Introduction

Infants less than 4 months of age are most at risk of adverse health effects from over exposure to nitrates and nitrites through ingestion of formula diluted with nitrate contaminated water [EPA 2007; WHO 2011a; WHO 2011b].

Although there is no nutritional indication to add complementary foods to the diet of a healthy term infant before 4 to 6 months of age, the American Academy of Pediatrics suggests that home-prepared infant foods from vegetables (i.e. spinach, beets, green beans, squash, carrots) should be avoided until infants are 3 months or older [Greer and Shannon 2005].

Gastroenteritis with vomiting and diarrhea can exacerbate nitrite formation in infants and has been reported to be a major contributor to methemoglobinemia risk in infants independent of nitrate/nitrite ingestion [Lebby et al. 1993; Gebara and Goetting 1994; Avery 1999; Nelson and Hostetler 2003; DeBaun et al. 2011].

In addition, the pregnant woman and her fetus might be more sensitive to toxicity from nitrites or nitrates at or near the 30th week of pregnancy [Gitto et al. 2002; Gordon 2012].

Individuals with glucose-6-phsphate dehydrogenase (G6PD) deficiency may have greater susceptibility to the oxidizing effects of methemoglobinemia inducers.

Infants Are at Highest Risk

Infants younger than 4 months of age who are fed formula diluted with water from untested rural domestic wells are especially prone to developing health effects from nitrate exposure [EPA 2007; WHO 2011a; WHO 2011b; Dusdieker and Dungy 1996]. They are more susceptible to developing methemoglobinemia for a number of reasons including:

Infant gut pH

• The high pH of the infant gastrointestinal system favors the growth of nitrate-reducing bacteria [Kross et al. 1992; Nelson and Hostetler 2003], particularly in the stomach and especially after ingestion of contaminated waters. The stomach of adults is typically too acidic to allow for significant bacterial growth and the resulting conversion of nitrate to nitrite.

HbF

• A large proportion of hemoglobin in young infants is in the form of fetal hemoglobin. Fetal hemoglobin (HbF) is more readily oxidized to MetHb by nitrites than is adult hemoglobin [Rehman 2001; Nelson and Hostetler 2003]. Over time, adult forms of hemoglobin gradually increase and HbF decreases [McKenzie 2010]. Infants with a higher proportion of fetal hemoglobin may have severely reduced oxygenation before cyanosis appears clinically [Steinhorn 2008]. Therefore, infants, especially premature ones, are particularly susceptible.

Impaired reduction of MetHb

• At birth, NADH-dependent methemoglobin reductase (also called cytochrome-b5 reductase), the major enzyme responsible for reduction of induced methemoglobin back to normal hemoglobin, has only about half the activity it has in adults [Hjelt et al. 1995; ATSDR 2004; Smith 1991; Nelson and Hostetler 2003; McKenzie 2010]. The level of cytochrome-b5 reductase does not reach adult levels until at least 4 months of age [Lebby et al. 1993; Nelson and Hostetler 2003].

Other factors

Infection and inflammatory reactions can increase endogenous synthesis of nitrate in both infants and adults [NRC 1995; Nelson and Hostetler 2003].

• Gastroenteritis with vomiting and diarrhea can exacerbate nitrite formation in infants. This has been reported as a major contributor to MetHb risk in infants independent of nitrate/nitrite ingestion [Lebby et al. 1993; Gebara and Goetting 1994; Avery 1999; Nelson and Hostetler 2003].
• Gastroenteritis can increase the in vivo transformation of nitrate to nitrite and systemic absorption of nitrite from the large intestine.
• Young infants can develop methemoglobinemia with systemic metabolic acidosis. The systemic metabolic acidosis is often caused by dehydration associated with diarrhea or sepsis, but it can occur with renal disorders as well [Nelson and Hostetler 2003; Hanukoglue and Danon 1996; Sager et al. 1995]. With sepsis, it is thought that nitric oxide is released and oxidizes hemoglobin as it is reduced to nitrate [Nelson and Hostetler 2003; Ohashi et al. 1998]. With acidosis, the NADH methemoglobin reductase system is affected leading to as much as 50% decrease in methemoglobin reduction [Nelson and Hostetler 2003].

These factors combine to place young infants with diarrhea, who are fed formula diluted with nitrate contaminated well water, at the greatest risk for toxicity [Johnson and Kross 1990; Zeman et al. 2002; EPA 2007; WHO 2011a, 2011b].

Pregnancy

The pregnant woman and her fetus represent another high-risk group.

Pregnancy is a high oxygen demand physiologic state. Due to the increased intake and utilization of oxygen, increased levels of oxidative stress are reasonably expected. The hematologic changes of pregnancy include a 40-50% increasing blood volume (plasma greater than RBC mass) expansion peaking at around 30 weeks [Gordon 2012]. With plasma volume increasing more than the RBC mass, the maternal hematocrit falls resulting in a "physiologic anemia of pregnancy" reaching a peak at 30 to 34 weeks [Gordon 2012].

Due to oxidative stress, methemoglobin is continually produced within red blood cells, but its levels are kept low (0.5% to 2.5% of total hemoglobin) by enzymatic pathways that work to reduce methemoglobin. Conditions such as pregnancy with its high oxygen demand and increased levels of oxidative stress may overwhelm the body's ability to reconvert methemoglobin back to hemoglobin, resulting in increased methemoglobin levels [Gitto et al. 2002].

Exposure to nitrates also increases oxidative stress and depletes antioxidant reserves. Thus, pregnant women may be more sensitive to the induction of clinical methemoglobinemia by nitrites or nitrates at or near the 30th week of pregnancy when oxidative stress peaks.

Reproductive outcome studies performed at sites with high nitrate levels in the water supply provide some evidence of maternal transfer of nitrate and nitrite [Manassaram et al. 2006; Tabacova et al. 1997 and 1998; Croen et al. 2001].

Others with increased risk

An increased risk of developing methemoglobinemia from exposure to oxidizing agents has been reported in individuals with coexisting

• Anemia, cardiovascular disease, lung disease, sepsis
• Glucose-6-phosphate deficiency (more common in
individuals of African, Asian or Mediterranean descent)
• Metabolic problems with pyruvate kinase and RBC methemoglobin reductase
• Presence of other abnormal hemoglobin species (structural abnormalities of the hemoglobin molecule itself) including carboxyhemoglobin, sulfhemoglobin and sickle hemoglobin (HbS) [Ash-Bernal et al. 2004; Skold et al. 2011]

Genetic factors may increase the risk of drug induced methemoglobinemia and hemolytic anemia [McDonagh et al. 2013].

Recreational drug users are at increased exposure risk, especially users of volatile nitrite inhalers and drugs like cocaine. Cocaine can be adulterated with a variety of substances including phenacetin and local anesthetics like benzocaine [Hunter et al 2011; Flomenbaum et al. 2006] (see Table 2).

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