol/L; range, 0.05-0.5 mmol/L]), peaked at 48-72 hours (median, 7.3 mg/dL; range, 1.0-25.0 mg/dL [median, 0.7 mmol/L; range, 0.05-2.4 mmol/L]), and decreased by 96 hours (median, 0.9 mg/dL; range, 0.5-16.7 mg/dL [median, 0.1 mmol/L; range, 0.05-1.6 mmol/L]). Compared with infants with plasma glucose concentrations above the median (median, 67 mg/dL [median, 3.7 mmol/L]), those with lower glucose had lower lactate concentrations in the first 12 hours and higher BHB concentrations between 24 and 96 hours. Lower interstitial glucose concentrations were also associated with higher plasma BHB concentrations, but only if the lower glucose lasted greater than 12 hours. Glucose contributed 72%-84% of the estimated potential adenosine triphosphate throughout the 5 days, with lactate contributing 25% on day 1 and BHB 7% on days 2-3.

Lactate on day 1 and BHB on days 2-4 may contribute to cerebral fuels in healthy infants, but are unlikely to provide neuroprotection during early or acute hypoglycemia.

The Australian and New Zealand Clinical Trials Registry: ACTRN12615000986572.