Fiber: An umbrella review
This week’s paper under review was published in March 2018. It was sent to me by someone curious to see if there is evidence for fiber being beneficial for general health. The paper is an “umbrella review”, which is a review of meta-analyses on a topic. In this case, the topic is dietary fiber and health outcomes. A meta-analysis pools together trials that meet certain criteria. When a number of meta-analyses have been undertaken on a particular topic area, it becomes possible to review the meta-analyses that reviewed the trials. John Ioannidis has issued guidance and caution about the methodology of such reviews, as they risk aggregating aggregations and over and under emphasizing findings (Ref 1).
What is fiber?
It’s spelled fibre in the UK and fiber in the US, but what is it? The carbohydrates that we eat fall into three categories: monosaccharides (single sugars); disaccharides (two sugars) and polysaccharides (many sugars). The single sugars are glucose, fructose and galactose. The disaccharides are sucrose, lactose and maltose (which break down into single sugars). The polysaccharides break down into digestible forms of carbohydrate (glycogen and starch) and indigestible forms of carbohydrate (collectively called fiber). The indigestible forms of carbohydrate comprise insoluble fiber (which doesn't dissolve in water) and soluble fiber (which dissolves, or swells, in water).
Requirements for fiber
Human beings have no requirement for fiber. It is not an essential nutrient i.e. something that must be consumed. This fact emanates from the fact that carbohydrates are not essential nutrients and fiber is a sub set of carbohydrates. The best reference for this is the US document “The panel on macronutrients”, which states: "The lower limit of dietary carbohydrate compatible with life apparently is zero, provided that adequate amounts of protein and fat are consumed" (Ref 2).
The Dietary Guidelines for Americans also explicitly state that fiber is not essential (Ref 3). On p91 of the 2015-2020 edition, the term essential nutrient is defined as: “Essential Nutrient – A vitamin, mineral, fatty acid, or amino acid required for normal body functioning that either cannot be synthesized by the body at all, or cannot be synthesized in amounts adequate for good health, and thus must be obtained from a dietary source. Other food components, such as dietary fiber, while not essential, also are considered to be nutrients.”
If you want to end an argument about fiber, you can simply point out that there is no requirement for the substance and leave it there. However, just because something is not required, doesn’t mean that it’s not healthy. I have used this example before – laughter is not essential, but I have little doubt that it’s healthy.
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