Most foods are combinations of carbohydrates, proteins and fats. Carbohydrates -- often nicknamed carbs -- are found in a wide variety of foods and usually contribute sweetness. This makes sense because most carbs are essentially sugars of varying lengths. The chemicals that convert complex chains of sugar into simple blood sugar are called enzymes, and you need more than one to do the job properly.
Carbs are compounds made of saccharides, which is a fancy biochemical name for sugars. Some polysaccharides -- such as fiber -- are long, complex chains that are very difficult to digest into usable sugar. Other types -- such as sucrose -- are shorter disaccharide molecules that are easily converted to glucose, which is the simple sugar that travels in your bloodstream and gets distributed to all your cells in order to power your body. Carb-rich foods such as baked goods and fruit tend to taste sweet and usually provide a quick boost of energy. On the other hand, eating too much carbohydrate at a time causes blood glucose and insulin levels to spike, which leads to the infamous “sugar crash.”
The conversion of carbs into smaller chains of sugar starts in your mouth with the release of saliva. Saliva is mainly water, but it also contains enzymes such as alpha-amylase. Alpha-amylase, also called ptyalin or salivary amylase, starts the conversion of starch -- a polysaccharide -- into a disaccharide sugar called maltose. Maltose cannot be absorbed and used by your body, so it continues its journey towards your stomach. Other disaccharides such as fructose, commonly called fruit sugar, and lactose -- the main sugar in milk products -- are essentially unchanged in your mouth.
Alpha-amylase stops working in the acidity of your stomach, but another similar enzyme called stomach or gastric amylase is activated and takes over. Stomach amylase finishes converting starch into maltose. Foods rich in starch include whole grains, brown rice, beans, lentils, potatoes and most other root veggies. Again, the other disaccharides are virtually unchanged in your stomach. Most carb-rich food such as bread, pasta and fruit spend an hour or less being digested in your stomach, although the presence of protein or fat significantly increases the transit time.
As the disaccharide sugars leave your stomach and enter your small intestine, many different enzymes are released and start to cleave and convert them into simple glucose. For example, maltose is converted by maltase enzyme, lactose by lactase and sucrose by sucrase. Glucose, which is the end-goal of carb metabolism, is readily absorbed through the intestinal wall and directly into your blood. Fructose is also absorbed, but it’s sent to your liver for further processing into glucose or storage as either glycogen or adipose fat.
- Human Biochemistry; Charles Dreiling
- Human Metabolism: Functional Diversity and Integration; J. Ramsey Bronk