Isothiocyanates from cruciferous vegetables: kinetics, biomarkers and effects

M. Vermeulen

Research output: Thesisinternal PhD, WU

Abstract

Cruciferous vegetables like cabbages, broccoli, mustard and cress, have been reported to be beneficial for human health. They contain glucosinolates, which are hydrolysed into isothiocyanates that have shown anticarcinogenic properties in animal experiments. To study the bioavailability, kinetics and effects of isothiocyanates from cruciferous vegetables, biomarkers of exposure and for selected beneficial effects were developed and validated.
As a biomarker for intake and bioavailability, isothiocyanate mercapturic acids were chemically synthesised as reference compounds and a method for their quantification in urine was developed. The validity of this biomarker was proven in a study in which three volunteers consumed 19 different raw and cooked vegetables and condiments. Urinary excretion levels of isothiocyanate mercapturic acids were higher after the consumption of raw vegetables and condiments (bioavailability 60%, range 8.2-113%) compared to cooked vegetables (10%, range 1.8-43%). In a second study, in which eight smoking men consumed cooked and raw broccoli, higher levels of sulforaphane were found when broccoli was eaten raw (bioavailability 37%) versus cooked (3.4%).
Also as a biomarker for kinetics, sulforaphane conjugates were determined in blood samples also collected in the second study. The area under the blood-concentration curve was higher when broccoli was eaten raw (0.50 µM•h, dose of 9.9 µmol sulforaphane) versus cooked (0.29 µM•h, dose of 61 µmol glucoraphanin). It is concluded that the bioavailability of isothiocyanates from glucosinolates is higher from raw than from cooked vegetables.
The induction of phase 2 metabolism enzymes via an electrophile responsive element (EpRE), that is present in the promotor region of the genes coding for these enzymes, was used as a biomarker for a beneficial health effect. Isothiocyanates containing a methyl-sulfur side chain (like sulforaphane, EC50 value 1.2 µM) are more potent inducers, in this in vitro assay, than aliphatic and aromatic isothiocyanates (e.g. allyl isothiocyanate, EC50 value 6.5 µM). The estimated concentrations of individual isothiocyanates in the body after consumption of cruciferous vegetables may amount to 0.04-4 µM. Since cruciferous vegetables contain several isothiocyanates which act synergistically, concentrations might thus reach these EC50 values for EpRE induction, and represent biologically active concentrations.
In conclusion, the biomarkers developed and validated in the present thesis show that higher and physiologically relevant amounts of isothiocyanates are absorbed after the consumption of raw cruciferous vegetables, compared to cooked vegetables. Broccoli (cress) is favorite since it contains high amounts of sulforaphane, which is, based on the results of the present studies, the most promising isothiocyanate because it shows the lowest EC50 for EpRE induction.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • van Bladeren, Peter, Promotor
  • Rietjens, Ivonne, Promotor
  • Vaes, W.H.J., Co-promotor, External person
Award date13 Feb 2009
Place of Publication[S.l.
Publisher
Print ISBNs9789085853121
Publication statusPublished - 2009

Keywords

  • leafy vegetables
  • glucosinolates
  • isothiocyanates
  • bioavailability
  • metabolism
  • biomarkers

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