P10. Nitrite and nitrate concentrations and metabolism in breast milk, infant formula, and saliva of term and preterm infants

Background: Nitrate and nitrite are precursors for nitric oxide in an NO-3 -> NO-2 -> NO axis that is important for protection of the adult gastrointestinal tract. Nitrate-reducing bacteria on the dorsal surface of the tongue convert salivary nitrate to nitrite which, when swallowed, is either converted to NO under acidic conditions in the stomach or absorbed where it contributes to circulating nitrite concentrations. Studies in adult humans and animal models suggest that NO in the gastrointestinal tract augments blood flow and mucus production and attenuates bacterial growth. Evidence also suggests that circulating nitrite improves exercise performance and protects against ischemic injury. The existence of the NO-3 -> NO-2 -> NO axis in newborn infants, a patient population particularly prone to gastrointestinal and ischemic disease, has not been characterized. These experiments were carried out to find whether nitrate/nitrite ingestion is less in infants compared to adults. Methods: To quantify dietary intake of nitrite of preterm and term infants we measured nitrite and nitrate levels in fresh and frozen-stored human breast milk and infant formula. We also measured nitrite and nitrate levels in saliva collected from term and preterm infants who were patients in the neonatal intensive care unit and from healthy term infants prior to discharge from the well-baby nursery. In experimental studies, we measured oral bacterial reductase activity in the saliva of newborns and adults . Saliva was collected by swabbing the back of the tongue with a cotton-tipped applicator. We then immediately placed the tip in 3.0 mL of sterile, anaerobic broth at 37 degreeC. To provide additional substrate, we added 333 lM nitrate to the broth and then measured the increase in nitrite concentrations at five minute intervals over 30 min. Nitrite and nitrate concentrations were measured by triiodide chemiluminescence. Results: Nitrite in milk of mothers of term infants was 0.13 +/- 0.02 lMand 61.5% less in milk of preterm infants (P < 0.05, ANOVA). Nitrite in several brands of infant formula ranged from less than to higher than breast milk. Nitrite in breast milk decreased 31% with freeze-thawing storage. Infants produced relatively little saliva and infant salivary nitrite levels were 33% of adult levels. Nitrate-reducing activity provided by oral bacterial flora of newborn infants in the first week of life was only ~1.3% of the activity in adults. Nitrite production was significantly slower in all saliva samples taken from infants, regardless of antibiotic treatment, than in samples taken from adults. Conclusions: We conclude that relatively little nitrite reaches the infant GI tract due to low nitrite concentrations in breast milk, relatively low volume of saliva production, and markedly less oral nitrate reductase activity. Given the importance of the nitrate-nitrite-nitric oxide axis in adult physiology, the acquisition of oral nitratereducing bacteria in infants may be a significant factor in the course of infant development.