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High fiber diets can produce inflammation of the gut as seen in gorillas due to overwhelmingly supporting the growth of large populations of bacteria. The type of fiber and the context in which it is consumed determines the result, but the more fiber there is, the more food there is for feeding bacteria instead of the human animal itself. Sugar—unlike fiber—is digestible by human intestines and can be absorbed in the upper intestines long before reaching the colon where most bacteria reside. Fiber for the sake of fiber is therefore misguided, but it can be tolerated (perhaps where gut inflammation isn't inevitable) in the larger context of a sane diet.
Sugar is a source of energy. The result of eating a diet high in sugar can be seen in the animals which consume high quantities of fruit and nectar. These include hummingbirds, fruit bats, bees, monkeys, etc. These animals are fast and/or skinny. One is then lead to believe sugar provides energy for sustaining a high rate of metabolism. Care must be taken to distinguish table sugar from fruit. While fruit contains many vitamins and minerals for the utilization of sugar, table sugar is refined and devoid of secondary nutrition.
Of animals on high fat diets, whales, walruses, and similar creatures stand out. They develop thick layers of blubber, but they tolerate such obesity due to being able to eat whole seafood instead of just fish fat. They also have remarkable intestines which develop in a lifelong absence of significant fiber.