Last week we looked at: what fiber is (indigestible carbohydrate); our requirement for fiber (none); the recommendations for fiber intake (high); and how we might get that intake. We then looked at an “umbrella review” of meta-analyses that examined fiber and different health conditions (heart disease, type 2 diabetes and various cancers) (Ref 1). Finally, we looked at the limitations of epidemiological evidence and the one large, long, randomised controlled trial that involved fiber as an intervention (and gave cause for concern). This week I want to look at plausible mechanisms – it is one thing to claim that fiber is associated with health outcomes; it is another to explain how this might happen. The Bradford Hill criteria set out that the strength of association should be double before an observed association might be worth exploring as possibly causal (Ref 2). One of the Bradford Hill criteria to explore for causation is a plausible mechanism. If A and B have been observed as associated with each other, is there any plausible mechanism by which A could cause B? Without getting anywhere near double for the strength of association between fiber and any health condition, last week’s umbrella review nevertheless explored plausible mechanisms by which fiber might confer health benefits. The hypotheses for possible mechanisms were (all are direct quotations from the umbrella review paper): 1) “A higher consumption of fiber seems to be associated with lower concentrations of serum inflammatory markers.” 2) “Dietary fibers are known to alter intestinal microbiota composition and function.” 3) “The consumption of fibers (particularly from vegetables) is associated with higher intakes of vitamins, minerals and phytoestrogens. These micronutrients seem to decrease the risk of the chronic diseases for which we observed a significant reduction.” 4) “…it was hypothesized that dietary fiber may inhibit cholesterol synthesis and consequently reduce serum cholesterol concentrations…” 5) “…it was hypothesized that dietary fiber may… promote weight loss by regulating energy intake…” 6) “…it was hypothesized that dietary fiber may… slow glucose absorption and thus improve insulin sensitivity.” Let’s look at each of these in turn. As we look at them, please consider an alternative hypothesis. This hypothesis is that dietary fiber, per se, does not confer benefit: some foods that contain dietary fiber – especially vegetables – might confer benefit.
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