The Chemical Structure of Carbohydrates

All carbohydrates have something in common: They are built out of sugar molecules. Sugar molecules can exist separately as single units, or they can join together in pairs to form double sugars. The scientific term for a single sugar is monosaccharide. The double-sugar units are known as disaccharides. Many sugar molecules can join together in long chains, and those are called polysaccharides. Starch and fiber are examples of polysaccharides.

Monosaccharides (single sugars) and disaccharides (double sugars) are also known as simple carbohydrates. Polysaccharides (many sugars linked together) are referred to as complex carbohydrates. See the figure for a closer look at the chemical structure of carbohydrates. Note that both starch and fiber are considered polysaccharides (complex carbohydrates) made out of chains of glucose molecules. The key difference is that starches are digestible and fiber is not. Digestive enzymes in the intestine easily cut the bonds that link the glucose molecules in starch. The enzymes can't cut through the stronger bonds that link the glucose chains in fiber.

A common question is "If carbs digest and turn into glucose, why not skip the carbs and just eat proteins and fats?" The flaw with that approach is that your body needs the nutrients found in healthy carb foods. Health experts warn against overconsumption of artery-clogging saturated fats. Plant foods are naturally low in saturated fat, while animal foods (such as meats, cheese, and butter) often have significant amounts of saturated fat, depending on which cuts of meat you choose. Plant foods such as whole grains, fruits, and vegetables offer fiber, while meat, cheese, eggs, and fat do not have any fiber. Every food group offers health benefits if you choose foods wisely. The best diets provide a variety of nutritious foods, from all food groups, as long as they're consumed in appropriate serving sizes.

Not all carbs are created equal when it comes to good nutrition, though. Whole grains, legumes, fruits, and vegetables are filled with the nutrients we need for health. Refined grains, desserts, sodas, and the seemingly endless supply of junk foods don't offer much in terms of healthy nutrition. Unhealthy food choices dump in carbs and calories and do more harm than good. Healthy, natural carbohydrate-containing food groups provide a diverse array of critical nutrients:

• Whole grains provide fiber, B vitamins, magnesium, phosphorus, manganese, iron, and vitamin E.
• Fruits and vegetables contribute fiber and are rich in vitamin A, vitamin C, potassium, folic acid, bioflavonoids, and antioxidants, just to name a few.
• Milk and yogurt provide protein, calcium, vitamin D, vitamin B12, phosphorus, and more.
• Legumes are not only high in fiber, but they also pack in the protein and offer several B vitamins, as well as magnesium, zinc, and iron.

You simply must have glucose traveling through your bloodstream at all times. Glucose is the preferred fuel for the brain, the central nervous system, and red blood cells. Glucose also provides fuel to muscles. You can also burn fat for energy, but you need to burn glucose and fat in the proper fuel mix to keep metabolism on an even keel. An appropriate amount of carb is needed to properly fuel your body. You can't drive a car without fuel, and you can't operate a human body without glucose in your bloodstream.