Certain foods are easier for your body to digest because their structures are less complex. Among the three types of carbohydrates -- starches, sugars and fiber -- sugars require very little digestion and are almost immediately absorbed into the bloodstream for energy. Starches have more complex structures that require your body to work harder to break them down. Fiber affects the speed of digestion. Whole grains are composed of mostly fiber and starch, which means your body has to work harder to metabolize them.
Whole grains, such as whole-wheat, brown rice and oats, retain all parts of the grain, including the bran, germ and endosperm, unlike refined grains, which are stripped of the bran and germ during processing. As a result, your body has to digest all the parts of the grain before its nutrients can be absorbed in the bloodstream. It must break down the fibrous outer hull, or bran, before it can get to the starchy endosperm and germ. Refined grains, since they lack the bran, pose much less of a challenge and are quickly metabolized into glucose, or blood sugar.
Metabolism in Detail
Whole grains begin to be digested in your mouth. The action of chewing breaks apart the hulls, and an enzyme called amylase released from your saliva begins to chemically break down the food into smaller pieces. Once you swallow, the partially digested food travels to your stomach, where little to no carbohydrate digestion takes place. In the small intestine, the grains are further broken down by enzymes into simple sugars ready to be absorbed into the bloodstream. The sugar is delivered to the liver, where it is doled out to cells that need it for energy or stored for later use. Although we think of fiber in whole grains as indigestible, that's not really true. When it moves into the large intestine, some of the fiber is fermented, or digested, by bacteria in the colon.
Whole Grains, Metabolism and Blood Sugar
Because your body has to work harder to break them down, whole grains may give your metabolism a little boost. Registered dietitian Sarah-Jane Bedwell told The Huffington Post that this can be attributed to the thermic effect of food -- the internal heat created when the body works to break down foods that can actually burn calories. Whole grains' high fiber contents also make them an important part of every meal. Fiber helps slow digestion so that your blood sugar rises more steadily. This prevents the blood sugar fluctuations that can lead to energy crashes, moodiness and food cravings.
Choosing whole grains over refined varieties offers myriad benefits to your health. According to a study published in the "European Journal of Health" in March 2005, intake of whole grains is inversely associated with metabolic syndrome, a collection of risk factors that increases your chance of heart disease, diabetes and stroke. In another study published in "Journal of the National Cancer Institute," researchers found an association between whole-grain intake and reduced risk of one type of colon cancer.