28.14 : Metabolic States of the Body: The Absorptive State

During the absorptive state, which lasts approximately four hours after a meal, the body absorbs nutrients from the gastrointestinal tract. The carbohydrates, proteins, and lipids we consume are broken down into monosaccharides, amino acids, and free fatty acids for absorption. While carbohydrates and proteins are absorbed as-is, lipids are absorbed in their broken-down forms and then re-esterified into triglycerides within enterocytes before being packaged into chylomicrons. These absorbed nutrients are used for immediate body requirements or stored for later use.

In the liver, absorbed monosaccharides are converted to glucose for energy production or stored as glycogen. Any excess glucose is converted to fat as very low-density lipoproteins (VLDLs) and is stored in the adipose tissue. VLDLs are then broken down by lipoprotein lipase, which releases free fatty acids. These fatty acids are taken up by adipose tissue, where they are re-esterified into triglycerides for storage. Apart from VLDLs, adipose tissues also store triglycerides after a small fraction of lipids is used for energy production.

Amino acids are used for protein synthesis, growth, and repair. Some amino acids can be deaminated in the liver to produce keto acids for ATP production or synthesis of glucose and fatty acids.

After a protein and carbohydrate-rich meal, there is a rise in blood glucose and amino acids, and the small intestine secretes glucose-dependent insulinotropic peptides. As a result, there is an increase in insulin production by the pancreas. Insulin helps to stabilize blood glucose levels by promoting cellular glucose uptake and glycogen synthesis while inhibiting glycogen breakdown and gluconeogenesis.