Abstract
The source of dietary nitrate (NO3) is mainly green, leafy vegetables, partially absorbed into blood through intestinal mucosa. The recycled nitrate is reabsorbed and concentrated by the salivary glands and then secreted into saliva. In 2012, sialin was first discovered as the mammalian membrane nitrate transporter in salivary glands and plays a key role in circulation of inorganic nitrate, providing a scientific basis for further investigation into the circulation and functions of nitrate. Dietary nitrate can be converted to nitrite (NO2) by oral commensal bacteria under the tongue or in the stomach, following which nitrite is converted to nitric oxide (NO) through non-enzymatic synthesis. Previously, nitrate and nitrite were thought to be carcinogenic due to the potential formation of nitrogen compounds, whereas the beneficial functions of NO3--NO2--NO pathway were ignored. Under conditions of hypoxia and ischemia, the production of endogenous NO from L-arginine is inhibited, while the activity of exogenous NO3--NO2--NO is enhanced. Recently, a greater amount of evidence has shown that nitrate and nitrite serve as a reservoir and perform positive biological NO-like functions. Therefore, exogenous dietary nitrate plays an important role in various physiological activities as an effective supplement of nitrite and NO in human body. Here we generally review the source, circulation and bio-functions of dietary nitrate.
Highlights
Circulation of nitrate and nitriteThe salivary glands and oral bacteria play an essential role in the circulation and conversion process of exogenous NO3--NO2--nitric oxide (NO) pathway
The source of dietary nitrate (NO3) is mainly green, leafy vegetables, partially absorbed into blood through intestinal mucosa
The salivary glands and oral bacteria play an essential role in the circulation and conversion process of exogenous NO3--NO2--nitric oxide (NO) pathway
Summary
The salivary glands and oral bacteria play an essential role in the circulation and conversion process of exogenous NO3--NO2--NO pathway. Dietary nitrate is absorbed almost entirely owing to its bioavailability in the stomach and the small intestine, and about 75% is excreted in urine, while the remaining amount is reabsorbed in the kidney, by biliary and in salivary glands [3, 30, 31]. In 2012, based on the organ model of salivary glands, sialin was discovered as the nitrate transporter in mammalian cell membranes, which provided the scientific foundation for the study of the biological effect and metabolism of nitrates in the body [8, 24, 33] Approximately 5%-7% of dietary nitrate is converted to nitrite in the oral cavity by commensal facultative anaerobic bacteria located in the deep crypts of the posterior part of the tongue [34, 35]. Thereafter, most nitrite is converted to nitric oxide in the stomach and absorbed systematically (Fig. 1)
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