Abstract
Broccoli consumption brings many health benefits, including reducing the risk of cancer and inflammatory diseases. The objectives of this study were to identify global alterations in the cecal microbiota composition using 16S rRNA sequencing analysis and glucoraphanin (GRP) hydrolysis to isothiocyanates ex vivo by the cecal microbiota, following different broccoli diets. Rats were randomized to consume AIN93G (control) or different broccoli diets; AIN93G plus cooked broccoli, a GRP-rich powder, raw broccoli, or myrosinase-treated cooked broccoli. Feeding raw or cooked broccoli for four days or longer both changed the cecal microbiota composition and caused a greater production of isothiocyanates ex vivo. A more than two-fold increase in NAD(P)H: quinone oxidoreductase 1 activity of the host colon mucosa after feeding cooked broccoli for seven days confirmed the positive health benefits. Further studies revealed that dietary GRP was specifically responsible for the increased microbial GRP hydrolysis ex vivo, whereas changes in the cecal microbial communities were attributed to other broccoli components. Interestingly, a three-day withdrawal from a raw broccoli diet reversed the increased microbial GRP hydrolysis ex vivo. Findings suggest that enhanced conversion of GRP to bioactive isothiocyanates by the cecal microbiota requires four or more days of broccoli consumption and is reversible.
Highlights
Broccoli belongs to the glucosinolate (GSL)-containing family of brassica vegetables
Even though many US consumers prefer cooked broccoli, epidemiological studies suggest that frequent consumption of brassica vegetables decreases the Nutrients 2017, 9, 262; doi:10.3390/nu9030262
Inactive nitriles [11,12], not bioactive ITC, are typically reported as products of microbial GSL hydrolysis ex vivo, as reviewed in [3], and ingestion of cooked broccoli typically provides only about one tenth the amount of SFN as that from raw broccoli hydrolyzed by plant myrosinase during digestion [13,14,15]. These findings suggest that if hydrolysis to SFN by microbiota could be enhanced, consumers might obtain greater health benefits from cooked broccoli
Summary
Broccoli belongs to the glucosinolate (GSL)-containing family of brassica vegetables. GSL in broccoli is glucoraphanin (GRP) which, upon hydrolysis, produces the bioactive isothiocyanate (ITC) sulforaphane (SFN), shown to slow or prevent cancer in humans [1]. One major mechanism of action of SFN is upregulation of several phase II detoxification enzymes, including NAD(P)H: quinone oxidoreductase 1 (NQO1) [2]. The formation of SFN from GRP is catalyzed by the plant enzyme myrosinase, active during chewing or crushing when broccoli is consumed raw or lightly steamed [3]. When broccoli and other brassica vegetables are cooked more vigorously, the plant myrosinases are inactivated and, are unable to hydrolyze GSLs [4]. Even though many US consumers prefer cooked broccoli, epidemiological studies suggest that frequent consumption of brassica vegetables decreases the Nutrients 2017, 9, 262; doi:10.3390/nu9030262 www.mdpi.com/journal/nutrients
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