The hydrogen ion concentration is a primary determinant of the structure of proteins, and as such animals must maintain the hydrogen ion concentration within narrow limits. In humans the blood is maintained within a range of around 7.35 to 7.45. Even a deviation of 0.5 pH can cause death to humans, and in order to prevent this occurring, complex buffer systems are used to allow maintenance of the pH at around 7.4. Acid is produced as a product of normal metabolic activity, and high protein high phosphorus diets can produce sulphuric and phosphoric acid, respectively. However, the main acid produced as a result of metabolism is CO2 from the lungs, and changes to the breathing rate can cause changes in the pH of blood for. Increased breathing can reduce the CO2 concentration of the blood, causing alkalosis, whereas reduced breathing rate can increase CO2
Plant diets contain high concentrations of salts of organic acids, and these are able to increase the bicarbonate concentration of the blood as a result of their metabolism. Diets high in plant foods therefore alkalinise the blood. In contrast, protein metabolism results in the formation of phosphate and sulphate ions and this can decrease the concentration of bicarbonate in the blood. Diets high in protein therefore acidify the blood. As with breathing, dietary factors therefore play a major role in determining the pH of the blood. Acute and chronic acidification of the blood results in an increased excretion of anions in the urine, as the kidney attempts to increase the excretion of acids to compensation for the increased hydrogen ion concentration of the blood. In this process glutamine is used to form ammonium ions which allow increased excretion of anions (conjugate bases of weak acids). In addition, the metabolism of glutamine also increase the production of bicarbonate ions which are released to the circulation to raise the pH of the blood.
Metabolic acidosis therefore increases the requirements for glutamine in the kidney to deal with the excretion of acids and the production of bicarbonate ions. As the pH of the blood falls plasma concentration of glutamine therefore rise to allow for an increased transport to the kidney. However, as the kidney metabolises glutamine, the glutamine blood concentration falls. Most of this available glutamine is released from the skeletal muscle, suggesting that significant amounts of nitrogen are catabolised from skeletal muscle during metabolic acidosis. However, the requirement of the intestine for glutamine also drops providing more glutamine to circulation. The amount of glutamine required by the kidney during chronic but small decreases in pH would be considerably less that during acute metabolic acidosis. However, the requirement of glutamine to counter increased hydrogen ions suggests that supplemental glutamine may be beneficial at preventing detrimental changes to the acid base balance caused by poor diet.
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