Plasma and urine diketopiperazine concentrations in normal adults ingesting large quantities of aspartame

In aqueous solution, aspartame can cyclicize to form its corresponding diketopiperazine (3-carboxymethyl-6-benzyl-2,5-diketopiperazine; DKP) and methanol. We measured plasma and urinary concentrations of DKP in samples obtained from six normal adult subjects ingesting 2.2 mg DKP/kg body weight. The DKP was administered as part of a dose of 200 mg aspartame/kg body weight. DKP concentrations in plasma were below the detection limit (less than 1 microgram/ml) of the high-pressure liquid chromatographic method at each time interval after ingestion at which they were measured. Mean (+/- SD) total urinary DKP excreted during the first 24-hr period after dosing was 6.68 +/- 1.30 mg (4.83 +/- 0.23% of the ingested DKP dose). Approximately 44% of the total DKP excreted was excreted in the first 4 hr after dosing.