Disposition of glucosinolates and sulforaphane in humans after ingestion of steamed and fresh broccoli.
Level 2 - randomized trial
Individual randomized crossover trial
PubMed 11525594 · doi:10.1207/S15327914NC382_5
What was done
Twelve male subjects participated in a crossover trial comparing the bioavailability and metabolic disposition of isothiocyanates after consuming 200 g of fresh versus steamed broccoli. Following a 48-hour washout avoiding dietary glucosinolates, participants consumed the broccoli meals. Blood and urine samples were collected over 24 hours. Total isothiocyanate equivalents in the broccoli, plasma, and urine were measured via high-performance liquid chromatography using a cyclocondensation assay.
What was found
Fresh and steamed broccoli had nearly identical baseline isothiocyanate potential following complete enzyme hydrolysis (1.1 versus 1.0 micromol/g wet weight). Mean 24-hour urinary excretion of isothiocyanate equivalents was 32.3 +/- 12.7% of the ingested amount for fresh broccoli compared to 10.2 +/- 5.9% for steamed broccoli. The sulforaphane metabolite sulforaphane-N-acetyl-L-cysteine comprised about 40% of urinary isothiocyanate equivalents (25.8 +/- 13.9 micromol for fresh versus 6.9 +/- 2.5 micromol for steamed). Plasma metabolites peaked between 0 and 8 hours, and urinary excretion peaked between 2 and 12 hours. Total isothiocyanate bioavailability from fresh broccoli was approximately three times higher than from steamed broccoli.
Why it matters
Thermal processing that inactivates myrosinase substantially diminishes the bioavailability of sulforaphane and other isothiocyanates in humans. Cooking methods significantly alter the effective biological dose delivered from dietary brassica vegetables.
Limits
The study included only 12 male participants, limiting generalizability to broader populations. Clinical health endpoints and long-term chemopreventive effects were not evaluated, only short-term pharmacokinetic markers over 24 hours.
Cited by
- supports When glucoraphanin is ingested alone without myrosinase, the average bioavailability in 24-hour urine collections is approximately 10%.