Absorptive state
Absorptive state is the period in which the gastrointestinal tract is full and the anabolic processes exceed catabolism. The fuel used for this process is glucose.
Nutrient processing in the absorptive state
Carbohydrates - Simple sugars are sent to the liver where they are converted to glucose. The glucose then travels to the blood or is converted to glycogen and fat (triglyceride). The glycogen and fat will be stored in the liver and adipose tissue, respectively, as reserves for the post-absorptive state. The remaining glucose is taken in for use by body cells or stored in skeletal muscle as glycogen.
Triglycerides - Chylomicrons, the main product of fat digestion, are first broken down to fatty acids and glycerol through hydrolysis using Lipoprotein lipase. This allows them to freely pass through capillary walls. Most of this will be reconstituted as triglycerides and stored in adipose tissue. The rest is used for energy in adipose cells, skeletal muscles, and hepatocytes. In a low carb environment, other body cells will also begin to use triglycerides as energy sources.
Amino Acids - The liver deaminates amino acids to keto acids to be used in the krebs cycle in order to produce ATP. They may also be converted to fat stores. Some are used to make plasma proteins, but most leave through liver sinusoids to be used by body cells to construct proteins.
References
- Marieb, Elaine M. Human Anatomy and Physiology. San Francisco: Pearson Benjamin Cummings, 2004., 972-973