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Fructose, Glucose, and Sucrose: How Your Body Digests These Common Carbohydrates

Molecular Structure Comparison of Glucose, Fructose, and Sucrose

Three of the most ubiquitous carbohydrates in our diet and their basic structures.
Three of the most ubiquitous carbohydrates in our diet and their basic structures. | Source

The Differences Between Fructose, Glucose, and Sucrose

There is a lot of attention being paid to low-carbohydrate diets nowadays, but what about the physiology behind some of the more common carbohydrates? Speaking from the atomic level, carbohydrates are huge molecules made of repeating carbon and hydrogen and oxygen groups, thus the carb + hydra- name (See image to right on Linear and Branched Macromolecules). Three of the more ubiquitous subunits of carbohydrates in our diet include glucose, fructose, and sucrose (See image to right on Molecular Structure Comparison). Let's take a look at each one.

  • Glucose This carbohydrate sub-unit is found in starchy foods such as pasta, potatoes, and breads. It is considered a 'monosaccharide', in that it is one carbohydrate molecule.
  • Fructose Fructose is another common carbohydrate. It is commonly found in honey, flowers, and fruits. Fructose is also a monosaccharide.
  • Sucrose Also known as table sugar, it is found in fruits and vegetables. Sucrose is a 'disaccharide', consisting of two sub-units: a fructose and a glucose molecule linked together. This changes how it is digested in your body, and can be considered the same as eating glucose and fructose, as it is broken down into these monosaccharides during digestion.


Diagram of Digestive System

Food travels down the esophagus into stomach, then into the small intestine where most absorption takes place. From the small intestine, nutrients travel via blood vessels to the liver. Remaining food travels to large intestine prior to excretion.
Food travels down the esophagus into stomach, then into the small intestine where most absorption takes place. From the small intestine, nutrients travel via blood vessels to the liver. Remaining food travels to large intestine prior to excretion. | Source

Digestion Inside Your Body

Let's say we have some toast with jam on it for breakfast. The toast, made of wheat, contains some glucose while the jam contains some fructose and some added sucrose. In your mouth, enzymes begin the process of chemically breaking down the macromolecules into smaller molecules.

(Note that chemically breaking down is different from mechanically breaking down, which would be like separating a raw egg into different bowls, whereas frying the egg results in a chemical breakdown of the egg proteins resulting in a white appearance.)

After the macromolecules travel through your esophagus and stomach, it enters the small intestine where it is digested further by pancreatic enzymes. Enzymes existing on the wall of the small intestine also breakdown the macromolecules into single subunits. Thus, sucrose is broken down into the fructose and glucose subunits. It is as this point that the body can then absorb the sugar molecules.

Enzyme Action and Hydrolysis of Sucrose

Absorption Inside Your Body

Absorption Cells in the walls of your small intestine contain channels that when presented with the sugar molecule, open and transport the molecule into the cell and out the other side into your bloodstream. Once in the blood, the monosaccharides fructose and glucose travel to the liver. In the liver, fructose is turned into glucose, one of the body's preferred sources of energy. Red blood cells can only use glucose for energy, and your brain cells prefer glucose over other energy molecules, such as ketones.

The liver has a very important role. It decides what to do with the glucose molecules based on amounts around the body. It has three options:

  1. The liver can store glucose as glycogen, a form of stored energy that can be used at a moment's notice. Glycogen can be broken down into glucose. There is only a limited amount of space in glycogen stores.
  2. The liver can convert excess glucose to fat, which is a more long-term storage. At this point, the glucose molecules are forever turned to fat and cannot be used again as glucose. Fat is a more steady, slow burning fuel. Unlike glycogen stores, glucose can be stored as fat in a limitless amount, which is where people can get into trouble.
  3. The liver can release glucose through the blood to other parts of the body that need it.

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Three Options Liver Has to do with Glucose

The liver can store excess glucose as fat or glycogen, or it can send it off to other cells in the body that need it.
The liver can store excess glucose as fat or glycogen, or it can send it off to other cells in the body that need it. | Source

Carbohydrates and Your Health

Now that you know an overview of how carbohydrates are absorbed into your body, let's examine some of the results of eating too much of too little carbohydrates.

Too Many Carbohydrates As mentioned earlier, glucose is an important source of energy for your body, especially for your red blood cells and your brain cells. But, too much can be a bad thing. With all food you eat, when you consume more than your body needs for energy, it is stored as fat. Too much fat is associated with a hefty list of health problems: heart disease, diabetes, joint problems, fatty liver disease, and on and on. A good way to avoid overeating carbohydrates is to eat unprocessed whole grains. For instance, groats (buckwheat kernels), whole corn, oatmeal, popcorn with limited salt and/or butter additions, barley, and quinoa are all examples recommended by the United States Department of Agriculture website, Tips to Eat Whole Grains. When you eat whole grains, you will fill up more quickly and eat less. Processed grains, such as in white bread, will be digested quicker and easier to eat mechanically as well. These characteristics will encourage you to eat more than you should.

Too Few Carbohydrates It is rare to see someone that consumes too few carbohydrates, and generally you only see the most worrying results in those with eating disorders or those with Type 1 diabetes, also known as insulin-dependent or as child-onset diabetes. In this disease, the chain of events that allows for glucose to enter cells to be used for energy has been disrupted. The body thinks it is starving, and unable to take up the glucose, it switches to a different system for energy, ketosis. Ketosis uses fat molecules in the form of ketones for energy. One effect of this is acidification of the blood, which can be fatal. This is extremely rare in those without type 1 diabetes, as glucose can be made from other fuel molecules, such as protein. There is no disruption in getting the glucose into cells, and thus it does not result in a total shift to ketosis (Volek & Phinney. The Art and Science of Low Carbohydrate Living. Beyond Obesity. p. 302.).

Sugar in Your Diet

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