We have already met this class of nutrients when we dealt with the section on nutrients. They are, essentially, carbohydrates characterised by the fact that they are indigestible.

Function

This has already been adequately discussed in the part where fibres were presented as nutrients. It is recalled here that dietary fibre can be classified into soluble and insoluble. Both contribute to reducing the bioavailability of nutrients taken with them (in the context of the same meal), although they do so in different ways: soluble fibres bind to and chelate the macronutrients in the digestive tract; insoluble fibres, on the other hand, being very hydrophilic, absorb a considerable amount of water, resulting in a gelatinous compound that incorporates all the macronutrients, making them non-bioavailable. The compound resulting from the absorption of water by the insoluble fibres also tends to have a significant mass. This implies a pressure on the stomach walls from which a feeling of satiety follows. Insoluble fibres, therefore, in addition to their function of hindering the absorption of macronutrients, also have an anorectic function. These functions, however, only in some studies resulted in actual loss of fat mass (e.g. Kovacs, 2001; Keithley, 2005; Birketvedt, 2005; Walsh, 1984), while in others this was not the case. In any case, individuals on a weight-loss diet have a higher than normal requirement for fibre, so it is certainly a good idea to consider taking a fibre supplement as part of such a diet, because while this may assist the slimming process, it will (almost) certainly benefit health.

Negative notes

Fibre is a nutrient in its own right and has no negative effects. Although there is no toxic dose, only rarely can large consumption give flatulence and diarrhoea.

Form and dosage

Supplements of this nutrient are commercially available in the form of tablets and soluble powder, the latter sometimes contained in single-dose sachets. As you may have already realised, dietary fibre is not a single class of nutrient. On the contrary, there are different types, even within the context of each of the two groups of soluble and insoluble fibres. Among the best known fibres in the context of supplementation products are fructo-oligo-saccharides (FOS), glucomannan and guar gum. The former have an important prebiotic action (i.e. they nourish the beneficial bacteria in the gut). Because of this, men and women who take 8g of FOS in the morning and evening reduce their energy intake by 5% compared to those who do not (for details of the mechanism, please refer to the section on the presentation of nutrients); glucomannan, also known as konjac root, increases its volume seventeenfold in the presence of water; and guar gum is used as a food additive with a thickening function in numerous products (foods and supplements) and has the advantage of being much cheaper than glucomannan. As for dosage, the recommended intake levels of fibre are:

• 0.5g/die/kg body weight, or a value (in g/day) within the range age plus 5 and age plus 10, during growth;
• 30g/day for adults.

Nonetheless, as we have already seen, while this food is not toxic even at high dosages, the need for it is increased in those on a weight-loss diet.