Citric acid, a weak organic acid found in citrus fruits, is a natural preservative and flavoring ingredient. Due to its acidic/sour taste, citric acid is added to many foods and drinks. As an intermediate in the citric acid cycle, it occurs in the metabolism of most living organisms and acts as an antioxidant. Higher concentrations of citric acid are found in lemons and limes.
As a preservative, citric acid increases the acidity of the bacterial environment, making it difficult for the microbes to survive or reproduce. It also binds the excess calcium and allows it to leave the body. This is quite beneficial for the prevention of kidney stones and other kidney ailments (reference 1). Although not widely regarded for fitness related benefits, a study done on the effects of citric acid on physical fatigue found that it reduces physiological stress and tempers physical fatigue (reference 2).
While generally considered safe, citric acid supplementation can cause a variety of side effects such as vomiting, diarrhea, nausea, dizziness, loss of blood pressure, and abdominal pain.
Though not an antioxidant itself, citric acid enhances the activity of many beneficial antioxidants which can help fend off free radicals that can trigger the growth of cancerous tumors. It also mixes with minerals and metals easily, facilitating their absorption in the body.
Alkalinizing effects of Citric acid supplementation have been shown in high doses of 5-10g per day.
2. J Clin Biochem Nutr. 2007 Nov; 41(3): 224–230. Effects of Citric Acid and l-Carnitine on Physical Fatigue. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2243251/