The hormone insulin is secreted by the pancreas in order to reduce sugar levels in the blood. An insulin spike is created when you eat foods with a high glycemic index (GI),that is a large amount of simple sugar. The simple sugars enter the bloodstream and insulin is needed to store the sugars as glycogen in the liver and the muscles. Insulin also stops protein breakdown after a workout and increases amino acid uptake into the body.
Muscles have a high demand for glucose and utilise an enzyme called hexokinase. This 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. When the glucose reaches the muscle, it remains there and is not released back into the bloodstream. Thus skeletal muscles do not need insulin in order to obtain much needed glucose, however any insulin secreted will cause additional glucose to be taken up.
Another enzyme called glucokinase found in the liver works when levels of glucose in the blood rise. Unlike skeletal muscle, the liver can release glucose when the cells require it. This enzyme only acts in the presence of high blood glucose levels.
Exercise, especially resistance training, has been shown to increase glucose uptake for skeletal muscle in the absence of insulin, so that you do not necessarily need to consume simple sugars in your post workout meal in order to cause an insulin spike so that your muscles will uptake glucose.
While insulin will certainly enhance the anabolic response of a meal post workout, glucose is not actually needed since skeletal muscle is already able to uptake glucose in the absence of insulin after a workout.
For optimum results however, consuming a meal post workout with simple sugars and protein, will bring glucose into the muscles and allow an increase the uptake of amino acids into the body.
Insulin not only controls the uptake of glucose into cells but also has an impact on fat oxidation and storage. When blood glucose and insulin levels are low, fat is the main fuel burned for energy. This is why a low intensity cardio workout on en empty stomach before breakfast can be so effective at tapping into the fat stores when glucose and insulin levels are low. But when blood glucose and insulin levels are high, fat burning is blunted and glucose oxidation is elevated.
When the body senses there is glucose in the bloodstream, it wants to return blood glucose levels back to a homeostatic level. In order to do this the body must get rid of the glucose, which is accomplished by increasing glucose oxidation and storage.
Since the body is focusing on storing nutrients, it would not make sense for fatty acids to be released from adipocytes because they would not be burned. Therefore it is important that blood glucose levels return to normal quickly so the oxidation of fat can once again become the primary source of energy. This can be done by controlling your carbohydrate intake and controlling your insulin secretion by consuming less high GI foods; simple sugars, and more slow release low GI complex carbohydrates.
In the presence of high blood glucose and insulin, hormone sensitive lipase (HSL), the enzyme that catalyzes the liberation of fatty acids from fat cells) cannot act on stored fat. Therefore, fatty acids cannot be liberated from fat cells and fat oxidation is put on the backburner while glucose oxidation and storage is made a priority. Insulin is termed an anti-lipolytic hormone because it blocks lipolysis – the breakdown of stored triglycerides fat into fatty acids.
In addition to blunting fat oxidation, insulin secretion stimulates fat synthesis in the liver and increases fat uptake by fat cells.
Insulin spikes can also be caused by prolonged periods of sitting, such as in an office environment. Studies show that a short walk to the bathroom or the water cooler can alleviate this by getting away from your desk every 20mins after one hour of sitting still.
As useful as insulin spikes are at aiding the uptake of amino acids into the body, a large number of spikes throughout the day can cause the storage of fat and decrease fat oxidation. Regular exercise and a diet consisting of complex carbohydrate allows the body a steady flow of glucose without spiking the blood and increasing fat storage.
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