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While all of the organs in the body work together, some organs have important roles in energy usage and storage.

Pancreas

The pancreas produces several hormones in specific cell areas known as pancreatic islets (islets of Langerhans).  Glucagon is produced in alpha cells and Insulin is is produced in beta cells.  Glucagon and insulin directly affect blood sugar and work opposite to each other.  Insulin performs an anabolic function (synthesizing larger more complex modules from simpler ones, ie storing fat) where it stores glucose into adipose tissue while glucagon performs a catabolic function (breaking down of complex modules into smaller simpler ones, ie, burning fat) where it draws lipids out of adipose tissue and where they are ultimately converted them into ketones.  Interestingly, insulin affects the entire body because pretty much every cell has an insulin receptor.  Glucagon has no effect on muscle tissue.

Generally, high serum glucose levels cause the pancreas to secrete more insulin, which then signals other tissues like muscle and adipose tissue) to remove it.  Low serum glucose levels cause the pancreas to secrete more glucagon, which signals adipose tissue to release lipids.

Muscle Tissue

One of the effects of diabetes is that muscle tissue has trouble getting glucose delivered across its cell membranes.  Insulin can be considered a key that helps glucose to unlock the boundary and cross that membrane into the muscle cell.  Glycolysis is the process that allows cells to convert glucose to ATP for cellular fuel.

Adipose Tissue

Fat in our bodies is stored in fat cells (Adipocyte) and the number of fat cells in adults is generally constant over their lifetimes.  The number is set during childhood and adolescence, although obesity can cause the number of adipocytes to increase and fasting can cause the number to decrease. As your body digests food, excess glucose is stored as glycogen in the liver and skeletal muscle tissue but neither has a high glycogen storage capacity.

High glucose levels cause an increase in insulin (the fat storage hormone) and a decrease in glucagon (the fat burning hormone), which triggers Lipogenesis to then store triglycerides in fat cells .  Insulin causes causes excess glucose to be stored in adipose tissue through lyposis.  Acetyl-CoA is produced by the breakdown of both carbohydrates and lipids, which is then used to manufacture and store fat in both adipose tissue and the liver through Lipogenesis.

Low glucose levels cause an increase in glucagon and a decrease in insulin, which triggers the release of lipids from adipose tissue.  As a result, the liver will manufacture ketones from those lipids through ketosis.  The ketones in turn have an additional beneficial metabolic effect on adipose tissue in that it causes white fat to behave more like brown fat. The body uses the mitochondria  in brown fat to generate heat and ketones cause the fat tissue to "waste" its stored energy as heat. Normally, mitochondria are "coupled" in that they only use as much fuel (glucose or ketones) as is necessary to manufacture ATP.  Ketones tend to cause adipose tissue mitochondria to be "uncoupled" so that that they instead generate heat.  Something to think about if your hands and feet are always cold.

Liver

The liver performs many tasks in managing energy usage in the body.  The liver manufactures glycogen through Glycogenesis when IGR is high, which is used as a glucose energy reserve.  When IGR is low, the liver uses Glyconeogenesis to manufacture glucose sourced from adipose tissue to maintain a minimum serum glucose level and it also uses the lipids in adipose tissue to manufacture ketones, which is the body's alternative fuel.  While most tissues can use either glucose or ketones as fuel, only erythrocytes (red blood cells) must rely on glucose for fuel because they lack mitochondria and this conserves the oxygen they carry.

The body also excretes ketones through respiration (high ketone levels can result in "ketone breath") and urine, which adds another aspect to weight loss.

Insulin to Glucagon Ratio

The Insulin to Glucagon Ratio (IGR) determines whether the metabolism has a anabolic or catabolic tendency.  The biochemical effects of insulin and glucagon are often antagonistic.

Process Insulin Dominates
(High IGR)
Glucagon Dominates
(Low IGR)
Glycogenesis Increased Inhibited
Gluconeogenesis Inhibited Increased
Glycolysis Increased [no effect]
Glycogenolysis Inhibited Increased
Ketogenesis Inhibited Increased
Lipogenesis Increased Inhibited
Protein Synthesis Decreased protein degradation Increased amino acid uptake by liver;
Decreased amino acids in plasma

 

Effects of Insulin and Glucagon on body tissues

Metabolic State Muscle Tissue Adipose Tissue Liver
Anabolic
High IGR
  • Glycogenisis
  • Protein Synthesis
  • Lipogenesis
  • Lipogenesis
  • Glycogenesis
Catabolic
Low IGR
  • [no effect]
  • Lipolysis
  • Brown Adipose Tissue Activation
  • Glycogenolysis
  • Gluconeogenesis
  • Lypolysis
  • Ketogenesis

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