Citrate ion, a weak acid, forms in the tricarboxylic acid cycle or can be derived from endogenous sources. The molecular weight of citric acid is 189 KDa and the values of the dissociation constant (pKa) are respectively 2.9, 4.8 and 5.6. At pH 7, citrate is predominantly (> 90%) in the form of trivalent anion, the bivalent form of the ion significantly increases to a pH more acidic.
Citrate present in plasma is filtered into the glomeruli, then reabsorbed mainly in the proximal tube. Approximately 10-35% of the filtered amount is eliminated in the urine. www.ncbi.nlm.nih.gov/pubmed/12957820
Among the organic acids and the anions present in the urine, citrate is the one present in greater quantity. The final concentration of citrate in urine depends mainly on its re-absorption and metabolism in the proximal tube, which seems to be the only trait involved in the metabolism of luminal or extracellular citrate.
With diet, 4 grams of citrate should be taken each day and in humans the administration of citrate results in an increase in plasma values within 30 minutes.
The food-based citrate is absorbed at the level of the small intestine in a manner similar to what happens in the kidneys. In addition, the same transport mechanism allows recovery of secreted citrate in pancreatic and gastric juices.
In foods, it is found as bicarbonate in all vegetables, predominantly leafy types, while in the form of citrate, it is found in oranges, and as citric acid in lemons. A citrus-rich diet can therefore help increase the level of urinary citrate, though it is necessary to pay attention to excess vitamin C (ascorbic acid) that undergoes a transformation into oxalate and can therefore facilitate precipitation of this type of stones. www.ncbi.nlm.nih.gov/pmc/articles/PMC4769668