Regarding digestibility of insoluble fiber: If we examine our excrement, we can find a partial answer to your question. If we eat seeds such as sesame or flax, a large amount of them are excreted in our solid waste, undigested. Our digestion is not so powerful that it can tackle and all the carbohydrates found in nature. Examples of such things would be grass (timothy hay), cotton, tree bark, and chitin. All of those are made of carbohydrates.
Ruminants have multiple stomachs and regurgitate their foods to chew them multiple times to break down the nutrients locked within. They also have a specialized set of bacteria to help the process along. Humans would starve trying to extract enough nutrients or calories from insoluble fiber sources. If this answer is not satisfactory, I can scare up a biochemist friend that can give a more detailed reason why we don't need to fret about fiber calories.
Ruminants have multiple stomachs and regurgitate their foods to chew them multiple times to break down the nutrients locked within. They also have a specialized set of bacteria to help the process along. Humans would starve trying to extract enough nutrients or calories from insoluble fiber sources. If this answer is not satisfactory, I can scare up a biochemist friend that can give a more detailed reason why we don't need to fret about fiber calories.
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