Carbohydrates are a natural part of the human diet, and have been for as long as we've been human, but the easy and cheap access to carbs is the number one cause of disease in Western civilization. Sound extreme? It is. Any consistent position is extreme; a compromise always means subverting a principle in favor of making nice with your enemies. And if the principle you want to follow is good health, then stay away from cheap carbs.
Before I get into the dangers of 'cheap carbs', such as sugar and wheat flour, I want to cover some carb basics -- the types of carbs and carbohydrate digestion.
There are three basic types of carbs: simple sugars, starches, and fiber. You can add alcohols to the list, but they're processed differently by the body and a bit more complicated so I'll leave them out of the discussion today.
The simplest carb is a single sugar molecule, such as glucose or fructose. A molecule containing just one sugar is called a monosaccharide. Some carbs are chains of sugar molecules, one after another bound together. Sucrose (common table sugar) is one molecule of glucose bound to one molecule of fructose. This combination of two sugars (saccharides) is called a disaccharide. Long chains of sugar molecules create starches, of which fiber is one subset. Fiber is differentiated by the fact that it is indigestible, and so does not contribute calories to diet.
Sugars are rapidly pulled into the bloodstream from the small intestine. Glucose directly becomes "blood sugar," whereas fructose and galactose are first transported to the liver for conversion. Since these sugars must be converted by the liver, high doses of these sugars tends to produce the same steatic effects as alcohol -- the liver stores excess converted sugars in vacuoles inside liver cells, and eventually these vacuoles grow so large that they rupture their containing cells causing cell death, which leads to tissue scarring (cirrhosis).
Disaccharides can't be absorbed into the bloodstream, so they first have to divided into their two constituent sugars. Disaccharides are broken down into monosaccharides in both the stomach and small intestine. Sucrose (table sugar) is broken down into glucose and fructose; Lactose (milk sugar) breaks down into glucose and galactose; and Maltose (malt sugar) breaks down into two units of glucose. Enzymes speed these reactions, although all will occur in the presence of acid (such as the hydrochloric acid in the stomach).
Starches, as chains of sugar molecules way longer than just the two sugars in disaccharides, must be broken down even further. Enzymes that promote this breakdown are called amylases. Amylase is found in saliva (which gives starchy foods some of their sweet taste) and is also produced by the pancreas. One genetic mutation in humans is the presence of multiple copies of the amylase gene, which means extra amylase in their saliva. If your ancestors came from the Baltic Sea, then chances are you'll have an easier time digesting starch (especially if you chew your food well).
Not all sugar chains can be broken down by our enzymes, though. That depends on how the sugar molecules are joined; there's actually many different ways to do it. Cellulose, which makes up 33% of most plant matter and 50% of wood, is indigestible to humans. Even though it's just chains of glucose molecules, they're joined by beta(1,4) bonds -- whatever that means, the consequence is that we don't have the chemical machinery to break that bond.
There are different types of long sugar chains; some are branched, some are linear, some contain different sugars, some have beta bonds others have alpha bonds, etc etc. Hence, the calorie contribution of long sugar polymers will vary, depending on how much of the sugar molecules are enzymes can break off. Generally, a food is called a "fiber" if it is primarily indigestible, and a "starch" if the amylase our bodies produce can break it down.
Cows can digest cellulose with symbiotic bacteria in their gut -- which is partly why cows have four stomachs. (Other ruminants, such goats, sheep, deer, and llamas have similar systems. They're all ruminants.) To us humans, though, such fibrous plants aren't food. You'll see "fiber" included on the labels in some foods, and the calories there included in the calorie count of the food, but you won't actually get those calories. That's like expecting to survive while eating wood chips. Your body can't do it. (Feel free to blame politicians and bureaucrats for the misleading labeling.)
In following posts in this series I want to cover the effect of carbs on metabolism, sugar's effects in disease through glycation, carb-loading before running, high-carb foods, and healthy carb consumption.