In order for a cell to acquire nutrients to use in energy production, the nutrients must be transported across the cell’s membrane. The hormone insulin is used to activate the transportation of nutrients into cells and is considered the “storage” hormone. Insulin secretion causes the uptake of amino acids, free fatty acids, and especially glucose to be
increased.
When one ingests carbohydrates, their blood glucose level, also referred to as their blood sugar level, is elevated, which causes insulin to be secreted. It is insulin’s job to return the blood glucose level back to a normal, homeostatic range.
When insulin is secreted, the breakdown of stored nutrients (glycolysis, lipolysis etc.) is turned off and the storage of nutrients is turned on. It would be counterproductive
to breakdown stored glycogen to obtain glucose when glucose has just been ingested and is now in the bloodstream. This fact is important because when insulin is secreted, fat breakdown and oxidation is turned off! Therefore when one wants to increase fat oxidation to its fullest, insulin secretion needs to be limited.
The amount of insulin needed to return blood glucose levels back to normal after ingesting a given amount of glucose is called insulin sensitivity. Individuals have differing insulin sensitivities based mainly on their diet, activity level, and genetic factors. In most cases, an ectomorph and mesomorph are more insulin sensitive than endomorphs; this is one of the primary factors that cause endomorphs to gain fat more easily than ectomorphs and mesomorphs. An endomorph will need to secrete more insulin to shuttle a given amount of glucose into cells and therefore fat oxidation will be halted for longer than an ectomorph or mesomorph. Therefore, an endomorph needs to
pay more attention to insulin secretion in order to keep gains lean.
The body does not like it when glucose is floating around in the blood stream, so after a meal the body increases the uptake and oxidation of glucose to get rid of it. Over time, consistently elevated blood glucose levels can lead to cells becoming insensitive to insulin or insulin resistant, meaning more insulin most be secreted to return blood glucose levels to normal and therefore fat oxidation is blunted longer If endomorphs must secrete more insulin to return blood glucose levels to normal, then they are at a disadvantage when its comes to staying lean while bulking because fat oxidation will be turned off longer for them than an ectomorph or mesomorph
and there is a greater chance for them to become more insulin resistant.
Before we continue I want to point out why there is great emphasis placed on muscle glycogen levels. Building new muscle proteins and adding inches to your arms is not a priority to the body. The body will not create new muscle proteins when it senses it is in need of energy. When muscle has a lot of glycogen, the body senses it has enough “extra” energy and can build new muscle proteins effectively. If muscle glycogen levels
are depleted, the body must replete glycogen stores in addition to increasing protein synthesis, both of which require energy and nutrients. Therefore, when gaining muscle is
your goal, you want to have adequate muscle glycogen stores so more energy can be focused on protein synthesis. Now this is an isolated examination of muscle growth because there are many other factors besides glycogen stores that govern whether one
gains muscle or not. Let’s take a deeper look at the metabolic factors affecting glucose
uptake.
Insulin promotes glucose uptake through the synthesis and translocation of the GLUT-4 glucose transporter, found on skeletal and cardiac muscle cells and adipocytes.
In the absence of insulin, the GLUT-4 transporters lay under the cell’s surface. When insulin is secreted, the GLUT-4 transporter translocates to the cell’s surface allowing glucose to enter into the cell. There are other glucose transporters, but the GLUT-4 transporter is our primary concern. Enzymes are protein molecules that catalyze (speed up) metabolic reactions. In the case of glucose uptake, there are two we will examine: hexokinase and glucokinase.
The enzyme hexokinase is found in skeletal muscle and promotes glucose uptake independently of blood glucose levels. Hexokinase has a high affinity for glucose, which allows muscle to take up glucose from the blood even when blood glucose levels are low. Once the muscle has the glucose, it keeps it for itself and the muscle does not release glucose back into the bloodstream. Insulin secretion further enhances glucose uptake in addition to hexokinase’s actions. The enzyme glucokinase is found in the liver and is activated when blood glucose levels are increased. Contrast to skeletal muscle, the liver is in service to all other cells of the body, so when it senses other cells need glucose it releases glucose and sends it to the other cells. Skeletal muscle holds on to its glucose for itself but the liver releases its stored glucose for other cells to use when they need it. Hexokinase is basically acting all the time to give muscle glucose but glucokinase is only acting in the presence of high blood glucose levels. What does all of this mean? It means that you do not need to jack insulin through the roof for your muscles to get glucose! In fact, it gets even better.
Exercise, especially resistance training, has been shown to increase GLUT-4 translocation on skeletal muscle in the absence of insulin, meaning after your lift weights you do not need insulin for your muscles to uptake glucose. While insulin will certainly enhance the anabolic response of a meal post workout, slamming 100 grams of dextrose (pure glucose) is not needed since skeletal muscle is already able to uptake glucose in the absence of insulin after a workout. Increasing the glucose content of skeletal muscle (in the form of glycogen) is beneficial for gaining muscle, but remember that GLUT-4 transporters also exist on fat cells and therefore insulin secretion promotes the storage of glucose in both skeletal muscle and fat cells. Therefore, one needs to increase the storage of glucose in skeletal muscle and decrease the storage of glucose in fat cells; this can be done by consuming low glycemic carbohydrates such as oatmeal post-workout instead of high glycemic carbs like dextrose.
In summary, it is not necessary to jack blood glucose and insulin levels through the roof in order to replenish glycogen and gain muscle. Skeletal muscle is able to uptake glucose whenever it needs it. In addition, skeletal muscle is primed to uptake glucose after exercise. By controlling your insulin levels you can gain lean mass while keeping fat gains to a minimum.
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