In an article published in April 2022, associations of diet with urinary melamine and derivative concentrations were evaluated among 123 children aged 4–6 years in the Global Alliance to Prevent Prematurity and Stillbirth cohort. Children’s diets on the day preceding urine collection were assessed using 24-h dietary recalls.
Melamine is a high-production-volume chemical with many industrial uses, including the manufacture of laminates, flooring, adhesives, insulation foams, and cleaning products. In high doses, ingestion of melamine can induce urinary stone formation. Crystal formation within nephrons leads to progressive tubular blockage and degeneration, potentially resulting in kidney injury and death. Studies have shown that low-level exposure to melamine is ongoing and can contribute to kidney stone formation and injury, particularly in the setting of other risk factors, such as small body size, chronic kidney disease, low fluid intake, or co-ingestion of cyanuric acid.
Diet is thought to be a primary pathway of melamine exposure. Melamine is present in food packaging materials and can migrate into foods from melamine-containing tableware or utensils.
The authors observed ubiquitous exposure to melamine and related compounds among children in the GAPPS cohort and identified red meat, certain starchy vegetables, and yogurt as potential dietary contributors.
Melamine exposure of children had been observed in several studies previously, suggesting pervasive ambient exposure among US children. A previous analysis suggested that urinary melamine concentrations greater than 7.1 µg/mmol creatinine represent significant exposure to melamine-tainted products. The mean concentration observed in the current study was well below this threshold, although melamine exposures of two participants exceeded 7.1 µg/mmol creatinine, indicating possible risk for melamine-related stones. Analysis of other childhood cohorts with similar mean cyanuric acid exposure to the present study found that cyanuric acid exposure was associated with early kidney injury.
Although a clear dose-dependent association of red meat intake with melamine exposure was not observed, this finding of increased melamine exposure among some higher consumers was consistent with previous reports. There are multiple routes by which animal-based food products may become contaminated with melamine or related compounds. Cyanuric acid is authorized for use in the US as a component in feed-grade biuret, a source of nonprotein nitrogen in animal feed. Cyanuric acid and melamine have been found ubiquitously in bovine feed from China, India, and the US. Several studies have documented that ingested melamine can subsequently be detected in the tissues of pigs, sheep, and chickens. Melamine in animal feed can also be transferred to the milk and tissues of dairy cows and the eggs of laying hens.
Another source of melamine exposure for agricultural animals is the pesticide cyromazine. Cyromazine is used as a feed-through fly control agent for horse and chicken manure, and can be metabolized in these animals to melamine. The fact that certain starchy vegetables were positively associated with urinary ammelide, may be explained by cyromazine use on agricultural crops (particularly root crops), as plants may also metabolize cyromazine to melamine.
It was unexpected to observe a negative association between some fruits and ammelide exposure. A higher consumption of these fruits may have displaced other foods in children’s diets which may be more prone to melamine contamination.
The study found no association between food source (’food at home’ versus ’food away from home’) and melamine exposure. Although ’food away from home’ is often associated with increased use of food packaging materials, which have been noted to be potential melamine sources, many ’food at home’ sources could have significant contact with similar food packaging materials, potentially containing melamine.
One important limitation of the study is that it could not account for participants’ potential use of melamine tableware. Moreover, it was also unable to differentiate between organic or conventionally grown foods in participants’ diets, which could influence exposure to melamine or related compounds because of cyromazine usage. The use of a single spot urine sample to assess melamine exposure is another limitation of this study. Although melamine concentrations of spot urine samples have been found to be highly correlated with excretion across several days, diurnal variations may exist due to the short half-life of melamine.
Additionally, this analysis was not able to account for numerous non-dietary sources of exposure to melamine. For example, dust is one principal source of exposure. Melamine and related compounds have also been found in mats used for children’s naps, and children may be exposed to melamine or its derivatives through other household items, cleaning products, and textiles used in children’s clothing.
Studies with larger sample sizes and increased monitoring of non-dietary sources of exposure should be conducted to confirm these findings.