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A human eats a large meal and indigestible carbs (IC) with such a timing that the IC arrive to the small intestine when it is already full and busy breaking down and absorbing the meal. What would happen to the IC? Would they stay there until the digestible meal is digested, or would they somehow be moved along? Would they start fermenting if they are stuck there for some time? Would they push the unabsorbed remnants of the meal out as the body tries to excrete IC? Would they hamper absorption or aid it?

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Indigestible carbs are known as dietary fiber. They slow down gastric emptying, which results in slower absorption of the absorbable nutrients in the small intestine.

Early research regarding soluble fiber demonstrated delayed gastric emptying and decreased absorption of macronutrients, resulting in lower postprandial blood glucose and insulin levels [44]. This is most likely due to the viscosity of soluble fibers inside the GI tract. Interestingly, different types of soluble fiber had varying effects on viscosity and nutrient absorption. Guar had the highest viscosity as well as the greatest effect at decreasing postprandial blood glucose. Therefore, it would be assumed that an increased level of soluble fiber would be associated with a decreased risk of diabetes. However, several recent studies have demonstrated the opposite showing no correlation between soluble fiber and a reduced risk of diabetes (Nutrients, 2010).

So, dietary fiber can slow down the absorption of macronutrients (carbohydrates, proteins, fats), but does not reduce the amount of macronutrients absorbed (at least not significantly).

Dietary fiber can be soluble (for example, stachyose and raffinose in legumes) or insoluble (for example, cellulose in fruit skins or grain hulls) (Linus Pauling Institute).

They are microbes, mainly in the large intestine (not in the healthy small intestine), that ferment soluble, but not insoluble fiber. Fermentation of a large amount of soluble fiber produces a lot of gas (flatulence), which typically occurs several hours after a meal.

Insoluble fiber can stimulate intestinal motility and thus help in constipation, but large amounts can be laxative, which could result in decreased absorption of nutrients; soluble fiber can trap some bile secreted by the gallbladder and may thus reduce the synthesis of cholesterol (Linus Pauling Institute).

Dietary fiber probably does not interfere significantly with the absorption of minerals and vitamins (Nutrients Review).

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