Nitrite and nitrate concentrations and metabolism in breast milk, infant formula, and parenteral nutrition

Nitrite in breast milk: roles in neonatal pathophysiology

Dietary nitrate has beneficial effects on health maintenance and prevention of lifestyle-related diseases in adulthood by serving as an alternative source of nitric oxide (NO) through the enterosalivary nitrate-nitrite-NO pathway, particularly when endogenous NO generation is lacking due to vascular endothelial dysfunction. However, this pathway is not developed in the early postnatal period due to a lack of oral commensal nitrate-reducing bacteria and less saliva production than in adults. To compensate for the decrease in nitrite during this period, colostrum contains the highest amount of nitrite compared with transitional, mature, and even artificial milk, suggesting that colostrum plays an important role in tentatively replenishing nitrite, in addition to involving a nutritional aspect, until the enterosalivary nitrate-nitrite-NO pathway is established. Increasing evidence demonstrates that breast milk rich in nitrite can be effective in the prevention of neonatal infections and gastrointestinal diseases such as infantile hypertrophic pyloric stenosis and necrotizing enterocolitis, suggesting that breastfeeding is advantageous for newborns at risk, given the physiological role of nitrite in the early postnatal period. IMPACT: The aim of this review is to discuss the physiological roles of nitrite in breast milk and its implications for neonates. Nitrite in breast milk may compensate for the decrease in nitrite during the early neonatal period until the enterosalivary nitrate-nitrite-nitric oxide pathway is established. Breast milk rich in nitrite may be effective in the prevention of neonatal infections and gastrointestinal diseases by providing nitric oxide bioavailability.

Assessment of nitric oxide metabolites concentrations in plasma, saliva, and breast milk and their relationship in lactating women

Nitric oxide (NO) plays a role in many biological mechanisms. The amounts of physiologically produced NO are associated with the concentrations of its metabolites nitrate and nitrite. This study investigated whether there is any association between the concentrations of NO metabolites nitrate, nitrite, and nitrosylated species (RXNO) in mature breast milk, saliva, and plasma in healthy lactating women (N = 30). We hypothesized that the NO metabolites concentrations in plasma are associated with those found in saliva and in breast milk. NO metabolites concentrations were measured using chemiluminensce-based assays. Nitrate concentrations in breast milk are twice as much as plasma concentrations, whereas nitrate concentrations in saliva are about eightfold higher (both P < 0.001). Similar differences were found when nitrite concentrations were taken into consideration. RXNO concentrations in breast milk were negligible, and RXNO concentrations in saliva were approximately sixfold higher than those found in plasma samples (P < 0.0001). Nitrate concentrations in plasma are associated with nitrate concentrations in saliva (rs = 0.474, P = 0.004). However, no significant association was found between nitrate concentrations in breast milk and in plasma (P > 0.05). Our results show a significant association between nitrate concentrations in plasma with those found in saliva, whereas all other relationships were not significant. In conclusion, this report shows for the first time that the physiological concentrations of NO metabolites in human breast milk are probably independent of circulating NO metabolites concentrations and may depend mostly on endogenous NO synthesis in the breast. These findings may have clinical implications for newborns and lactating women.

Airway nitrite is increased in extremely preterm infants with bronchopulmonary dysplasia

RATIONALE

Bronchopulmonary dysplasia (BPD) is the most common complication of prematurity and significantly contributes to mortality and morbidity with few predictive biomarkers. Given that nitrites have been implicated in pathways associated with lung disease, we hypothesized that nitrite levels would be altered in the airways of premature infants diagnosed with BPD.

METHODS

This was a prospective cohort study of extremely low birth infants (< 28 weeks' gestation) at the University of Alabama at Birmingham. Nitrite levels from tracheal aspirates (TAs) were compared between intubated and ventilated infants with BPD and gestation matched full term (FT) controls. TA derived nitrite levels from day one after birth were also compared between preterm infants who did and did not develop BPD.

RESULTS

Infants with BPD were found to have significantly elevated nitrite levels in their tracheal aspirates compared to gestation matched FT controls (p < 0.05). There was a trend for increased nitrite levels on postnatal day one in infants that developed BPD compared to infants that did not develop BPD (p = 0.05).

CONCLUSIONS

In conclusion, nitrite levels are significantly increased in airways of infants with BPD. Data from a larger cohort are needed to further support the utility of nitrite for BPD prediction.

