data have shown associations across many population groups in Asia, Western Europe, and North America. Animal studies have shown that both cruciferous vegetable extracts and isothiocynate and indole isolates are chemopreventive. Various mechanisms have been proposed to explain the association, including decreased inflammation and oxidative stress, induced cell differentiation and apoptosis, and improved carcinogen metabolizing capacity.
From a human dietary perspective, one of the challenges to deriving chemopreventive benefit from cruciferous vegetables is that isothiocynate, the active component that actually imparts the protective benefit, is difficult to access. It exists in the plant as a glucosinolate, which the plant enzyme myrosinase cleaves into isothiocynate. However, myrosinase is active only in raw vegetables. Cooking inactivates it. On the basis of measurements of excreted total isothiocyanates in urine, Shapiro et al. (2001) reported that chewing uncooked broccoli results in much greater recovery of isothiocynates than swallowing unchewed sprouts and that cooking decreases isothiocyanate availability relative to both chewed and unchewed raw broccoli. They also found that pretreating cooked broccoli with myrosinase dramatically increases availability (i.e., to a point where excretion is more than double what it is with chewed uncooked broccoli). Lampe noted that one could thus argue the value of pretreating all cruciferous vegetables with myrosinase, but that would pose yet another challenge—that is, free isothiocyanate in the diet tends to cause gastritis, with nausea and vomiting, in some individuals. Also, it is not really clear where in the gastrointestinal (GI) tract, or how, isothiocyanates exert their chemopreventive effect and whether introducing pure isothiocyanate would be ideal.
Given that very few human populations regularly consume raw cruciferous vegetables, how are humans deriving the chemopreventive benefit of broccoli and other cruciferous vegetables? The answer, Lampe said, is in our gut microbiome. Lampe and her colleagues reported a wide range of recovery of isothiocynates (ITCs) in urine after eating 200 grams of cooked broccoli (Li et al., 2011). Some individuals excreted almost no ITC (i.e., the ITC was not available to them), whereas others excreted nearly 30 percent (i.e., indicating high availability). To determine whether the gut microbiome might be contributing to this variation, researchers analyzed fecal samples from the low- versus high-ITC excretors and observed that fecal bacteria in the low-ITC excretors have a lower capacity to degrade glucoraphanin compared to the high-ITC excretors. So clearly there is something going on at the level of the gut microbiome, Lampe remarked, with high excretors containing the enzymatic machinery necessary for cleaving glucosinolate into ITC. The researchers did not find any major taxonomic differences in bacterial composition. However Li et al. (2011) was a pilot study, Lampe