How Metabolic Pathways Work

The metabolic pathways in the body are involved in the digestion and absorption of the carbohydrates, proteins and fats we eat, and are critical in the maintenance of our health and well being.

These pathways are utilized differently depending on whether the body is in a specific metabolic state. There are three states:

  • "fed state" (just after a full meal) or a
  • "fasting state" (no food for 2-10 hours)
  • "starved state" (no food for more than 2 days).

Metabolic Pathways

The "Fed" State

After you consume a meal, the three main nutrients in food (carbohydrate, fats and protein) are broken down in separate metabolic pathways and the associated processes:

  • Carbohydrates are broken down into glucose by various enzymes. Some are burned for immediate energy, but overall the level of glucose in the blood stream rises, which triggers an insulin release by the pancreas. The insulin acts to move the glucose into the cells for storage as glycogen or fat in the fat cells (adipose tissue).
  • Fats are digested in the small intestine through the action of pancreatic enzymes and bile from the liver. Pancreatic lipase breaks fat molecules into smaller pieces and bile acts as an emulsifier to facilitate the absorption of the resulting fatty acids. Complete digestion of one molecule of fat (a triglyceride) results in 3 fatty acid molecules and one glycerol molecule. Fatty acids can be packaged into lipoproteins in the liver, and then sent around the body to repair cell membranes, act as an energy source, make hormones, and act as a substrate of other body chemicals and tissues.. Any excess gets stored in the fat cells as triglycerides. When fats are used as an energy source, they are broken down in cells through a process called beta-oxidation.
  • Proteins are broken down into individual amino acids by various enzymes such as pepsins and trypsins. The amino acids are absorbed into the bloodstream and then used in body cells to form new proteins for energy. Amino acids that are in excess of the body's needs are converted by liver enzymes into keto acids and urea. Keto acids may be used as sources of energy, converted into glucose, or stored as fat. Urea is excreted from everyone’s body in sweat and urine.

The "Fasting" State:

The three main nutrients in food (carbohydrate, fats and protein) are broken down in separate processes, but all three nutrients are metabolized to a common product called acetyl-CoA. Acetyl-CoA is a major metabolic pathways player, and is an important part of the process which creates the energy molecule called ATP (adenosine triphosphate) in the Kreb’s cycle (or citric acid cycle) in each of the trillions of cells in the human body.

The fasting state begins about 2-4 hours after a meal, when blood glucose drops toward it's normal baseline. This drop in blood sugar causes insulin levels to also decline. Another hormone, glucagon, is released in response to the drop in glucose, and it triggers the release of fuels from storage:

  • Liver glycogen is released via glycogenolysis and it supplies glucose to the bloodstream.Glucose is used by the brain and red blood cells first.
  • Triglycerides flow out of the fat cells via lipolysis and free fatty acids are formed. The longer the fast, the more fatty acids are released. Fatty acids are the main fuel used overnight for the muscles and liver as we sleep. The liver also creates a small amount of ketone bodies from the fatty acids, and these can be used for fuel in the kidneys and muscles if needed.

The "Starved" State

  • After about 3 days of fasting, the stored glycogen in the liver and muscles runs out, and the liver will begin a process called gluconeogenesis. In this process, the liver breaks down lactate, the glycerol part of fatty acids, or amino acids from muscle tissues to create the glucose that the red blood cells (and to a lesser extent the brain) need.
  • The liver will also begin to create more ketone bodies through ketosis. The muscles continue to burn fatty acids, but decrease their use of ketones. The ketone bodies then build up in the blood stream to a level at which the brain begins to oxidize them for fuel. As the brain uses the ketones, it needs less glucose, so the liver decreases the rate of gluconeogenesis. This helps preserve muscle tissue as the body doesn't need to break down the amino acids to convert them to glucose.
  • Because of ketosis and the ketones, the human body can survive for long periods without eating.

How a Ketogenic Diet Uses the Metabolic Pathways

  • During a ketogenic diet, the lack of carbohydrate (glucose) being consumed mimics the condition of the "starvation state" metabolic pathway in the body, in that the liver creates more ketones since less glucose is available. And the best part is that there's no hunger, but you get the benefits of burning fat for fuel.
  • Protein intakes stay within normal ranges in the "fasting state", and so muscle loss is minimal, and lean body mass stays within healthy ranges.
  • Your blood lipids improve, because when you cut your carbohydrates, the triglyceride levels in your blood will drop significantly. And as you eat more saturated fat, your HDL cholesterol levels will go up. That's a good thing because it improves the ratio of triglycerides to HDL, which is the most important marker for heart disease risk. The closer your Triglyceride/HDL ratio is to 1.0, the better your cardiovascular health.

Done with Metabolic Pathways, back to Ketosis

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