TRIAL REGISTRATION

Not applicable.

The role of gasotransmitters in neonatal physiology

The gasotransmitters, nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are endogenously-produced volatile molecules that perform signaling functions throughout the body. In biological tissues, these small, lipid-permeable molecules exist in free gaseous form for only seconds or less, and thus they are ideal for paracrine signaling that can be controlled rapidly by changes in their rates of production or consumption. In addition, tissue concentrations of the gasotransmitters are influenced by fluctuations in the level of O₂ and reactive oxygen species (ROS). The normal transition from fetus to newborn involves a several-fold increase in tissue O₂ tensions and ROS, and requires rapid morphological and functional adaptations to the extrauterine environment. This review summarizes the role of gasotransmitters as it pertains to newborn physiology. Particular focus is given to the vasculature, ventilatory, and gastrointestinal systems, each of which uniquely illustrate the function of gasotransmitters in the birth transition and newborn periods. Moreover, given the relative lack of studies on the role that gasotransmitters play in the newborn, particularly that of H₂S and CO, important gaps in knowledge are highlighted throughout the review.

Sodium nitrite augments lung S-nitrosylation and reverses chronic hypoxic pulmonary hypertension in juvenile rats

Deficient nitric oxide (NO) signaling plays a critical role in the pathogenesis of chronic neonatal pulmonary hypertension (PHT). Physiological NO signaling is regulated by S-nitrosothiols (SNOs), which act both as a reservoir for NO and as a reversible modulator of protein function. We have previously reported that therapy with inhaled NO (iNO) increased peroxynitrite-mediated nitration in the juvenile rat lung, although having minimal reversing effects on vascular remodeling. We hypothesized that sodium nitrite (NaNO2) would be superior to iNO in enhancing lung SNOs, thereby contributing to reversal of chronic hypoxic PHT. Rat pups were exposed to air or hypoxia (13% O2) from postnatal days 1 to 21. Dose-response prevention studies were conducted from days 1–21 to determine the optimal dose of NaNO2. Animals then received rescue therapy with daily subcutaneous NaNO2 (20 mg/kg), vehicle, or were continuously exposed to iNO (20 ppm) from days 14–21. Chronic PHT secondary to hypoxia was both prevented and reversed by treatment with NaNO2. Rescue NaNO2 increased lung NO and SNO contents to a greater extent than iNO, without causing nitration. Seven lung SNO proteins upregulated by treatment with NaNO2 were identified by multiplex tandem mass tag spectrometry, one of which was leukotriene A4 hydrolase (LTA4H). Rescue therapy with a LTA4H inhibitor, SC57461A (10 mg·kg−1·day−1 sc), partially reversed chronic hypoxic PHT. We conclude that NaNO2 was superior to iNO in increasing tissue NO and SNO generation and reversing chronic PHT, in part via upregulated SNO-LTA4H.

Re-evaluation of sodium nitrate (E 251) and potassium nitrate (E 252) as food additives

The Panel on Food Additives and Nutrient Sources added to Food (ANS) provided a scientific opinion re-evaluating the safety of sodium nitrate (E 251) and potassium nitrate (E 252) when used as food additives. The current acceptable daily intakes (ADIs) for nitrate of 3.7 mg/kg body weight (bw) per day were established by the SCF (1997) and JECFA (2002). The available data did not indicate genotoxic potential for sodium and potassium nitrate. The carcinogenicity studies in mice and rats were negative. The Panel considered the derivation of an ADI for nitrate based on the formation of methaemoglobin, following the conversion of nitrate, excreted in the saliva, to nitrite. However, there were large variations in the data on the nitrate-to-nitrite conversion in the saliva in humans. Therefore, the Panel considered that it was not possible to derive a single value of the ADI from the available data. The Panel noticed that even using the highest nitrate-to-nitrite conversion factor the methaemoglobin levels produced due to nitrite obtained from this conversion would not be clinically significant and would result to a theoretically estimated endogenous N-nitroso compounds (ENOC) production at levels which would be of low concern. Hence, and despite the uncertainty associated with the ADI established by the SCF, the Panel concluded that currently there was insufficient evidence to withdraw this ADI. The exposure to nitrate solely from its use as a food additive was estimated to be less than 5% of the overall exposure to nitrate in food based on a refined estimated exposure scenario. This exposure did not exceed the current ADI (SCF, 1997). However, if all sources of exposure to dietary nitrate are considered (food additive, natural presence and contamination), the ADI would be exceeded for all age groups at the mean and the highest exposure.

