Metabolic flexibility refers to the ability of the body to switch its primary fuel source. Specifically it is the ability to transition between the two primary energy sources which is lipids (fats) and carbohydrates (sugars).
Normally, after a (carbohydrate rich) meal, the primary goal of the body is to normalize the hyperglycemia (elevation of blood sugar), which was driven by dietary carbohydrates. Logically, the body shifts towards a carbohydrate storage and carbohydrate metabolic state. A few hours postprandial, blood glucose reaches homeostatic (normal) values and the body relies primarily on glycogen (stored glucose chains) and therefore remains in a carbohydrate dominant metabolic state. Once glycogen stores begin to become depleted, the body shifts towards fat metabolism. Circulating stored fatty acids are broken down for energy in most cells of the body. Additionally, the liver breaks down lipids and converts them into ketones and some glucose for the brain and red blood cells, as these cells cannot carry out lipolysis.
In a healthy, or metabolically fit individual, these shifts happen readily and on a continuum. Unfortunately. metabolic inflexibility is a growing issue in North America which is very under looked.
What is metabolic inflexibility and why does it occur?
Inability to carry-out the metabolic shifts described above defines metabolic inflexibility (MI). MI individuals constantly remain in a carbohydrate driven metabolic state with impaired or defective fatty acid metabolism. It is stimulated by western eating habits and the Standard American Diet (SAD). When one ops to eat carbohydrate rich meals every few hours, the body is forced to use carbs. even the average 8-10 hour fast at night is not enough time to deplete glycogen. When this pattern is repeated for a period of years, the body learns to rely on only carbohydrates and completely down-regulated the never used lipolysis enzymes. Basically, if you don’t use it you lose it. The first sign of this is extreme hunger, hanger, fatigue weakness or other symptoms when skipping a meal. Despite having large quantities of stored energy as fat, the body cannot function optimally without a constant supply of glucose.
Why is metabolic inflexibility an issue?
One reason may be quite obvious; the inability to burn fat clearly leads to issues with weight management and obesity. Additionally, MI inflexibility is associated with several disease states including insulin resistance, metabolic syndrome, heart disease, diabetes and cancer. perhaps the most interesting of these is cancer. Cancer cells are largely metabolically inflexible and rely on glucose as its primary fuel source. Restoring metabolic flexibility, restores the body’s advantage over cancer cells.
What can be done?
Our body was designed to burn fat and glucose, therefore fat burning enzymes can be upregulated and restored by increasing the need for them. This is done through low-carb diet and fasting. The adaptation is not pleasant as it requires starving the body of the only macronutrient it knows how to use. However, onece metabolic flexibility is reversed, so is the risk of many diseases consistent dependence on food.