What Happens When Carbohydrates Are Digested?

Carbohydrates often get a bad rap. Blamed for ills such as weight gain and bloat, diabetes and cavities, carbohydrates are in fact your body's primary energy source. Carbs provide your cells with a ready-made form of fuel that powers biochemical reaction, pumps your heart and propels your body in motion. True, not all carbohydrates are created equal. Carbs packed with water and fiber provide more sustained energy than simple sugars, which can cause blood glucose to rise and then plummet, giving rise to cravings and weight gain. Regardless of type, carbohydrates are used as fuel or can be stored as an energy source for later use.

A woman is eating a bowl of granola. (Image: puhhha/iStock/Getty Images)

Digestion of Carbohydrates

Most carbs consist of large chains of sugar molecules. Your body must first break these chains down before they can be absorbed into your blood and used. Digestion starts in your mouth, where the enzyme salivary amylase breaks down starch, or complex sugars, into smaller molecules. Starch is the type of sugar that exists in foods such as grains and potatoes. Carbohydrates pass through your stomach essentially unchanged and enter your small intestine, where additional enzymes convert them into the simple sugars, glucose and fructose. These sugars then enter your bloodstream.

Used for Fuel

Of the macronutrients, protein, fat and carbohydrates, carbs are your body's primary energy source. Protein and fat can be used as energy, but only after your body burns the carbohydrate portion of your meal. As glucose and fructose, carbs travel through the bloodstream and are absorbed in all the tissues of your body. Fructose is freely absorbed, while glucose requires the help of a hormone called insulin, which drives glucose into your muscle cells so it can fuel muscle contraction. Within cells, glucose and fructose combine with oxygen to produce ATP, or adenosine triphosphate, the energy currency of the cell.

Saved for Later

Once your body's energy requirements are fulfilled, remaining dietary carbohydrate travels to the liver and muscle. There it is converted to glycogen. Glycogen is a long chain of glucose molecules, strung together. Glycogen provides your body with a storage form of energy to meet your energy needs between meals or during fasting. Glycogen is also used to maintain normal blood sugar levels and prevent hypoglycemia. Hypoglycemia, or low blood sugar, produces symptoms such as sweating and anxiety.

Stored as Fat

Your body stores about 500 g of glycogen in the liver and muscle. This is equivalent to 2,000 calories, and is able to meet your body's energy requirements for up to 18 hours without food. Dietary carbohydrate not required for glycogen storage is converted into fatty acids, which are stored in your fat cells as triglycerides. Triglycerides provide a concentrated source of energy. Your body burns stored fat only once glycogen is depleted. Your body evolved this mechanism to protect against prolonged periods of fasting. Now that we live in a time of plenty, fasting, or depriving yourself of calories, is usually intentional, in the form of dieting.

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