The Role of Mucosal Immunity in the Pathogenesis of Necrotizing Enterocolitis

Necrotizing enterocolitis (NEC) is the most devastating gastrointestinal disease of prematurity. Although the precise cause is not well understood, the main risk factors thought to contribute to NEC include prematurity, formula feeding, and bacterial colonization. Recent evidence suggests that NEC develops as a consequence of intestinal hyper-responsiveness to microbial ligands upon bacterial colonization in the preterm infant, initiating a cascade of aberrant signaling events, and a robust pro-inflammatory mucosal immune response. We now have a greater understanding of important mechanisms of disease pathogenesis, such as the role of cytokines, immunoglobulins, and immune cells in NEC. In this review, we will provide an overview of the mucosal immunity of the intestine and the relationship between components of the mucosal immune system involved in the pathogenesis of NEC, while highlighting recent advances in the field that have promise as potential therapeutic targets. First, we will describe the cellular components of the intestinal epithelium and mucosal immune system and their relationship to NEC. We will then discuss the relationship between the gut microbiota and cell signaling that underpins disease pathogenesis. We will conclude our discussion by highlighting notable therapeutic advancements in NEC that target the intestinal mucosal immunity.

Dietary nitrate and nitrite: Benefits, risks, and evolving perceptions

Consumers have an illogical relationship with nitrite (and its precursor, nitrate) in food. Despite a long history of use, nitrite was nearly banned from use in foods in the 1970s due to health concerns related to the potential for carcinogenic nitrosamine formation. Changes in meat processing methods reduced those potential risks, and nitrite continued to be used in foods. Since then, two opposing movements continue to shape how consumers view dietary nitrate and nitrite. The discovery of the profound physiological importance of nitric oxide led to the realization that dietary nitrate contributes significantly to the nitrogen reservoir for nitric oxide formation. Numerous clinical studies have also demonstrated beneficial effects from dietary nitrate consumption, especially in vascular and metabolic health. However, the latest wave of consumer sentiment against food additives, the clean-label movement, has renewed consumer fear and avoidance of preservatives, including nitrite. Education is necessary but may not be sufficient to resolve this disconnect in consumer perception.

Changes in plasma and urinary nitrite after birth in premature infants at risk for necrotizing enterocolitis

BACKGROUND

Plasma nitrite serves as a reservoir of nitric oxide (NO) bioactivity. Because nitrite ingestion is markedly lower in newborns than adults, we hypothesized plasma nitrite levels would be lower in newborns than in adults, and that infants diagnosed with necrotizing enterocolitis (NEC), a disease characterized by ischemia and bacterial invasion of intestinal walls, would have lower levels of circulating nitrite in the days prior to diagnosis.

METHODS

Single blood and urine samples were collected from 9 term infants and 12 adults, 72 preterm infants every 5 d for 3 wk, and from 13 lambs before and after cord occlusion.

RESULTS

Nitrite fell 50% relative to cord levels in the first day after birth; and within 15 min after cord occlusion in lambs. Urinary nitrite was higher in infants than adults. Plasma and urinary nitrite levels in infants who developed NEC were similar to those of preterm control infants on days 1 and 5, but significantly elevated at 15 and 20 d after birth.

CONCLUSION

Plasma nitrite falls dramatically at birth while newborn urinary nitrite levels are significantly greater than adults. Acute NEC is associated with elevated plasma and urinary nitrite levels.

Maternal intake of fatty acids and their food sources during lactation and the risk of preclinical and clinical type 1 diabetes in the offspring

AIMS

We examined maternal dietary intake of fatty acids and foods which are sources of fatty acids during lactation and whether they are associated with the risk of preclinical and clinical type 1 diabetes in the offspring.

METHODS

The subjects comprised a cohort of 2,939 mother-child pairs from the prospective Type 1 Diabetes Prediction and Prevention Study. Composition of maternal diet during the third month of lactation was assessed by a validated food frequency questionnaire. Among the children with HLA-conferred susceptibility to type 1 diabetes, 172 developed preclinical and 81 clinical diabetes. Average follow-up for preclinical type 1 diabetes was 7.5 years (range 0.2-14.0 years) and for clinical type 1 diabetes 7.7 years (0.2-14.0 years).

