Cortisol is a steroid hormone that the adrenal gland produces and releases in response to stress at various times throughout the day. Its main functions are to increase blood sugar, regulate the immune system and aid in the metabolism of energy. In response to intense and prolonged exercise, cortisol acts to preserve carbohydrate stores by impairing the entry of glucose into skeletal muscle and increasing alternate sources of fuel. Low-carbohydrate diets can be a problem under these circumstances.
Glucose Levels
Glucose is the main source of energy for the human body because it burns quickly and efficiently. However, the glucose stores in your body are quite small, so most of the glucose must come from carbohydrates in your diet. If glucose levels fall too low during exercise, your body can convert amino acids and fatty acids into glucose, thereby increasing blood sugar through a process known as gluconeogenesis.
Effects
Cortisol is generally undesirable under normal conditions. It robs amino acid stores and stimulates muscle catabolism, which is the breakdown of muscle protein for energy. Protein breakdown increases by as much as 5 to 20 percent in the presence of excessive cortisol. This is one reason why overtraining is generally discouraged. Cortisol also counteracts the effects of insulin and contributes to a state of insulin resistance. It plays a role in fat distribution and may cause excess abdominal fat, depending upon your genetic predisposition. Finally, cortisol is known to decrease bone formation.
Aerobic Exercise
Aerobic exercise that exceeds a duration of 45 minutes may automatically trigger the release of cortisol if you do not consume an adequate amount of carbohydrates during this time. The solution, therefore, is either to truncate your exercise sessions or maintain your blood glucose at reasonable levels through the ingestion of solid or liquid carbohydrates, even if it means raising your daily carbohydrate intake. This will counteract or diminish the effects of cortisol. For shorter periods of exercise, cortisol may not necessarily take effect as long as your carbohydrate stores provide enough energy.
Research
In two separate studies --- one published in the July 2002 issue of "Metabolism" and the other in the December 2008 issue of "Nutrition Research" --- low-carbohydrate diets did not significantly affect blood cortisol levels over long periods of time in a group of normal-weight individuals. One study lasted six weeks; the other lasted more than a year. This suggests little if any adverse metabolic consequences due to cortisol levels. Maintaining carbohydrate intake during long periods of endurance exercise appears to be the main concern for adherents of a low-carbohydrate diet.
References
- University of New Mexico: The Role of Cortisol in Concurrent Training; Rob Robergs and Len Kravitz; 2005
- "American Journal of Physiology - Endocrinology and Metabolism"; Effect of Cortisol on Energy Expenditure and Amino Acid Metabolism in Humans; B.J. Brillon, et al.; March 1995
- "Science Daily"; Stress May Cause Excess Abdominal Fat In Otherwise Slender Women, Study Conducted At Yale Shows; November 2000
- "Metabolism"; Body Composition and Hormonal Responses to a Carbohydrate-Restricted Diet; J.S. Volek, et al.; July 2002
- "Nutrition Research"; Long-Term Consumption of a Carbohydrate-Restricted Diet Does Not Induce Deleterious Metabolic Effects; P. Grieb, et al.; December 2008
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