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). 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. Last week’s “umbrella review” offered six plausible mechanisms for how fiber might be of benefit. In summary, the six hypotheses (and it’s important to note that these are just hypotheses) suggested by the paper were:
1) Higher fiber intake seems to be associated with a lower level of an inflammatory marker in the body (something called C-reactive protein).
2) Dietary fiber alters our gut flora.
3) Higher fiber intake (particularly from vegetables) is associated with higher intakes of vitamins and minerals. These micronutrients seem to decrease the risk of chronic disease.
4) Higher fiber intake may reduce blood cholesterol levels.
5) Higher fiber intake may promote weight loss by regulating energy intake.
6) Higher fiber intake may slow glucose absorption and thus improve insulin sensitivity.
You can see the full review of the hypotheses below. In brief, the six hypotheses don’t stack up for the following reasons (and I invite you to consider an alternative hypothesis as we go through these – the notion that dietary fiber, in itself, does not confer benefit: some foods that contain dietary fiber – especially vegetables – might confer benefit):
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