What Does Cortisol Do to Help Maintain Blood Glucose Levels During Fasting

Cortisol is often regarded as the "stress hormone." It is important to realize that fasting, especially prolonged fasting, is a form of stress—and stress results in the release of cortisol from the adrenal glands along with epinephrine mentioned in the previous question. Cortisol also supports the breakdown of glycogen and the conversion of amino acids, lactate,

Table 4.5 Actions Of Insulin, Glucagon, Cortisol, and Epinephrine in Carbohydrate Metabolism

Insulin Increases the uptake of glucose by our muscle and fat cells

Increases the synthesis of glycogen in our muscle and liver Increases fatty acid synthesis from excessive diet carbohydrate Decreases fat breakdown and mobilization from our fat tissue

Glucagon Increases glycogen breakdown in our liver

Increases liver glycogen-derived glucose release into our blood

Increases glucose manufacturing in our liver

Increases fat breakdown and mobilization from our fat tissue

Epinephrine Increases glycogen breakdown in our liver and skeletal muscle

(adrenaline) Increases liver glycogen-derived glucose release into our blood

Increases fat breakdown and mobilization from our fat tissue

Cortisol Increases muscle protein breakdown to amino acids which can

(stress circulate to the liver and be used for glucose production hormone) Increases liver glycogen-derived glucose release into our blood

Increases fat breakdown and mobilization from our fat tissue and glycerol to glucose in our liver. Because cortisol also promotes the breakdown of our body protein, especially skeletal muscle protein, it ensures a supply of amino acids for conversion to glucose in our liver (Figure 4.5).

Exercise promotes the breakdown of carbohydrate stores in muscle.

What Happens to Stored Carbohydrate (Glycogen) During Exercise?

The hormone picture that develops during exercise is similar to the one discussed regarding a fasting period; however, there are relative differences. Epinephrine is released from our adrenal glands as a direct effect of exercise.

Adrenal glands / Cortisol

Liver

Glucose

AA AA AA

AA" AA

Skeletal muscle

I Glucose

Circulation

Figure 4.5 During fasting and endurance exercise (at least moderate intensity) cortisol causes the breakdown of muscle protein and some amino acids can be used to make glucose in our liver.

Quite simply, the greater the exercise intensity, the greater the epinephrine release. Epinephrine stimulates the breakdown of muscle cell glycogen (see Table 4.5 and Figure 4.3). This makes glucose available for the muscle cells hard at work. Epinephrine also promotes the breakdown of glycogen to glucose in the liver. Some of this glucose will then circulate to working muscle to provide support. Cortisol may also be released in response to moderate to intense exercise, particularly as the exercise becomes prolonged (for example, endurance cycling and running). Cortisol will also support the breakdown of glycogen as well as gluconeogenesis in our liver.

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