Dietary fibre is a heterogeneous group of non-digestible polysaccharides. Increasingly dietary fibre is being shown to provide health benefits beyond the traditional viewpoint that it provides bulk to the chyme in the gut. The health benefits of dietary fibre appear to centre on the beneficial glycaemic effects that it can cause, which explains the detrimental effects of refined starches on the insulin system. The current recommendations are therefore to consume more fibre. However, the exact amounts and types of fibre that confer the best health effects are not known. Negative effects of high fibre diets have also been suggested, with the most commonly postulated problem being the reduction in the absorption of minerals that might occur with high fibre intakes. Generally it is thought that components in dietary fibre such as phytate, may be able to bind to particular minerals removing them from solution and decreasing their absorption. A number of studies have tested the mineral binding capacity of fibre.
For example, in one study1 healthy subjects were fed a control diet with a low fibre content for one month. The diet contained 23 % protein, 30 % fat and 47 % carbohydrate, as well as 15 grams of fibre (9.5 grams of neutral detergent fibre), 585 mg of calcium and 276 mg of magnesium. The subjects then switched to a high fibre diet that was similar to the control diet but with the addition of 16 grams of refined cellulose (Solka Floc brand) (total neutral detergent fibre 23.5 grams) as a supplement made from processed wood pulp. The macronutrient and micronutrient intakes were maintained at similar levels to the original control diet. The treatment diet was consumed for a further month. Following the treatment diet the stool weight of the subjects and the defecation frequency increased significantly from baseline. The fibre supplements also decreased the transit time of two of the subjects that at baseline, has particularly exaggerated transit times. The fibre therefore provided bulk to the chyme.
In terms of mineral absorption, the fibre treatment diet significantly increased the faecal excretion of calcium and magnesium. In particular, cellulose ingestion increased magnesium excretion by 54 mg per day (115 to 144 mg per day; 56 % in the faeces and 44 % in the urine), whilst it also increase calcium excretion by 161 mg per day (from 328 to 489 mg per day; 100 % in the faeces). Therefore the cellulose was able to negatively affect the absorption of both calcium and magnesium. This supports other studies that show fibre binding effects for dietary fibres. However, the fibre used in this study was in a supplemental form. It is recommended however that fibre should come from food. While high fibre diets may increase excretion of certain minerals, those plant foods containing the fibre are also often very good source of minerals, particularly calcium and magnesium. Nuts, seeds and legumes for example are high in fibre, but also high in calcium and magnesium, offsetting any increased mineral excretion.
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