Background & aims: Current self-report methods to monitor dietary intake are often unreliable. As part of a dietary intervention study, we investigated whether adding a common food flavor (vanillin) to test diets and measuring the major metabolic end product vanillic acid in urine, could provide assessment of compliance with dietary supplements. Methods: After baseline urine was collected 10 subjects (6 control and 4 study patients) consumed 1.3 g of vanillin in a liquid test meal as the last food at bedtime and collected the first morning urine. Next a kinetic excretion study was performed in which 6 controls consumed a vanillin spiked drink with continued urine sampling at 30-min intervals for 5 h. Vanillic acid concentrations were measured by reverse phase high-performance liquid chromatography. Results: The test diet was consumed just before bedtime; the first morning void vanillic acid concentration gave a reliable indication of compliance (3 males 0.224±0.041 and 3 females 0.290±0.099 mg/ml; mean±SD). Thirty-minute sampling of vanillic acid excretion for 6 controls was maximal 1 h after the test diet, returning to baseline after 4 h. Conclusion: Vanillin is a useful, inexpensive and non-toxic biochemical marker for confirming compliance with experimental diets. © 2007 European Society for Clinical Nutrition and Metabolism.
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