Citric acid in lemon juice provides a clinically recognized benefit for kidney stone prevention. Citrate, the form citric acid takes once metabolized, inhibits calcium oxalate crystal formation by binding urinary calcium and preventing crystal growth. A daily intake of approximately 4 oz (125ml) of pure lemon juice has been shown to significantly raise urinary citrate levels, producing effects comparable to pharmacological potassium citrate therapy for hypocitraturic calcium stone formers. This is specific and well-established enough that urologists refer to it as “lemonade therapy.”
One widely repeated claim, that lemon juice has a general “alkalizing effect” on the body, should be understood narrowly. The body tightly regulates blood pH within a range of 7.35–7.45 regardless of diet; dietary choices do not meaningfully shift systemic pH. What is accurate is that citrate metabolism raises urinary pH and citrate excretion, which is specifically protective against kidney stone formation, a real and targeted effect, not a systemic alkalizing of body chemistry.
Vitamin C is heat- and oxygen-sensitive, so bottled and pasteurized lemon juice contains significantly less than fresh-squeezed. For vitamin C and kidney stone prevention purposes, fresh juice is preferred. At high intake levels, citric acid may aggravate acid reflux in susceptible individuals. Some medications, blood pressure drugs, diabetes medications, and certain statins, may interact with citrus components at high doses.