Effects of the relative intensity of exercise on fuel selection by the working muscles
SPORTS NUTRITION & PERFORMANCE
Relative intensity of exercise significantly influences the working muscles capacity to breakdown macronutrients in the form of carbohydrates, fats and to a minor scale, protein as a fuel source. There is a mix of fuel selection throughout varying intensities, although one might be utilized more than the other at different stages. Other factors that determine fuel selection are the ‘aerobic training status of the individual, general diet, food intake before and during exercise, age and gender’ (Henrikkson, 1995). For the purpose of this paper, a focus on relative intensity measured by percentage of VO2 max will be discussed. Occasionally, relative intensity will be measured from the percentage of heart rate maximum (%HR max), however, ‘the error in estimating %HR max from VO2 max equals about 8%’ (Mcardle et al. 2006)
Main stored fuels that provide energy for muscle ATP re-synthesis during exercise are liver and muscle glycogen primarily derived from carbohydrate consumption (CHO), and triacylglycerols (TG) within adipose tissue and active muscle, primarily derived from fats’ (Mcardle et al. 2008). ‘Amino acids in proteins also have a smaller contribution, although there is conflicting evidence on the amount that it actually does play with energy requirements’ (Mcardle et al)
At rest, fatty acids (FA) produced from the mobilisation of adipose triacylglycerol (lipolysis), ‘provide a major portion of the energy requirements for skeletal muscle’ (Burke & Deakin, 2006) at approximately 60% of the total fuel use. ‘The rate of lipolysis usually exceeds energy requirements of skeletal muscle, so when there is an increase from rest to low intensity, an increase in FA oxidation can occur without an instant increase in lipolysis’ (Burke et al) At this intensity a small percentage of the muscles fuel comes from carbohydrates (approximately 35%) and this consists almost entirely of plasma glucose.
‘During light exercise (25 – 40% of VO2 max), the rate of FA mobilisation in plasma matches the rate of FA oxidation and therefore there is an increase in FA’s utilization, allowing fats to continue as the main energy source (approx. 55%). The increase of FA utilization is partly due to an increase in epinephrine and glucagon which rise with intensity. Epinephrine activates enzyme hormone-sensitive HSL’ (Burke et al. 2006), the chemical regulator in lipolysis, further assisting fat oxidation. Although fat is the dominant macronutrient throughout light intensity, ‘glycogen stored in muscle is the predominant CHO energy source and contributes to approximately 40% of the total energy expenditure’ (Mcardle et al. 2008). Blood glucose also slightly increases. At this intensity fuel selection does not change considerably even if it lasts between 1-2 hours because ‘muscle energy requirements can be met exclusively from oxidation of the FA mobilized from adipose TG stores’ (Burke et al.), which comes with an almost unlimited source.
MODERATE TO HIGH INTENSITY
‘With an increase from 40% to 65% of Vo2 max, total fat oxidation reaches its peak, despite a slight decline in plasma FA’ (Burke et al. 2006). This increase reflects an increase in the oxidation of intramuscular triacylglycerol (IMTG). At this time, ‘fat contributes to 55% of total fuel, with CHO supplying 40% and protein between 2-5 %. Once intensity reaches its peak at 65% of VO2 max, there is very little, if any, further increase in lipolysis’ (Mcardle et al. 2006) as the increasing ‘blood glucose inhibits lipase and reduces lipolysis’ (Burke et al.)
Once intensity rises past 65% of VO2 max, muscle glycogen usage significantly increases and the ‘liver releases more glucose for use by active muscles, while muscle triacylglycerol and plasma free fatty acid use decrease’ (Mcardle et al. 2008). Simultaneously, lipolysis is suppressed, and the contribution of FA oxidation to total energy requirement of exercise declines’ (Burke et al. 2006). There is also evidence from Lambert (1994) that ‘lactic acid further inhibits FA oxidation by skeletal muscle during high intensity exercise’. As can be seen from graph 1.1, carbohydrates are the most dominant fuel at this intensity with approximately 70% of fuel use, while fat utilization is only 15%.
‘When intensity reaches 85% of Vo2 max, there is a further decline in total FA oxidation compared to moderate intensity due to the insufficient blood flow from adipose tissue to the blood stream which reduces plasma FA’ (Mcardle et al. 2006) and overall fat oxidation. Henriksson, 1995 further explains that fat use is inhibited because ‘the primary site of control of FA oxidation during moderate to high intensity exercise resides at the muscle tissue level, and even an infusion of fat could not increase oxidation for this reason’. Additionally, CHO’s have a faster rate of supplying ATP for working muscles than fat, which is why it is the preferential fuel during high intensity aerobic exercise. According to Mcardle et al., ‘it has two times more rapid energy transfer compared with that of fat and protein’.
Graph 1.1 (Mcardle et al. 2008) ‘This graph does not account for the alterations in liver and muscle glycogen depletion from prolonged, intense exercise’.
The body can only maintain a high intensity exercise for a certain amount of time (Approx. 90 minutes). ‘Eventually, plasma glucose decreases due to the liver’s glucose output not meeting the muscles needs’ (Mcardle et al. 2006). With carbohydrate depletion, exercise intensity decreases to a level governed by the body’s ability to mobilize and oxidize fat’ (Mcardle et al.) at around 50%, and fat once again becomes the muscles main energy source.
During intense, all out exercise, ‘oxygen fails to meet energy demands, and muscle glycogen becomes primary energy contributor because it can provide energy without oxygen (Wahren et al. 1971). Therefore, it is no surprise to see carbohydrates supply almost 95% of fuel selection during anaerobic activities such as sprinting.
In summary, both carbohydrate and fat oxidation increased as the exercise intensity increased, with fat being the predominant fuel selection up until 65% VO2 max. With further increments in relative intensity, both muscle glycogen and plasma glucose oxidation rate increased significantly and fat oxidation markedly decreased. The decrease in fat fuel selection was due to the inability of fat oxidation to keep up with energy muscle demands, blood glucose inhibiting lipase and in effect reducing lipolysis, and there is some evidence to suggest that lactic acid production might have a role in slowing the process down also. Carbohydrates are the dominant fuel past 65% VO2 max, and continue to be for up to 90 minutes in duration until muscle and liver glycogen and blood glucose are exhausted, in which fat once again becomes the main energy supplier. Anaerobic activity is predominantly CHO regulated with very little influence from fat or protein. Past research has shown protein to be a small contributor to energy demands; however, it is still unclear whether it plays a more significant role.
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