As mentioned, monosaccharides (glucose, fructose, and galactose) are absorbed into the body by crossing the wall of the small intestine and entering circulation via a special blood vessel called the portal vein. As the portal vein carries blood from the digestive tract directly to the liver, the liver gets the first shot at the absorbed monosaccharides. The liver is able to pull most of the galactose and fructose from our blood as well as a respectable portion of the glucose (see Figure 4.2). However, much of the glucose continues past our liver and enters the general circulation where other tissue will have a shot at it. This increases the concentration of glucose in the blood from a normal or "fasting" level of 70 to 100 milligrams up to 140 milligrams of glucose per 100 milliliters of blood or higher.
How Does Our Body Respond to the Rise in Blood Glucose?
The concentration of glucose in the blood is very tightly regulated. When the level of circulating glucose climbs above the normal fasting level, the pancreas releases the hormone insulin (see Figures 4.2 and 4.3). Insulin will interact with receptors on muscle cells and fat cells and promote the movement of glucose into these cells. Because skeletal muscle and fat cells together tend to make up more than half of our total body mass, the net effect is a fairly rapid lowering of the glucose concentration. Insulin increases the movement of glucose in these cells by increasing the number of glucose transport proteins on their plasma membranes. As the level of glucose returns to the normal fasting level, the pancreas responds by releasing less insulin into circulation.
All cells in our body will continuously take glucose from our blood throughout the day to help meet their need for energy. However, after a meal, the liver, muscle, and fat cells will take a lot more glucose out of the blood than they immediately need. This allows blood glucose levels to quickly return to a normal fasting concentration.
Increased blood glucose levels causes the release of insulin to process, use, and store carbohydrate.
What Does Our Body Do with the Glucose from a Meal?
Insulin directs muscle, fat tissue, and the liver to use glucose, fructose and galactose as the primary fuel. This allows for a lot of carbohydrate entering the body from a meal to be used for energy immediately. In
Figure 4.3 Relative levels of the major metabolic hormones during and right after a meal (fed), more than 8 to 12 hours after a meal (fasting) and during sustained moderate to higher intensity exercise. (Glucagon levels may increase during exercise if blood glucose levels decline.)
addition, insulin directs muscle and liver, and to a lesser extent other tissue, to store extra carbohydrate as glycogen. Glycogen is composed of large branching links of glucose and is very similar to plant starch. However, only so much glycogen can be made and stored, since it is meant to be a short-term not a long-term energy reserve.
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