Test Your Knowledge

Most carbohydrates are divided into two classes: Simple carbohydrates and complex carbohydrates.

Simple carbohydrates are digested quickly and often taste sweet. They are found in milk products, fruit, candy, soda table sugar, honey and other sweets. Fruit and milk products are important sources of valuable nutrients, vitamins and minerals. Candy, soda and sweets provide few or no nutrients. Although these carbohydrates help provide fuel for energy, they are empty calories, with little or no nutritional value. An excess intake of calories can lead to undesired weight gain.

Simple carbohydrates are made up of either a single unit (monosaccharide) or two units (disaccharide) of sugar. There are three different types of monosaccharides:

• Glucose. The most common form of sugar found in the bloodstream. In foods,  it has a mildly sweet flavor.
  
  
• Fructose. The sweetest of the sugars, it occurs naturally in fruits and honey. It is also found in high-fructose corn syrup (in soft drinks) and is a major source of sugar in the diet of Americans.
  
  
• Galactose. Found in gums, sugar beets and the lactose of dairy products. It has very little sweet taste.

When two of these monosaccharides join together, they become disaccharides. The following are the three disaccharides found in foods:

• Sucrose (glucose + fructose). Naturally occurs in sugar cane, sugar beets, honey and many fruits and vegetables. It is refined to make table sugar.
  
  
• Lactose (glucose + galactose). Naturally occurs in milk and dairy products.
  
  
• Maltose (glucose + glucose). Also known as malt sugar, it is the least common disaccharide in nature. It naturally occurs in barley.

All carbohydrates are chemical chains of hydrogen, oxygen and carbon. Complex carbohydrates also contain fibers that have large molecules not sensed by the sweet taste receptors on taste buds. They are found in whole grain breads and cereals, legumes and starchy vegetables that provide nutrition in the form of vitamins, minerals and fiber. Fiber in complex carbohydrates such as oatmeal protects starchy carbohydrates from digestive enzymes. This slowed absorption rate allows for improved digestive system function. Fiber can also help control blood cholesterol levels.

Complex carbohydrates are composed of at least three glucose units. In some cases, they may be composed of many monosaccharides strung together in compounds known as polysaccharides. The three polysaccharides that are crucial for nutrition are:

• Glycogen. Storage form of energy in the body made up of glucose units. Limited amounts are found in meat, but it is not found at all in plants. Glycogen is known primarily as the stored form of glucose and is not a significant food source of carbohydrates. The primary storage sites for glycogen in the human body are the liver and the muscles.
  
  
• Starches. Storage form of energy in plants made up of glucose units. Grains are the richest source of starch and provide the foundation of food energy for much of the world’s population. Starches are found in grains such as wheat and rice, root crops and tubers such as yams and potatoes, and legumes such as beans and peas.
  
  
• Dietary fiber. Provide structure in stems, trunks, roots, leaves and skins of plants. They are made up of a variety of monosaccharides and other carbohydrate derivatives. Fibers are found in all plant-derived foods, including fruits, grains, legumes and vegetables.

Dietary fiber differs from starches because the bonds that hold together the sugar units in fiber cannot be broken down by digestive enzymes. Although some of the bacteria that live naturally in intestines can convert small amounts of the fiber into fatty acids, dietary fiber generally is not considered a source of energy.

Each type of carbohydrate contains four calories per gram. Gram for gram, simple and complex carbohydrates eventually add the same amount of glucose to the bloodstream. However, complex carbohydrates are broken down into glucose more slowly than simple carbohydrates. Complex carbohydrates provide important sources of essential nutrients for a healthy diet.

Some carbohydrates are considered to be neither simple nor complex. The starches and grains that are in the typical American diet are mostly highly refined grains, also called refined carbohydrates. This means that the fiber-rich outer layer or bran of the grain and the nutrient-rich inner part (also called the germ) are removed during the milling process, leaving only the endosperm, or middle part of the grain.

In contrast to refined grains, foods made with whole grains are digested and absorbed into the blood stream gradually, minimizing a quick rise in blood sugar. The protein, fat and fiber content of whole grain foods provide increased satiety (feeling full) and decrease feelings of hunger. Therefore, experts recommend eating a variety of whole grain foods instead of refined grains and simple sugars as part of a heart healthy diet.