PT on the Net Research

Metabolic Efficiency Training for Endurance Athletes


The combination of eating less quality foods at less ideal times of the year combined with poor training methods can have a negative impact on performance. Most endurance athletes, whether coached or self coached, follow a methodical physical training program leading up to competitions. However, their nutrition plan does not hold the same importance as their physical training plan and unfortunately is typically only given special attention the week before a competition.

Proper nutritional interventions offered at certain times during a training year will provide an endurance athlete the ability to attain their specific physiological goals associated with each of their training cycles. Typical physical periodization cycles include preparatory, competition and transition, and each cycle has different energy demands. A sound nutrition plan, following the concepts of nutrition periodization, will support the fluctuating volume and intensity shifts experienced throughout each training cycle so the athlete will be able to maximize training adaptations.

Metabolic Efficiency Training

To further understand how the nutrition and training plan work in concert with one another, the concept of Metabolic Efficiency Training (MET) will be discussed. MET offers specific nutrition and physical training alterations that will manipulate cellular processes in order to improve the body’s ability to utilize macronutrients. The main benefit of MET is that it allows the body to more efficiently use macronutrients, which has a profound effect on training status.

While some endurance athletes methodically plan their training program and follow it as closely as possible, others leave training to chance. This may work for a small percentage of athletes, but the latter typically does not schedule enough aerobic training for fear of not having enough time to improve their overall speed. In essence, some endurance athletes skip or significantly reduce aerobic training cycles or sessions so more interval training can be accomplished. Aerobic training is sometimes thought of as less beneficial to some athletes because they cannot see the immediate physical benefits as they can when they do speed work. This can be extremely detrimental to their future success because it can develop metabolic inefficiencies, which will have a negative impact on their performance during their competition season.

Aerobic training induces cellular changes that improve the body’s efficiency in using macronutrients, specifically fatty acids. Mitochondria, which produce ATP, increase in size and number as a result of aerobic training. Mitochondrial enzymatic activity also increases. More specifically, those associated with the Kreb’s Cycle and respiratory chain, the shuttle system that transfers protons developed through glycolysis into the mitochondria for use in the respiratory chain and fatty acid metabolism. The latter is most important as it pertains to MET as this allows the body to use more available fats for energy to fuel exercise.

The oxidation of free fatty acids by the mitochondria predominates energy supply when exercise intensity is not very high. This consequence of aerobic training is known as “carbohydrate sparing.” Because glycogen stores can deplete rather quickly (after about three hours of continuous training) and exogenous carbohydrate supplementation cannot provide adequate energy for endurance athletes when duration increases above eight to 10 hours due to the high rate of energy expenditure, it is beneficial for the body to become more metabolically efficient in using fats as an energy source. These metabolic changes can typically happen in 12 to 16 weeks and are most beneficial early in an endurance athlete’s program, most notably in the preparatory (base) training cycle.

Aerobic training is important to induce positive cellular changes but just as important is the quality of food eaten during certain times of an athlete’s yearly physical periodization program. Nutrition training that will help promote a higher degree of metabolic efficiency is important to implement during an athlete’s preparatory training cycle and compliments the cycle’s physiological goals of improving cardiovascular endurance.

While this concept has not received scientific attention as of yet, it is easy to utilize a well known physiological term to understand the impact that this nutrition training along with physical training can have on metabolic efficiency.

The crossover concept (see Figure 1 below) describes the relationship of fat and carbohydrate oxidation to the intensity of exercise. As exercise intensity increases, the body prefers to use carbohydrate for energy. The crossover point is the intensity, typically a percentage of VO2 max, where fat and carbohydrate intersect with the energy from fat decreasing and the energy from carbohydrate increasing. The crossover point can be manipulated with proper aerobic training due to the positive mitochondrial adaptations discussed previously but only if intensity is maintained in the athlete’s aerobic training zones. Training at higher intensities will improve the athlete’s lactate threshold, economy and possibly VO2 max but will not induce macronutrient partitioning that improves fatty acid metabolism during training. An athlete who is more aerobically conditioned can use more fat for energy at higher intensities and this will in turn induce a glycogen sparing effect.

Figure 1. The bottom graph indicates an athlete who is more aerobically fit and thus is able to use more fat as energy at higher intensities. The green line represents the oxidation of fat as intensity increases from the left to the right and the purple line represents carbohydrates.

Re-enter the rossover concept. Many endurance athletes do not know that the crossover point can be further manipulated through proper macronutrient partitioning. Eating a higher carbohydrate diet will lead to an increase in carbohydrate oxidation, and the same is true with fat although “fat loading” regimens have not been entirely supported in the scientific literature at this point. While the benefits of eating a higher carbohydrate diet are certainly justified during a competition season where intensity is high and volume may be moderately high due to the amount of competitions, this type of eating can work to disadvantage for athletes during the preparatory cycle. Eating and therefore oxidizing more carbohydrates decreases the body’s ability to oxidize fat at higher intensities. Thus to properly teach the body to utilize free fatty acids more efficiently, carbohydrate intake should be less than it was during competition season. This is not a recommendation for a low carbohydrate diet. It is simply a strategy to decrease the total amount of calories and therefore carbohydrate since energy expenditure isn’t as high as it was during the race season.

The following case study describes the impact of proper physical training combined with proper nutrition training over a period of 12 weeks.

This is an example of metabolic inefficiency. As can be seen, the athlete was tested running on a treadmill at different paces after an adequate (15 minute) warm up. Each stage was five minutes in length, and gas exchange (respiratory exchange ratio or RER) was collected by use of a metabolic cart. This athlete utilizes a large amount of carbohydrates (black line) from the beginning of exercise and is very inefficient at oxidizing fat, even at lower intensities, as can be seen by the declining white line throughout the exercise session.

This second example is the same athlete three months later following the exact same protocol and is an example of the effects of proper aerobic training and nutrition periodization. During these three months, this athlete decreased total daily carbohydrate intake by 2.3 grams per kilogram of body weight (or eight percent of total calories) and used a heart rate monitor to train effectively and solely in aerobic training zones. This athlete achieved a significant difference in the ability to improve macronutrient partitioning during exercise in a very short period of time.

To summarize, here are some interventions for endurance athletes in order to increase metabolic efficiency:

  1. Maintain a strict 12 to 16 week training program that includes ample training time in aerobic heart rate zones. This doesn’t mean that athletes cannot train with increased intensity. It simply emphasizes the importance of including aerobic training and not straying from the plan.
  2. Decrease the amount of calories and carbohydrates as compared to what was eaten during competition season. Since energy expenditure is lower, these calories and nutrients are not necessary.
  3. Eat to train, don’t train to eat. Try not to use food as a reward for positive training sessions.

The impact of a nutrition only intervention on the metabolic efficiency and crossover point is not entirely known and has not been scientifically scrutinized. However, proper nutrition planning in addition to well planned aerobic training early in an endurance athlete’s training year can quite possibly have a more profound effect on the cellular adaptations than only one of the variables by itself.