Overview
This lecture reviews the bioavailability of sulforaphane from glucoraphanin-rich broccoli and the crucial role of endogenous myrosinase in enhancing sulforaphane absorption in humans.
Background and Rationale
- Diets high in cruciferous vegetables (like broccoli) are linked to reduced cancer and chronic disease risk due to glucosinolate content.
- Glucosinolates are converted to active isothiocyanates (e.g., sulforaphane) by the enzyme myrosinase or gut microflora.
- Sulforaphane activates cell defense pathways (Keap1-Nrf2-ARE) and protects against disease.
- Delivering stable, bioavailable sulforaphane for human consumption is challenging due to its reactivity.
Broccoli Preparations and Study Methods
- Tested preparations: broccoli sprout extract (BSE, with/without myrosinase), freeze-dried broccoli sprouts (FDBS, with myrosinase), and broccoli seed powder (BSdP, with myrosinase).
- Myrosinase activity, glucoraphanin, and sulforaphane content measured by HPLC and spectrophotometry.
- Human subjects avoided crucifers prior to dosing; 24-hr urine collection measured sulforaphane metabolites as a bioavailability marker.
Key Findings—Bioavailability Results
- GR-rich BSE (no myrosinase): ~10% of glucoraphanin dose recovered as sulforaphane metabolites in urine.
- Commercial GR-rich supplements (no myrosinase) had similar low bioavailability (~10%).
- FDBS and BSdP (with active myrosinase): ~33–41% of glucoraphanin dose bioavailable as sulforaphane.
- Pre-hydrolyzed FDBS in juice (with myrosinase) had highest bioavailability (~40%).
- Delivery method (juice vs. gel-caps) did not significantly affect bioavailability when myrosinase was present.
- Including active myrosinase reduced inter-individual variability in sulforaphane conversion.
Conclusions
- Active endogenous myrosinase in the preparation greatly increases sulforaphane bioavailability from glucoraphanin-rich broccoli.
- The food matrix and delivery method are less important than the presence of active myrosinase.
- Commercial supplements lacking myrosinase provide much lower sulforaphane bioavailability.
Key Terms & Definitions
- Glucoraphanin (GR) — A stable glucosinolate precursor found in broccoli, converted to sulforaphane.
- Sulforaphane (SF) — A reactive isothiocyanate with chemoprotective properties.
- Myrosinase — An enzyme (from plants) that converts glucoraphanin to sulforaphane.
- Bioavailability — The proportion of a nutrient or bioactive compound absorbed and available for use or storage.
- Dithiocarbamates (DTC) — Sulforaphane metabolites measured in urine to assess bioavailability.
- Broccoli Sprout Extract (BSE) — Preparation derived from broccoli sprouts, with or without active myrosinase.
Action Items / Next Steps
- Review mechanisms of the Keap1-Nrf2-ARE pathway.
- Study the role of gut microflora vs. plant myrosinase in glucosinolate conversion.
- Know methods for measuring bioavailability using DTC in urine.