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Carbohydrates are large molecules that come in several main categories:

•Monosaccharides

Single-molecule carbohydrates. These include glucose, fructose, and galactose.

•Disaccharides

Two-molecule combinations of monosaccharides used to form a single, large molecule. These include sucrose (a glucose and a fructose bonded), lactose (a glucose and a galactose bonded), and maltose (two glucoses bonded).

•Polysaccharides

Longer strings of monosaccharides chained together. These include starch (a digestible form of many glucoses linked together), cellulose (fiber which is mostly indigestible by humans and made up of glucose molecules), and glycogen (an irregular matrix of connected glucose molecules which is the most common form for carbohydrate stored in muscle tissue and the liver).

All the listed carbohydrates (aside from fiber) can be converted into glucose and used for the following, usually in this order of priority:

•Transported to cells and broken down for immediate energy use.

•Transported to the blood to circulate and provide glucose to needy cells, such as neuronal cells that do not store much fuel of their own and prefer glucose.

•Transported to the liver to be assembled as glycogen for storage. Liver glycogen can be broken down to release glucose into the blood when blood glucose levels fall too low.

•Transported to skeletal muscles to be assembled as glycogen for storage. When skeletal muscles are working at higher effort intensities (anything as hard as a jog or harder), they rely heavily on this stored glycogen to provide the energy to power contractions.

Energy-needy cells get first priority for incoming glucose. Not until most cells are energy-satiated will carb consumption result in increased blood glucose. Once blood glucose is at an appropriate level, liver glycogen synthesis becomes priority. Only when all the above carbohydrate needs are attended to will muscle glycogen start to be synthesized in any meaningful amount.

Originally, “simple” carbs were defined as monosaccharides and disaccharides and “complex” carbs were meant to denote polysaccharides. It was previously thought that simple carbs digested quickly, tasted sweet, were more addicting, and were worse for health whereas complex carbs were opposite in every respect. Unfortunately, this model for carbohydrates was fundamentally flawed. For example, fructose is a simple carbohydrate, but is incredibly slow digesting. In contrast, starch is a complex carbohydrate that, in its pure form, is digested and absorbed even faster than glucose. Furthermore, simple carbs are no more addicting than starches. When consumed appropriately, simple sugars are no worse for health than starches and can have some distinct timing-related benefits for training. There is no reason to assign “good” or “bad” monikers to different carb sources based on their molecular complexity alone.

As you may have already inferred, the primary role of carbs in the human diet is for use as an energy source. Proteins are mainly used as building blocks for tissue and only used for energy on occasion (when carbohydrates and fats are lacking). Carbohydrates are the raw materials for energy metabolism and are used only in limited forms as structural components. In other words, the body’s primary use for carbs is to power cells, and carbs are particularly important in powering the operation and contraction of muscle cells. As energy substrates, carbohydrates have no equal–they easily and rapidly provide energy, especially for high-volume users like nervous system cells and muscle cells.

Figure 3.4 Carbohydrate have the second largest impact on body composition and performance compared to the other macronutrients and thus must be prioritized after protein in a structured diet.