The concept that carbohydrates vary in their degree of complexity, the extent to which they are digested, and the readiness with which they yield glucose is actually rather easy for consumers to comprehend and to adopt into their food selection and diet plans.

Carbohydrates, which have now become a four-letter word, "carb," include sugars, starches, and fibers. Not all carbohydrates, however, are equal and it is a myth that all carbohydrates contribute to weight gain and obesity.

The concept is actually rather simple: Not all carbohydrates are the same — they differ naturally in their levels of structural complexity, digestibility and the extent to which they produce glucose.

Carbohydrates in Nature

Carbohydrates occur abundantly in nature and range from simple sugars (monosaccharides and disaccharides) through oligosaccharides and polysaccharides to saccharide complexes and starch. Made up primarily of carbon, hydrogen, and oxygen, carbohydrates are essentially polymers [a substance made of many repeating chemical units or molecules] of simple sugar molecules (saccharides) and may be visualized as a string of beads. Single beads represent simple sugars (monosaccharides and disaccharides) such as glucose, fructose, and sucrose. Strings of few, more and many more beads represent oligosaccharides, polysaccharides, and starch, respectively.

The polymers may be made up of short linear chains such as those of amylase, or consist of longer branched chains such as those of amylopectin. While monosaccharides and disaccharides are sweet tasting as such, they lose their sweetness when strung together in chains. Polysaccharides and starches are not sweet tasting in nature.

The type and number of sugar molecules in the polymer, and the manner in which they are linked together contribute significantly to the physical and biological properties of the carbohydrate. Each carbohydrate type exhibits characteristic physiological and nutritional functions — in both in vivo (in living systems) and in vitro (test tube) systems, making one wonder why the Food and Drug Administration (FDA) continues to use the single term "carbohydrate" to cover the whole range that are encountered in foods.

Dietary Fiber in Nature

Dietary fibers are also long chain polymers of saccharides and often contain several different types of saccharide building units. They may be linear or branched and soluble or insoluble in water. Some soluble fibers are viscous when dissolved and can therefore slow the speed and therefore, the extent of digestion and also the rate of passage through the gastrointestinal tract. Insoluble fibers such as cellulose and hemicellulose, on the other hand, have no affect on the speed of digestion.

Fiber molecules are generally large. They differ from starches in that they are not broken down by human digestive enzymes and instead pass through undigested to the large intestine, where they are broken down by bacterial action.

Starch is the major energy storage carbohydrate and is largely digestible while gums / mucilages / hydrocolloids and celluloses contribute to cellular structure and are generally called non-starch polysaccharides (NSP). Simple carbohydrates are important in various metabolic pathways and glucose — the simplest of them all - is the universal energy currency for all living matter.

Carbohydrate Digestibility

In humans, starches and sugars must be broken down into simple forms (i.e. glucose) for the cells to absorb and utilize. This process is called digestion. The glucose thus produced is absorbed by the body and used to create energy and to trigger other important metabolic reactions. Digestion starts in the mouth where enzymes in the saliva break down starch molecules into short chain polysaccharides such as maltose and maltodextrins. Digestion proceeds at different levels as the food continues its way into the remainder of the gastrointestinal tract.

Starch, long considered to be fully digestible, was recently discovered by scientists to be partly indigestible in the GI tract of humans. The fractions resisting in vivo digestion are referred to as "non-digestible fiber" or "resistant starch" (RS). Of these, the latter is gaining nutraceutical importance as the result of their fermentative capacity in the lower gut and the ability to yield small chain fatty acids (SFA) such as butyric acid which have been discovered to be critical for overall health.

Most carbohydrates are digested and absorbed into the body through the small intestine. The rate at which they are broken down and absorbed depends to a great extent on their level of complexity. Monosaccharides such as glucose and fructose are simple carbohydrates that are made up of a single sugar molecule and are simply absorbed through the intestinal wall and enter the blood stream. Disaccharides such as sucrose and lactose are broken down relatively quickly and the resulting monosaccharides are then absorbed through the intestinal wall and enter the blood stream. Digestible polysaccharides, such as starches, are more complex and need to be converted into simpler and simpler forms before they can be absorbed by the body. Other polysaccharides resist digestion and pass through to the large intestine where they may be fermented to a small extent by intestinal microorganisms or they may simply pass through as undigested and unabsorbed material. These are the non-digestible fibers and resistant starches.

Non-digestible fibers and resistant starches are different in their composition and structure and are non-digestible for different reasons. In fiber, the sugar units are linked (bonded) together in such a way that human enzymes cannot disrupt the bonds; therefore, they cannot be broken down into simpler forms and be absorbed. This is why these fibers transit through the small intestine and arrive intact at the large intestine. Resistant starches — although starches — are resistant to enzymatic digestion in the small intestine for a number of reasons including denseness and a highly crystalline structure. Consequently, these types of starch act more like fiber than starch, and travel through the intestinal tract until they reach the large intestine where, like fiber, they may be fermented by the bacteria in the colon. Resistant starches have certain advantages over traditional fibers — they don't impact the taste and texture of foods like dietary fibers do.

Resistant starches exist in nature in legumes, raw potatoes, and plantains — to name a few plant based food stuffs. But Nature is not the only one to make these treasures. Resistant starches may also be commercially manufactured in a number of ways. The RS2 kind is produced from unripe bananas, uncooked potatoes, or Hi-maize corn by extraction and concentration. The RS4 kind of resistant starches are not found in nature and manufactured by chemically altering the structure of starch found in certain cereals and tubers.

Another innovative approach to make virtually any kind of regular starch resist digestion is to simply coat the carbohydrates with hydrocolloids such as xanthan and pectin gums and thereby, render them resistant to digestion. The soluble polysaccharides (the coating) swell to form a viscous layer which is impermeable to the enzymes of the gastrointestinal tract. Thus protected, these carbohydrates are not easily digested in the small intestine and therefore, do not contribute sugars to the blood.

Carbohydrate choice is largely influenced by nutrition and health considerations and also by cultural acceptability and individual likes and needs. The chemical complexity of carbohydrates — in terms of its readiness to contribute glucose in the system — is becoming increasingly important in human physiology.