Is dietary fiber a negative or positive nutrient?

Not all types of fiber are the same, and it is important to appreciate the difference between viscocity and fermentability.

Dietary fiber is a classic example of Dr. Jekyll and Mr. Hyde, at least in the strange world of nutritionists. Of course, the term “fiber” incorporates a varied group of chemical compounds that differ widely in viscocity and fermentability, whilst they are all largely indigestible by the animal.

Nevertheless, it has been long recognized that dietary fiber, in general, impairs digestibility of major nutrients, like energy and amino acids, and as such it is considered a rather undesirable dietary component. If tolerated, it is only because it abounds in all vegetable-derived feedstuffs, and a minimum of fiber is required for gastric motility. But, recently, dietary fiber is being seen as a versatile ingredient that can be used to modulate gastrointestinal microflora; hence the term “functional fiber.” But, not all types of fiber are the same, and to this end, it is important to appreciate the difference between viscocity and fermentability.

Viscocity is the property of creating a gel-like mixture of digesta through the absorbing of water and the swelling of certain types of fiber. The best example is the sticky nature of breakfast oat porridge that is due to the high viscocity value of cooked oats. High viscocity is not a desirable trait because it reduces gut motility leading to:

  1. A more stable environment for pathogens to proliferate
  2. Soft, sticky feces

Fermentability is the degree by which gut microbes can use fibers as nutrient. As most pathogenic bacteria thrive on protein, a high concentration in fermentable fiber usually favors the growth of beneficial bacteria that produce volatile fatty acids leading to:

  1. Strong antibacterial action against pathogenic microbes
  2. Positive energy balance that can amount up to 5 percent of total daily energy needs

Yet, it is not all that simple. An example is a classic study by Hooda (2011; British Journal of Nutrition, 106:664) where the interaction between viscocity and fermentability was studied. In that study, animals were fed four diets in a 2×2 factorial design with the main factors being fiber viscocity (high or low) and fermentability (high or low). Results indicated that high viscocity and low fermentability were associated with the highest digestibility of energy and amino acids, mainly due to reduced passage of time. In contrast, low viscocity and high fermentability led to the greatest losses of nutrients for the animal, again due to reduced time for digesta to be properly digested by enzymes produced by the animal.

It is clear then how confusing fiber can be. For enhanced nutrient digestibility, we would prefer a diet rich in viscous fibers that are also low in fermentability. This would be the case for healthy animals under intense levels of productivity. But, for enhanced gut health, we would prefer a diet low in viscocity and high in fermentability (the opposite of the above case), as is the case for young animals, or those reared in antibiotic-free schemes. No wonder, thus, why fiber can be such a misconstrued nutrient.