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Definition of Absorptive State: The absorptive state refers to the period when nutrients from a meal are being absorbed into the bloodstream. This occurs shortly after eating.
Post Absorptive State: This state occurs when food is no longer being eaten, and the body relies on stored energy.
Energy Utilization: In the post-absorptive state, the body starts to use stores of energy, primarily glucose and fats. If energy is still needed and glucose stores are low, the body also processes fats and proteins for energy.

Liver: Plays a critical role in metabolism, utilizing nutrients and converting them as necessary.
- Nutrients such as monosaccharides and amino acids are delivered to the liver via the hepatic portal vein.
- Lipids (chylomicrons) from the small intestine do not directly enter the blood; they enter the lymphatic system before reaching the bloodstream and liver.

Energy Store Utilization:
- In the absence of immediate energy (food), the body taps into glycogen stores, then mobilizes fats when glycogen is depleted.
- Gluconeogenesis: This is the synthesis of glucose from non-carbohydrate sources, primarily occurring in the liver and kidneys. This involves converting fats and amino acids into glucose, which is crucial for organs such as the brain and kidneys that primarily use glucose for energy.
- Beta-Oxidation: Fatty acids undergo this process to produce energy for the cells. Laurates are broken down, releasing free fatty acids into the bloodstream, while glycerol is converted to glucose in the liver.

Carnivores and Omnivores: The metabolism involves primarily glucose and some other carbohydrate forms (galactose and fructose) that are converted to glucose in the liver.
Herbivores: Continuous nutrient absorption means less distinction between the absorptive and post-absorptive states; they primarily utilize fiber fermentation products for energy.

Newly born mammals primarily rely on glucose; hence, fats are not the main source of energy in growing or lactating animals.

Amino Acids: Derived from proteins; some are converted to ketoacids and subsequently to urea by the liver, which is excreted in urine. In ruminants, urea is secreted in saliva to help synthesize proteins in the rumen.
Glucose Production: The liver converts amino acids and glycerol into glucose for energy, which then enters the bloodstream. Excess glucose is converted to glycogen or fat for storage.

State of Energy Needs: In this state, when the body has not consumed food, it mobilizes stored nutrients.
Liver Function: Important for maintaining plasma glucose levels; it releases glucose from glycogen stores and produces new glucose through gluconeogenesis.
- Main substrate for gluconeogenesis includes lactate and certain amino acids. It is vital during periods of fasting or starvation.
Glucose Usage: Most cells depend on glucose for energy; especially brain cells and red blood cells.

Definition: The synthesis of glucose from non-carbohydrate sources.
Role of Amino Acids: Amino acids can be converted into glucose or intermediates for energy production.
Carnivores vs. Herbivores: Different metabolic pathways in gluconeogenesis.
- Carnivores primarily rely on fats and proteins for energy due to a diet low in carbohydrates.
- Herbivores utilize fatty acids generated from fermentation processes.

Hormonal Regulation: Two main hormones involved in metabolism are insulin and glucagon.
- Insulin: Produced in the absorptive state. It promotes glucose uptake and storage as glycogen in liver and muscles, and thus acts as an anabolic hormone.
- Glucagon: Predominates in the post-absorptive state, activating the breakdown of glycogen to release glucose.
Adrenaline: This hormone increases during stress or exercise, leading to increased glucose mobilization from glycogen stores.

Definition: This cycle describes how glucose released from liver glycogen is utilized by muscles and converted back to lactate, which is then sent back to the liver for further processing.

Metabolic Pathway: During the absorptive state, excess glucose is stored as triglycerides. The body mobilizes these triglycerides during starvation, using lipid breakdown to produce energy.
- Very Low-Density Lipoproteins (VLDLs): Carry fatty acids from adipose tissue to the liver for oxidation and energy production.
- Ketone Bodies: In prolonged fasting, excessive fat mobilization can lead to increased ketone bodies, impacting blood pH and potentially leading to ketosis.

Insulin facilitates the storage and synthesis of nutrients, while glucagon promotes the mobilization of energy stores. The balance of these hormones regulates metabolic pathways effectively across different physiological states.