When assessing diet-disease risk relationships, Johanna Lampe said, “We really can’t ignore the contribution of the gut microbiome.” Not only do gut microbes influence host energetics, as Peter Turnbaugh elaborated, they also play key roles in multiple other areas of host metabolism. Microbes contribute to host fermentation, reduction of nitrate and sulfate, esterification, aromatic fission, and hydrolysis and deconjugation (i.e., not just of glycosides in our plant food, but also of steroid hormones and other endogenous compounds that are excreted in bile and end up in the colon). Qin et al. (2010) identified a number of host metabolic pathways handled by gut microbes, many of which are involved with carbohydrate or amino acid metabolism or xenobiotic biodegradation. But how do microbes influence metabolites? And how do microbial metabolites of dietary components contribute to disease prevention and disease risk?
Lampe addressed these questions using xenobiotic degradation of phytochemicals as an example. There are an estimated 25,000 phytochemicals, with both negative and positive effects. As an example of a phytochemical with negative effects, most people associate nitrates with processed meats, but in fact vegetables are a major source of nitrates in the human diet. Water is another major source. Nitrates can be converted into nitrites, which in turn can interact with a number of different compounds to form nitrosamines, nitrosamides, and nitrosoguanidine; these, in turn, can form DNA adducts and cause DNA damage, creating the potential for carcinogenesis. On the flip side, there are a whole host of dietary bioactive phytochemicals with potential beneficial human health effects (Scalbert et al., 2011). These include the phenolics (phenolic acids, stilbenes, curcuminoids, chalcones, lignans, flavonoids, isoflavones), terpenoids (phenolic terpenes, carotenoids, saponins, phytosterols), organosulfurs (thiosulfinates), and nitrogen-containing compounds (glucosinolates, indoles). Lampe focused on three specific phytochemicals: glucosinolates in cruciferous vegetables, soy isoflavones, and plant lignins.
Glucosinolates and the Human Gut Microbiome
Cruciferous vegetables—that is, broccoli and its vegetable relatives—are the poster children of cancer-preventing vegetables. Both epidemiological and animal data show consistent associations between intake of cruciferous vegetables, whether broccoli, cauliflower, cabbage, or something else, and lower risks of various cancers, primarily of epithelial origin (e.g., lung, colorectal, breast, prostate, and pancreatic cancers). The epidemiological
2 This section summarizes the presentation of Johanna Lampe.