Dietary fibres are a particular class of carbohydrates, whose main characteristic is that they are indigestible. Since they cannot be converted into glucose, they do not enter the bloodstream and do not provide calories. They are therefore eliminated from the body via the faeces. The fact that fibre provides no calories, however, should not lead one to believe that it has no function on body composition. A classification is in order to introduce their treatment:

1. soluble fibres, which have the ability to solubilise in water;
2. insoluble fibres, which, by contrast, do not solubilise.

Soluble fibres

  Soluble fibres have the ability to take up, in the digestive system, part of the macronutrients taken in the context of the same meal and chelate them (chelation is a special chemical reaction). Now, when macronutrients are chelated, they become non-bioavailable. In other words, they become like fibre: indigestible and impossible to absorb. They therefore provide no calories and are excreted with the faeces.

Insoluble fibres

  Insoluble fibres also have a similar function to soluble fibres. In fact, insoluble fibres also have the ability to make macronutrients taken in the same meal non-bioavailable, but they do so in a different way. This particular type of dietary fibre, in fact, has the property of absorbing a large amount of water. As a result of this, they result in the formation of a gelatinous compound that incorporates all the neighbouring macronutrients, which then become non-bioavailable. Furthermore, the gelatinous compound resulting from the absorption of water by the fibres occupies a very large portion of the stomach and presses against the walls of this organ. This pressure causes the perception of satiety. Hence, insoluble fibres also have an anorectic effect.

The effect of fibre on intestinal flora

  As is well known, the food taken in is digested, i.e. broken down into its various constituent nutrients, inside the stomach. It is here that food takes on a form suitable for circulation in the bloodstream. Once the food has been digested inside the stomach, it is sent to the intestine, which absorbs it. In other words, for the most part, it is in the gut that the nutrients that make up the food consumed are carried to the bloodstream. Now, the gut is populated by a great many bacteria, most of them beneficial. It is said that the bacteria that inhabit the gut (the intestinal bacterial flora) are our symbionts, that is, living organisms that must inhabit the human body in order to survive and, in doing so, benefit it. Well, going back to fibre, since it cannot enter the bloodstream, it is not a source of calories, but this does not mean that it cannot nourish the gut bacteria. When fibre is consumed, in fact, the intestinal flora is well nourished and thrives as a result. Now, healthy, flourishing gut bacteria not only imply that the gut is healthy too, but also have implications for body composition: the gut bacterial flora, in fact, feeds itself by ‘stealing’ part of the food taken in with the diet. And the more bacteria that constitute it, the more calories they will steal. You might think that the amount of calories needed to feed the intestinal bacteria would be minimal, negligible. Well, this is not the case.

Fibre and anti-nutrients

  Apart from the effect on body composition and athletic performance, excessive fibre consumption should be avoided. This is because dietary fibres contain substances belonging to the class of so-called anti-nutrients. These substances, in fact, make not only macronutrients, but also certain micronutrients (more on micronutrients later), which are essential for maintaining good health, non-bioavailable. More specifically, fibre can hinder the absorption of calcium, selenium, iron and zinc. Precisely for this reason, it is often recommended to limit the amount of fibre consumed in the diet to a maximum of 25-35g per day (perhaps to be taken in meals other than those in which foods rich in these micronutrients are consumed).