The metabolic effects are dietary fibres are diverse in humans. Fibre may lower postprandial glycaemia, improve colonic health, provide energy in the form of short chain fatty acids, modulate lipoprotein levels, cause weight loss and also affect mineral balance. However, the exact effects of fibre varies between different fibre types, and because most foods contain other components that may also have metabolic effects, it is not always easy to ascertain the precise role of the fibre. Studies have attempted to get around this problem by supplementing with isolated fibre fractions, both synthetic and natural, but from such studies it is becoming clear that isolated fibre does not provide the same metabolic effects as the original whole fibre found in foods. It is therefore very difficult to predict the metabolic effects of particular dietary fibres. So generally while high fibre foods provide particular health benefits that are well characterised, it is unclear as to the mechanisms by which the fibre component may exert these effects.
Soybeans have been particularly problematic in this regard because while it has been relatively easy to assess the effects of whole soybeans, investigations into the components responsible for these metabolic effects have not been easy to interpret. In fact, soybean fibre is a complex mixture of non-digestible carbohydrates, many of which have different metabolic effects. The effects of soybean fibre have been investigated in the nutritional literature because of the known cholesterol lowering properties of soybeans. For example, in one study1, researchers supplemented the diet of subjects with 21 grams of dietary fibre from two different preparations of soybean fibre. One fibre was a whole extract of non-purified soybean pulp containing 39 % dietary fibre while the other was a purified soybean extract containing 79 % fibre. These fibres had been prepared in different ways through different manufacturing processes. The sugar profiles of the two preparations was very similar, but the non-purified soybean fibre had a higher phytic acid content and a higher ratio of soluble to insoluble fibre.
The ingestion of the fibre caused increases in the stool weight of the subjects, and magnesium, calcium and iron excretion were increased following the ingestion of both fibres. Faecal steroid excretion did not change, but the increase in stool weight lowered the concentration of steroids in the stool. In addition, the non-purified fibre caused a 21 % increase in faecal short chain fatty acids due mainly to increases in deoxycholic acid, which increased 32 % over the control diet. However, oral glucose tolerance only improved following ingestion of the purified soybean fibre. Interestingly, the soybean fibre preparations did not modify the lipoproteins of the subjects as expected. In fact only the purified fibre caused changes in lipoproteins. In this regard the fibre increased total and low density lipoprotein (LDL) plasma levels, which meant that the high density lipoprotein to low density lipoprotein ratio was lowered by 12 % However, this was not a significant difference from the control diet.
Dr Robert Barrington’s Nutritional Comments: Fibre is likely a conditionally essential nutrient in humans. Eating carbohydrates necessitates an intake of fibre to prevent disease. Refined carbohydrates devoid of their fibre are a likely driver of Western lifestyle disease. The effects of fibre are diverse and it is difficult to identify the benefits of individual fibres. A range of dietary fibres from a range of whole plant sources should therefore be incorporated into the diet. This study illustrates that fibre is likely not the cause of the beneficial lipoprotein effects seen in humans who consume soybeans.
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