Section 3 The hormonal response to aerobic exercise

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Several hormones are affected by aerobic exercise depending on exercise intensity and duration. Overall, the hormonal response to aerobic exercise is very similar to what is seen during a ketogenic diet. Levels of anabolic hormones, such as insulin, go down while levels of catabolic hormones, such as the catecholamines, cortisol and growth hormone, go up. The major hormones which are affected by aerobic exercise are discussed below.

The catecholamines are adrenaline and noradrenaline and both are involved in energy production. The catecholamines raise heart rate and blood pressure, stimulate fat breakdown , increase liver and muscle glycogen breakdown, and inhibit insulin release from the pancreas (49). Both adrenaline and noradrenaline increase during exercise although in differing amounts depending on intensity of exercise. Noradrenaline levels rise at relatively low exercise intensities. This stimulates FFA utilization in the muscles but has little effect on the breakdown of liver and muscle glycogen.

Adrenaline levels increase more slowly with increasing exercise intensity until the lactate threshold (LT) is reached at which time levels increase quickly (3). This point is sometimes called the adrenaline threshold and corresponds very well with the lactate threshold (50). As adrenaline is one of the primary hormones responsible for stimulating the liver to release glycogen, raising adrenaline levels by training at or above the LT is one way to quickly empty liver glycogen to establish ketosis (this topic is further discussed in chapter 21).

After exercise, adrenaline levels decrease quickly but noradrenaline levels may stay elevated for several hours depending on the intensity and duration of exercise. Noradrenaline stimulates calorie burning in muscle cells and the elevations in NA following exercise may explain part of the post-exercise calorie burn (see chapter 22 for more details on this topic).

During aerobic exercise, insulin levels drop quickly due to an inhibitory effect of adrenaline on its release from the pancreas (3,49). The drop in insulin allows FFA release to occur from the fat cells during exercise. Lowering insulin is also important for establishing ketosis. Despite a decrease in insulin levels during exercise, there is an increased uptake of blood glucose by the muscle. An increase in glucose uptake with a decrease in insulin indicates improved insulin sensitivity at the muscle cells during exercise.

Increased insulin sensitivity occurs because muscular contraction causes a specialized receptor called the glucose transporter-4 (GLUT-4) receptor to move to the cell membrane. For individuals suffering from hyperinsulinemia (overproduction of insulin), the increase in insulin sensitivity means that carbohydrates can be consumed during exercise with a minimal increase in insulin.

Insulin levels can not go up during exercise when glucose is consumed due to the inhibitory effect of adrenaline on insulin secretion. As soon as exercise ceases, insulin returns to baseline depending on blood glucose levels. So the amounts of carbs consumed will have to be determined through trial and error to avoid a insulin reaction after exercise. This topic is discussed in greater detail in chapter 11.

As the mirror hormone of insulin, glucagon levels increase during aerobic exercise (49). Thus the overall response to aerobic exercise is pro-ketogenic in that it causes the necessary shift in the I/G ratio to occur.

As duration and intensity of aerobic exercise increases, the body releases cortisol to further stimulate liver glycolysis (to maintain blood sugar) and stimulate FFA release. Growth hormone is also released to help stimulate FFA release (49).

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