If you put an egg into a sausage machine it will try to make sausages out of it because that's its job. Does the digestive system attempt to digest everything it receives? Furthermore, does it attempt to derive nutrients from everything it receives? How does the digestive system determine what is nutritious? Can "particles" (e.g., microscopic dirt) pass through to the bloodstream if they are small enough to be absorbed?
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Does the digestive system attempt to digest everything and from everything it receives and how it determines what is nutritious?
Human digestive system is genetically determined and can digest only those substances for which it has digestive enzymes and absorb only those that can be absorbed through the specific "channels" in the intestinal lining.
Human digestive system can absorb nutrients "from everything" (not literally, though), for example, sugar, but not cellulose, from raw sugar cane.
Before absorption, complex nutrients (starch, sucrose, lactose, proteins and triglycerides), need to be broken down by the digestive enzymes into simpler nutrients, but some nutrients need no digestion before absorption. The following nutrients can be absorbed in the small intestine:
- Monosaccharides: glucose, fructose, galactose...
- Fatty acids
- Amino acids
- Minerals: calcium, potassium...
- Polyphenols, plant sterols and stanols and various other plant-derived nutrients
- Sugar alcohols: erythritol, glycerol
Nutrients that are not absorbed in the small intestine:
- Sugar alcohols: isomalt, lactitol, maltitol, mannitol, sorbitol, xylitol
- Oligosaccharides: raffinose, fructooligosaccharides, etc.
- Certain polysaccharides (soluble fiber): beta-glucan, guar gum, etc.
These nutrients travel to the large intestine, where they are broken down (fermented) by normal intestinal bacteria into absorbable nutrients, mainly short-chain fatty acids (SCFAs), and gases.
Can "particles" (e.g., microscopic dirt) pass through to the bloodstream if they are small enough to be absorbed?
There is a countless list of substances that are not nutrients but can be absorbed:
- Medicines and other drugs
- Various random substances ("dirt")
And there is another countless list of substances that cannot be absorbed, for example, insoluble fiber, such as cellulose, and certain artificial sweeteners, such as aspartame and steviol glycosides from stevia.