In order to produce energy, your body cells need the molecular building blocks of macronutrients, which are proteins, carbohydrates, and fats. Most body cells can use any of the macronutrient building blocks to produce energy, but some cells -- including brain cells -- rely specifically upon simple carbohydrates, which you can obtain from several sources.
Carbohydrates
There are two kinds of carbohydrates: simple and complex. Simple carbohydrates include the monosaccharides, which are sugars made up of a single sugar unit, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Examples of monosaccharides include fructose and glucose. Disaccharides -- two sugar units chemically bonded together -- are also simple carbohydrates. Table sugar is a disaccharide. Complex carbohydrates are starches, or polysaccharides. They're made up of many sugar units chemically bonded together.
Use of Carbohydrates
Your cells don't use complex carbohydrates or disaccharides to produce energy, because you have no mechanism for absorbing disaccharides or polysaccharides into the bloodstream. Your intestine absorbs only monosaccharides, which the cells then take up and chemically burn, explains Dr. Lauralee Sherwood in her book "Human Physiology." All body cells can burn monosaccharides such as glucose for energy. For some body cells, glucose is only one of many possible fuels that the cell can burn. For other body cells, glucose is the primary or only fuel that the cell uses.
Pure Monosaccharides
There aren't many sources of pure monosaccharides in food -- most foods contain a mixture of monosaccharides, disaccharides and polysaccharides. Fruit is a particularly rich source of both glucose and fructose in their monosaccharide forms. There are also processed food additives used as sweetening agents that contain either pure glucose or a mixture of monosaccharide glucose and monosaccharide fructose. Corn syrup is pure glucose, while high fructose corn syrup is a mixture of glucose and fructose. You can absorb the monosaccharides from these sources directly into the blood.
Eating Carbohydrates
While you can certainly obtain glucose and other monosaccharides, including fructose and galactose, by eating pure sources of the monosaccharides, there's no cellular benefit to eating free monosaccharides. Your digestive tract is capable of breaking polysaccharides and disaccharides into monosaccharides that you can absorb, meaning that any time you consume sugar or any sort or starch, you're providing your cells with a source of the monosaccharides -- especially glucose -- that they need for energy.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
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