Where there is no energy, there is death. A fundamental requirement for maintaining our physical existence is the ability to generate energy for the body. Let’s talk about all the sources our body uses to create energy.
Let me walk you through 4 metabolic processes that the body uses to create energy. Energy is so crucial to survival that the body has redundant systems in the event of failure of any one system.
When we understand how the body generates energy we can manipulate this information to inform diet, lifestyle and medication choices, if necessary, to optimize blood glucose levels.
4 Energy Generating Processes
Glucose as Energy
The body’s primary source of energy is glucose. To support biological processes there should be glucose in the bloodstream at all times.
When we eat (consumption) the digestive tract breaks down complex food into simple molecules. Glucose is derived from the metabolism of carbohydrates in the diet. Once absorbed into the bloodstream the body is in a constant process of maintaining a strict concentration of blood glucose.
After we eat (post prandial) the normal concentration of blood glucose, in an otherwise healthy adult, should be less than 140mg/dL.
When there is no consumption (fasting) the normal blood glucose is maintained at 70-99mg/dL.
This delicate balance of receiving and eliminating glucose is maintained by insulin and glucagon.
Insulin: How the Body Recognizes Glucose
Glucose is useless to the body without insulin. Your blood could literally be saturated with glucose yet your cells could be starved if there is not enough insulin present (as we see in diabetes).
Glucose is the body’s energy source but insulin allows access to create energy.
Insulin is a hormone that is secreted by the beta cells of the pancreas in response to the presence of glucose in the blood.
Since there needs to be a constant presence of glucose in the bloodstream there is also a constant, basal release of insulin from the pancreas.
When we eat, blood glucose levels increase. The pancreas will also increase its release of insulin to maintain those ideal blood glucose concentrations.
Insulin binds to insulin receptors on cells changing their conformation in a way that allows them to express the glucose transporter GLUT 4 on the cell surface. This makes it possible for cells to take up glucose from the bloodstream. The glucose transporter GLUT4 is found primarily in muscle, adipose and liver cells. Without insulin, glucose will not enter these cells.
This is why insulin resistance is also harmful to the body.
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3 Pathways of Glucose
When glucose is available the body will meet its immediate energy needs and store any extra glucose for later use as either glycogen or fat.
- Immediate use in muscle
- Stored in liver as glycogen
- Stored in adipose cells as fat
Glucagon: The Low Glucose Alarm
There can be an actual lack of glucose being supplied to the body in cases of starvation or a perceived lack of glucose when there is glucose in the bloodstream but the body is unable to utilize it (insulin absence or resistance).
Regardless of the cause, the body will activate one of its many redundant systems for creating energy starting with an increase in the release of glucagon (also from the pancreas) when blood glucose is too low.
This article provides an in depth look into the relationship between insulin and glucagon in regulating the body’s energy sources.
Like glucose there is a basal release of glucagon even when blood glucose is normal. Normal blood glucagon level is a very low: “pico” concentration (25-50pg/ml).
Increased glucagon concentration initiates a series of energy generating processes, the first being glycogenesis.
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Glycogenesis
Any “extra” glucose not needed for immediate use by the body is first stored as glycogen in the liver.
The body is limited in the amount of glycogen it can store. The average person can store about 500grams of glycogen which translates to ~2000 calories. The average caloric need per day ranges from 1600-3000 kcal.
Glucagon will signal the liver to convert stored glycogen into glucose to generate energy in the body. This is glycogenolysis. Glycogenolysis will maintain blood glucose concentrations in the absence of consumption.
Once we have depleted glycogen stores the body’s secondary response will be gluconeogenesis as a source of energy.
Gluconeogenesis
Gluconeogenesis is the formation of new glucose molecules from non- carbohydrate sources including fatty acids, certain insulin sensitive amino acids, glycerol and lactate.
It occurs primarily in the liver and too a lesser extent in the kidneys.
If gluconeogenesis is prolonged the body will start to breakdown fat tissue (lipolysis) to access more fatty acids so that the body can continue to use it in gluconeogenesis as an energy source. Gluconeogenesis is the basis of a calorie deficit diet for weight loss which allows the body to create energy from fat instead of getting it from food consumption.
Ketogenesis
If there continues to be an absence of exogenous glucose the body will transition from using fatty acids as a substrate for glucose production to using fatty acids as a direct source of energy.
In ketosis, fatty acids are broken down by the liver into ketones that are released into the bloodstream and used instead of glucose to generate energy in the body.
Survival Pathways
Insulin ensures that blood glucose is not too high, glucagon ensures that blood glucose is not too low. This is why glucagon is referred to as counterregulatory to insulin.
These alternative sources of energy for the body (glycogenesis, gluconeogenesis and ketogenesis) are intended to be temporary buffers for maintaining blood glucose levels. These metabolic processes keep us alive until we can get to back to the obtaining glucose from exogenous sources. They are survival pathways.
When glucose levels are low our bodies go into “fight or flight” mode. In addition to glucagon other counterregulatory hormones like cortisol, vasopressin and adrenaline as also released. These are stress hormones.
Our bodies are not designed to function in these pathways for prolonged periods of time. In healthy adults short periods of ketosis has been used as a strategy for weight loss. In patients with diabetes, prolonged ketosis as an energy source leads to acute and life threatening ketoacidosis.
If you’ve found this unit helpful, I would love to hear from you. Leave a question or comment below.
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The information on this website is intended to be used solely for educational and informational purposes. While the content may be about specific medical and health care issues, it is not a substitute for or replacement of personalized medical advice and is not intended to be used as the sole basis for making individualized medical or health-related decisions.
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