Cellulose and starch are examples of carbohydrates. Cellulose is the most common organic material on the planet and the most important food source of the animal kingdom. Starch has become the major nutrient of man but few other animals.
Both carbohydrates are made up of thousands of glucose molecules but differ in the type of bond that link them. Animals do not have enzymes that can break the bonds of cellulose but they do have enzymes that rapidly break down starch into its component glucose molecules, on the other hand, gut microbes transform cellulose it into useful nutrients, mostly short chain fatty acids (SCFAS) but no glucose. Over tens of millions of years of evolution, our human ancestors have relied on hind gut fermentation for much of their energy needs as starch and sugars are relatively scarce in the natural world. In fact, we now have evidence that SCFAS are essential nutrients for human gut lining cells and also important signaling molecules in metabolism and immunity.
Fiber, the common term for digestion resistant carbohydrates such as cellulose, the most important macronutrient of plant eating animals and the main component of the much touted 5 a day servings of vegetables. Fibers are food for symbiotic gut microbes that extract energy and leave behind nutrients including short chain fatty acids which we are only recently learning their important role in gut health, immunity, glucose metabolism, cell signaling and energy homeostasis.
Flash flooding is a good analogy for the adverse effects of refined carbs, too much rain over a short time overloads the drainage systems leading to backlogs and damage. The toxic effects of simple sugars occur when the flux (mass/time) of sugars entering the liver and the blood stream overloads the systems responsible for metabolizing them. In the case of fructose entering liver cells, too much causes energy depletion, cell dysfunction and fat infiltration, whereas too much glucose entering the bloodstream demands very high insulin levels that lead to increasing fat storage and eventually insulin resistance (pre-diabetes) by exhausting the insulin receptor pathways. The average consumption of high glycemic carbs in the USA is 500 grams a day and that includes 70 grams of fructose: consumption at these levels explain why it is inevitable that 70% of the US population is overweight while 35% have fatty liver and pre-diabetes.
Glucose is the most common basic building block of carbohydrates and although there are many other simple sugars such as fructose from fruit and galactose from milk, glucose is the only sugar allowed to pass through the liver into the bloodstream, every other sugar in the diet is filtered out and metabolized on the first pass through the liver after digestion. Of the simple sugars, glucose is the least reactive with proteins and this probably explains its exclusive role in animal metabolism, nevertheless glucose must be controlled within a narrow range to prevent harmful consequences. The amount of free glucose in the body is only 10 grams so it is easy to see how 300 grams of glucose absorbed rapidly from an average meal of the standard American diet can seriously overload the system but the glucose and insulin spikes shown in the figure below repeats every day for years on end. Hitherto scientists have accepted this as normality but we now know it is far from normal and is producing our insulin resistance (diabetes) epidemic with disastrous public health implications.