What is Endurance Exercise?
Endurance exercise, better known as aerobic exercise or cardio, involves activities which are known to raise your blood pressure in order to meet oxygen demand for respiration. Activities such as walking, running, swimming and cycling all fall under this category.
Endurance exercise is usually adopted as a weight-loss technique, and has recently been increasingly used as a form of management for a variety of health conditions.
Energy Sources & Usage During Endurance Exercise
The human body uses different fuels as sources of energy depending on the intensity and duration of exercise. The three fuels used are:
When an individual is performing short-term, intense exercise, anaerobic metabolism of carbohydrates is the primary energy source. This means either glycolysis or the ATP-PC system is adopted as energy-providing methods (Powers and Howley, 2015).
The ATP-PC system can supply all the energy required for intense, short-term exercise lasting up to five seconds; energy dependence on glycolysis gradually increases as the intense exercise is prolonged from 5-45 seconds. Any intense exercise lasting longer than 45 seconds utilises aerobic and anaerobic systems as sources of energy; the ratio shifts as the duration increases (Powers and Howley, 2015). As the duration of exercise increases the aerobic system becomes more abundant.
Long-term exercise (lasting longer than ten minutes), uses aerobic metabolism as a primary source of energy (Powers and Howley, 2015). Protein may be used as an energy source if the exercise session lasts between 3-5 hours. The contribution of protein may reach up to 10%.
During long-term exercise carbohydrates and fats are used as alternative sources of energy. The following factors affect this usage:
- Duration and intensity,
- Whether or not the individual performing the exercise is trained.
For example, during low-intensity (30% VO2 max – VO2 is the maximal oxygen uptake) (Harris-Fry, 2019), prolonged exercise, the amount of fat oxidised by working muscles increases (Powers and Howley, 2015). Endurance-trained individuals use more fat compared to carbohydrates than less-fit individuals during prolonged, low-intensity exercise.
However, during high-intensity exercise (70% V02 max), carbohydrate metabolism is the primary source of energy. When the exercise intensity increases, there are more fast twitch fibres in use and they are better adapted for carbohydrate metabolism (they are abundant in glycolytic enzymes rather than lipolytic enzymes), which explains why it is preferred over fat as a fuel during high-intensity exercise (Powers and Howley, 2015).
Furthermore, during high-intensity exercise blood levels of adrenaline increase (Powers and Howley, 2015). We are aware that the adrenaline hormone works alongside glucagon to activate an enzyme for glycogenolysis. This means blood glucose concentration will rise, promoting carbohydrate metabolism.
Romijn et al. (1993) used stable isotope tracers and indirect calorimetry to evaluate the regulation of fat and carbohydrate metabolism. They found that glycogenolysis and glucose uptake in tissue increased as exercise became more vigorous. In addition to this, a study conducted by Bergman et al. produced results which support the claim that carbohydrates are preferred over fat during high-intensity exercise (Hawley and Leckey, 2015).
What does this all mean for Type II Diabetics?
Insulin is an important factor for storage of excess glucose in the circulation. Individuals with type II diabetes (T2D) are either unable to produce enough insulin or have developed some sort of insulin resistance. It is very clear that endurance exercise could be beneficial for T2D, as glucose in the blood is used up during periods of activity.
In a study conducted by Roden (2012), it was found that the concentration of glucose in the blood was 7-12% lower for up to 24 hours after exercise. Colberg et al. (2010) also said that a single session of endurance exercise increased insulin sensitivity and glucose tolerance for up to 24 hours, but less than 72 hours. This shows us that T2D individuals should be training every two to three days minimum since the effects don’t appear to be long-term.
Winnick et al. (2008) conducted a study to test the effect of endurance exercise on peripheral (relating to tissues) and hepatic insulin sensitivity. The participants were split into two groups. Group 1 was a control group which consumed an isocaloric (50% carbohydrate, 30% fat, and 20% protein) for 15 days. Group 2 were given a similar diet for 15 days and they were then asked to perform 50 minutes of treadmill walking at 70% VO2 for the last 7 days. There was no significant difference recorded for the control group, however, group 2 showed a significant increase in both whole-body insulin sensitivity (P < 0.05) and peripheral insulin sensitivity (P < 0.0001) when individuals were taking a high-dose of insulin.
These findings illustrate that although insulin sensitivity improvements are only related to uptake in tissues (peripheral), it is still of interest to type 2 diabetics to take part in regular, high-intensity endurance exercise since there is still significant improvement in insulin sensitivity.
Borghouts, L. (2000) conducted a study to test the long-term effects of endurance training on individuals with type II diabetes. Twelve participants started the program, but only 9 completed it. Monitoring was minimised during the second year of the study, which resulted in a 50% decrease in exercise.
He found that insulin responsiveness was improved by 27% after the first year, but dropped back to pre-training levels after the second year. The expression of certain enzymes, transporters and cholesterol mimicked this. With all that being said, we can conclude that endurance training does not have a long-term effect on type II diabetes patients, therefore endurance training should be life-long.
In conclusion, I believe patients with T2D should be exercising at least three times a week, with a maximum break of three days in between each session, for a minimum of 150 hours at moderate intensity (45-60% VO2 max) or higher (Colberg et al., 2010).
Written by Igor S.; published on 13/12/2019.
Bergman, B., Butterfield, G., Wolfel, E., Casazza, G., Lopaschuk, G. and Brooks, G. (1999). Evaluation of exercise and training on muscle lipid metabolism. American Journal of Physiology-Endocrinology and Metabolism, 276(1), pp.E106-E117.
Borghouts, L. (2000). Exercise and type 2 diabetes. Maastricht: Universiteit Maastricht.
Colberg, S., Sigal, R., Fernhall, B., Regensteiner, J., Blissmer, B., Rubin, R., Chasan-Taber, L., Albright, A. and Braun, B. (2010). Exercise and Type 2 Diabetes: The American College of Sports Medicine and the American Diabetes Association: joint position statement. Diabetes Care, 33(12), pp.e147-e167.
Harris-Fry, N. (2019). What Is VO2 Max And How Do You Improve It?. [online] Coach. Available at: https://www.coachmag.co.uk/fitness/6987/what-is-vo2-max-and-how-do-you-improve-it [Accessed 9 Dec. 2019].
Hawley, J. and Leckey, J. (2015). Carbohydrate Dependence During Prolonged, Intense Endurance Exercise. Sports Medicine, 45(S1), pp.5-12.
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Powers, S. and Howley, E. (2015). Exercise physiology. 9th ed. p.67,73,78,292&507.
Roden, M. (2012). Exercise in type 2 diabetes: to resist or to endure?. Diabetologia, 55(5), pp.1235-1239.
Romijn, J., Coyle, E., Sidossis, L., Gastaldelli, A., Horowitz, J., Endert, E. and Wolfe, R. (1993). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology-Endocrinology and Metabolism, 265(3), pp.E380-E391.
Winnick, J., Sherman, W., Habash, D., Stout, M., Failla, M., Belury, M. and Schuster, D. (2008). Short-Term Aerobic Exercise Training in Obese Humans with Type 2 Diabetes Mellitus Improves Whole-Body Insulin Sensitivity through Gains in Peripheral, not Hepatic Insulin Sensitivity. The Journal of Clinical Endocrinology & Metabolism, 93(3), pp.771-778.