Twenty-four-hour temporal trend of melamine and its derivatives in urine in association with meal consumption: a panel study in Shanghai, China.

Timing of sampling is important for the exposure assessment of melamine (MEL) and its derivatives. This study aimed to investigate whether MEL and its derivatives in spot urine can effectively represent individual exposure levels throughout the day in adults and to explore their temporal trend before and after meal consumption for helping understand the timing of sampling and for assessing the potential exposure risk. This is a 2-day panel study with 43 college students being enrolled to provide urine specimens in 24 h (from the morning of the first day to the second day) and to answer a questionnaire on demographic characteristics, physical measurements, and time of having meal. Spearman correlation and Wilcoxon rank-sum test were used to examine the associations of the urinary concentrations of MEL and its derivatives in different sampled times and compare the concentrations' differences before and after meal consumption. Urinary concentrations of MEL and its derivatives (ammeline (AMN), ammelide (AMD), and cyanuric acid (CYA)) in the first-morning urine at the second day and randomly selected spot urine were positively associated with the average concentrations in the previous 24-h urine (all P ≤ 0.002). Urinary MEL concentration increased rapidly after meal consumption, reaching a maximum at approximately 3 h and then decreased gradually towards baseline (P = 0.006). Two subjects (4.65%) had a cumulative daily intake exceeding the severest tolerable daily intake. MEL and its three derivatives in spot urine can effectively represent the average concentrations in the previous 24-h urine in adults. Meal consumption is still a notable source of exposure to MEL for humans. These findings are important for choosing a better sampling strategy of performing exposure assessment. Meanwhile, the acute elevation in urinary MEL concentration following meal consumption may pose a potential health risk.