  Scope: Biomarkers of food intake (BFIs) are useful tools for objective
assessment of food intake and compliance. The aim of this study was to discover
and identify urinary BFIs for onion. Methods and results: In a randomized
controlled cross-over trial, 6 overweight participants (age 24-62 years)
consumed meals with 20 g/d onion powder or no onion for 2 weeks. Untargeted
UPLC-qTOF-MS metabolic profiling analysis was performed on urine samples and
the profiles were analysed by multilevel-PLSDA, modified PLS, and nearest
shrunken centroid to select features associated with onion intake. Eight
biomarkers were tentatively identified; six of them originated from
S-substituted cysteine derivatives such as isoalliin and propiin, which are
considered the most specific for onion intake. Most of the biomarkers were
completely excreted within 24 hours and no accumulation was observed during 2
weeks indicating their ability to reflect only recent intake of onions.
Receiver-operator curves were made to evaluate the performance of individual
biomarkers for predicting onion intake. The area under the curve values for
these biomarkers ranged from 0.81 to 1. Conclusion: Promising biomarkers of
recent onion intake have been identified in human urine. Further studies with
complex diets are needed to validate the robustness of these biomarkers.