RESULTS

Maternal intake of fatty acids during lactation was not associated with the risk of type 1 diabetes in the offspring. After adjusting for putative confounders, maternal total consumption of red meat and meat products during lactation was associated both with increased risk for preclinical [hazard ratio (HR) 1.19, 95 % CI 1.02-1.40, p = 0.038] and clinical type 1 diabetes (HR 1.27, 95 % CI 1.06-1.52, p = 0.025). In particular, consumption of processed meat products showed an association with increased risk for type 1 diabetes (HR 1.23, 95 % CI 1.02-1.48, p = 0.045). Maternal use of vegetable oils was associated with increased risk for preclinical type 1 diabetes (HR 1.21, 95 % CI 1.03-1.41, p = 0.023).

CONCLUSIONS

Maternal consumption of red meat, especially processed meat, during lactation may increase the risk of type 1 diabetes.

Nitrate reduction to nitrite, nitric oxide and ammonia by gut bacteria under physiological conditions

The biological nitrogen cycle involves step-wise reduction of nitrogen oxides to ammonium salts and oxidation of ammonia back to nitrites and nitrates by plants and bacteria. Neither process has been thought to have relevance to mammalian physiology; however in recent years the salivary bacterial reduction of nitrate to nitrite has been recognized as an important metabolic conversion in humans. Several enteric bacteria have also shown the ability of catalytic reduction of nitrate to ammonia via nitrite during dissimilatory respiration; however, the importance of this pathway in bacterial species colonizing the human intestine has been little studied. We measured nitrite, nitric oxide (NO) and ammonia formation in cultures of Escherichia coli, Lactobacillus and Bifidobacterium species grown at different sodium nitrate concentrations and oxygen levels. We found that the presence of 5 mM nitrate provided a growth benefit and induced both nitrite and ammonia generation in E.coli and L.plantarum bacteria grown at oxygen concentrations compatible with the content in the gastrointestinal tract. Nitrite and ammonia accumulated in the growth medium when at least 2.5 mM nitrate was present. Time-course curves suggest that nitrate is first converted to nitrite and subsequently to ammonia. Strains of L.rhamnosus, L.acidophilus and B.longum infantis grown with nitrate produced minor changes in nitrite or ammonia levels in the cultures. However, when supplied with exogenous nitrite, NO gas was readily produced independently of added nitrate. Bacterial production of lactic acid causes medium acidification that in turn generates NO by non-enzymatic nitrite reduction. In contrast, nitrite was converted to NO by E.coli cultures even at neutral pH. We suggest that the bacterial nitrate reduction to ammonia, as well as the related NO formation in the gut, could be an important aspect of the overall mammalian nitrate/nitrite/NO metabolism and is yet another way in which the microbiome links diet and health.

Dietary intake and bio-activation of nitrite and nitrate in newborn infants

Nitrate and nitrite are commonly thought of as inert end products of nitric oxide (NO) oxidation, possibly carcinogenic food additives, or well-water contaminants. However, recent studies have shown that nitrate and nitrite play an important role in cardiovascular and gastrointestinal homeostasis through conversion back into NO via a physiological system involving enterosalivary recirculation, bacterial nitrate reductases, and enzyme-catalyzed or acidic reduction of nitrite to NO. The diet is a key source of nitrate in adults; however, infants ingest significantly less nitrate due to low concentrations in breast milk. In the mouth, bacteria convert nitrate to nitrite, which has gastro-protective effects. However, these nitrate-reducing bacteria are relatively inactive in infants. Swallowed nitrite is reduced to NO by acid in the stomach, affecting gastric blood flow, mucus production, and the gastric microbiota. These effects are likely attenuated in the less acidic neonatal stomach. Systemically, nitrite acts as a reservoir of NO bioactivity that can protect against ischemic injury, yet plasma nitrite concentrations are markedly lower in infants than in adults. The physiological importance of the diminished nitrate→nitrite→NO axis in infants and its implications in the etiology and treatment of newborn diseases such as necrotizing enterocolitis and hypoxic/ischemic injury are yet to be determined.

Evidence-based feeding strategies before and after the development of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a devastating disease of premature infants and is associated with significant morbidity and mortality. While the pathogenesis of NEC remains incompletely understood, it is well established that the risk of disease is increased by the administration of infant formula and decreased by the administration of breast milk. This review will focus on the mechanisms by which breast milk may serve to protect against NEC, and will review the evidence regarding various feeding strategies that may be utilized before and after an episode of NEC.