The Basics: High-Intensity training goals and fuel utlilization
Energy is metabolised from the three macronutrients namely, carbohydrates, fats and protein. As explained in part 1 (click here) these macronutrients provide energy simultaneously via the aerobic and anaerobic system. However, the degree to which fats and carbohydrates are metabolised for energy is dependent on the intensity and duration of the training session.
Figure 1: Energy utilisation shift with increased training duration.
The available fuel sources during training includes:
Muscle glycogen: stored glucose (carbohydrates) in the muscle fibres.
Muscle triglycerides: stored fat in the muscle fibres.
Plasma Free Fatty Acids (FFA): Fats available in the blood from dietary intake and adipose tissue (fat cells).
Plasma Glucose: Glucose available in the blood from dietary intake during training and glycogen stored in the liver.4
Figure 1 shows that at high intensity exercise levels (85% of the VO2max), most of the fuel utilised comes from muscle glycogen, and less from other sources. At low exercise intensities, however, much less fuel is derived from plasma glucose and glycogen and most of the fuel is derived from plasma FFA, derived from adipose tissue or dietary intake during training (MCTs in Octane Gel
Figure 2: Energy utilisation shift with increased training duration
As evident in Figure 2, the longer the event lasts at a specific work load (75% of the VO2max), the more fat contributes towards energy production.
High intensity events typically last between 10 seconds – 80 minutes. The type of fuel sources and metabolic pathways used to breakdown nutrients depend on duration and intensity of the specific high-intensity sport. The importance of the fuel source and metabolic pathways for high-intensity athletes in descending order are:
Anaerobic Glycolytic system – CARBOHYDRATES
Lactic acid system (400 m sprint) – Fuel source = CARBOHYDRATES and Lactate
At high levels of 70-80% of VO2Max carbohydrates may contribute more than 80% of the energy sources.
It is thus important that athletes have high muscle glycogen levels, to support high intensity exercise.
Metabolic thresholds (indicated in table 1) are used to describe the point (or exercise intensity) where the source your body uses to fuel activity changes significantly and measurably. It is indicative of the various metabolic cross-over points and demarcate specified minimum – maximum training zones respectively. The metabolic thresholds consist of:
Fatty acid threshold (FAT) – The exercise intensity where the maximum amount of fat is burned.
Aerobic carbohydrate threshold (ACT) – The upper limit of exercise intensity at which the exercise is almost exclusively fuelled by the aerobic glycolytic system (carbohydrates).
Lactic acid threshold (LT) – The beginning of the anaerobic metabolism. The exercise intensity where lactate levels have risen above baseline.
Onset of Blood Lactate Accumulation (OBLA) – The point where lactate production exceeds lactate clearance and starts to accumulate in the blood.
Table 1: Metabolic thresholds for fuel utilisation.
The workload can be defined in terms of Objective Metabolic variables such as heart rate (HR) and VO2max and/or
an athlete’s perceptions such as the rate of perceived exertion and effort.
As seen in Figure 3, with specific periodization of training and nutrition athletes can gain metabolic adaptations, relating to a shift in all the metabolic thresholds to a higher workload for the same heart rates at all exercise intensity levels therefore improving in athletic performance.
Figure 3: Metabolic shifts with training
Thus, the training and nutritional goals of high-intensity athletes are:
Optimise the anaerobic glycolytic and lactic acid system.
Delay the onset of intracellular lactic acid appearance.
Increase intracellular hydrogen and ammonia buffer clearance capacity.
Increase the clearance of blood lactate.
Sufficient substrate for metabolic reactions.
Sufficient sport specific aerobic glycolytic and lipolytic capacity.
Shortened recovery time.
Maintain mental alertness and decision-making ability.
Ensure biomechanical efficiency.
Maintain high power output.
*NB: Degree to which athletes can adapt to these goals depends on genetic predispositions such as muscle fiber type (fast twitch/slow twitch), recruitment of these fiber types, and other genetic factors.