The Basics: Endurance training goals & fuel utilization
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.
The available fuel sources during training includes:
Glycogen: stored glucose (carbohydrates) in the muscle fibers and liver.
Fat: stored fat in the muscle fibers (triglycerides) and adipose tissue.
Exogenous carbohydrates: carbohydrates consumed during training.
As evident in Fig 1, the longer the event lasts at a specific work load (75 % of the VO2max), the more fat contributes towards energy production.
Figure 1: Energy utilisation shift with increased training duration.
Endurance events typically last between 90 min-17 hours or longer in duration. The importance of the fuel source and metabolic pathways for endurance athletes in descending order are:
Aerobic lipolytic system – Fuel source = FAT stored in muscle fibres (triglycerides) and adipose tissue.
Aerobic glycolytic system – Fuel source = STORED GLYCOGEN and CARBOHYDRATES consumed during training (exogenous carbohydrates)
Lactic acid system – Fuel source = STORED GLYCOGEN and LACTATE.
Therefore, about 50-60 % of energy for long endurance events comes from fat. The rest comes from carbohydrates. It is thus important that endurance athletes have adequate muscle triglycerides, stored glycogen and exogenous carbohydrate intake to support both moderate and high intensity phases, (like hill climbs or sprint efforts) to enhance their endurance capacity. The rate at which exogeneous carbohydrate during physical exertion can be oxidised (absorbed) is limited.
The previously held belief was that the upper limit of carbohydrate intake is approximately 60 -90 g per hour (using a variety of carbohydrate sources). Research confirms this is not the case. World-class endurance athletes can train their bodies, in specifically their gut to tolerate 100 – 120 g per hour allowing them to perform at higher exercise intensities (after glycogen has been depleted during endurance events) that would otherwise not be possible.
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 2, with specific periodization of training and nutrition, endurance athletes can gainmetabolic adaptations, relating to a shift in all of the metabolic thresholds to a higher workload for the same heart rates at all exercise intensity levels therefore improving in athletic performance.
Figure 2: Metabolic shifts with training
Thus, the training and nutritional goals of endurance athletes are:
Optimization of aerobic capacity of the lipolytic and glycolytic system
Enhanced ability to burn fat (lipolysis) & carbohydrates aerobically
Sufficient substrate for metabolic reactions
Delayed onset of blood lactate accumulation
Clearance of blood lactate
Biomechanical efficiency etc.
*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.