The crossover concept, the idea that the balance of fat and carbohydrate utilization during exercise depends on the interaction between exercise intensity and a person’s endurance training status, dates back to the 1930s and has been widely accepted among exercise physiologists. Some of the many scientifically based definitions of the crossover concept include the following:
“Recognition of energy flux, as determined by relative exercise intensity, is the major factor in determining the balance of substrate utilization during exercise.”
“Represents an attempt to integrate the seemingly divergent effects of exercise intensity, nutritional status, gender, age and prior endurance training on the balance of carbohydrate and lipid metabolism during sustained exercise.”
“An interaction between exercise intensity and endurance training status with the net effect of these two opposing influences determining the relative contributions of carbohydrate and fat to energy metabolism during exercise.”
As you can see, there are different interpretations of the crossover concept among researchers, and there is conflicting evidence for the use and validity of the concept as a whole.
For example, it is argued that trained versus untrained individuals rely less on carbohydrate during high intensity exercise as seen from respiratory gas exchange data (respiratory quotient or respiratory exchange ratio) where training has been seen to decrease carbohydrate oxidation and increase fat oxidation even during intense exercise.
The rebuttal states that energy flux, as determined by exercise intensity, is the major factor in determining the balance of substrate utilization during exercise. Additionally, proponents of the crossover concept indicate that the pattern of substrate utilization does not differ in trained versus untrained individuals as long as exercise is at the same percentage of VO2max.
The important aspect to realize is not which researcher or group of researchers adheres to which definition, but rather how they differ in their interpretations and use of the data gained from the crossover concept as a whole.
As exercise intensity increases, the body prefers to use carbohydrate for energy. The crossover point is the intensity, typically a percentage of VO2max, where fat and carbohydrate intersect with the energy from fat decreasing and the energy from carbohydrate increasing. However, contrary to popular belief, some people do not reach a crossover during exercise. These examples are evident in the field working with clients. The pattern of substrate utilization at any point during exercise depends on the exercise intensity induced responses, which increase carbohydrate oxidation and endurance training induced responses, which can increase fat oxidation.
Prior endurance training results in muscular biochemical adaptations that can increase fat oxidation and decrease sympathetic nervous system activity. These adaptations can obviously improve the ability to utilize all of the energy substrates, but overall, the adaptations will favor more fat rather than carbohydrate oxidation. With higher intensities of exercise, certain biochemical adaptations contribute to the increased use of carbohydrate. These include contraction induced muscle glycogenolysis, increased recruitment of more skeletal muscle with a greater proportion of fast twitch fibers and increased sympathetic nervous system activity. Norepinephrine may stimulate both liver glucose production and lipolysis. Epinephrine intensifies the contraction induced rate of muscle glycogenolysis, which leads to a higher lactate formation. The acidic effect of lactate inhibits free fatty acid mobilization, which reduces uptake into the muscle.
Regarding the role of energy flux on the pattern of substrate utilization, as exercise intensity increases, energy demand increases. Energy release must meet the need of a higher exercise intensity, which causes the metabolic crossover to carbohydrate as a fuel source. This shift is due to more glycolytic versus lipolytic enzymes in skeletal muscles and the pattern of muscle fiber recruitment to involve more fast twitch fibers. The metabolic characteristics of these specific muscle fibers increases glycogenolysis and glycolysis, leading to more lactate production rather than fat oxidation.
It is well accepted that prior endurance training increases fat oxidation during submaximal exercise. The four main areas of support of this include training and muscle mitochondrial mass, training and the respiratory exchange ratio, muscle glycogen sparing and blood lactate concentration.
Endurance training increases mitochondrial mass, which results in an increased ability to oxidize fats. Specifically, training increases fatty acid enzyme action, the beta-oxidation pathway and the electron transport chain, all crucial in fat metabolism. The respiratory exchange ratio (RER) is improved (lower RER value) with endurance training. As the RER nears 0.70, the body becomes more efficient at utilizing fat, and as the RER nears 1.0, the body relies more on carbohydrate. It is beneficial for most individuals to have lower RER values. The improved training effect allows the body to spare muscle glycogen due to a blunting of glycogenolysis. Lastly, training has a positive impact on lactate accumulation in that blood lactate concentration is lower at submaximal exercise intensities in a well trained versus untrained state.
Now the question of “so what” takes shape. The science is crucial to understand prior to manipulating a client’s exercise program, but it is the application of the science to the art of exercise prescription that becomes the critical piece of the puzzle. Metabolic efficiency should be the goal that you as a fitness professional strive for in your work with clients. Exercise training will exhibit positive effects on substrate utilization as you read previously, but you must know how and when to adapt the exercise program to reap the rewards of the crossover concept.
The crossover point can be manipulated with proper aerobic training due to the positive mitochondrial adaptations 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 VO2max but will likely 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. Take the following graphs as an example. They represent metabolic testing done on an elite mountain biker during his initial meeting and then three weeks after a training and nutrition intervention.
As can be seen, the athlete did not achieve a true, textbook crossover during his initial test. It was concluded that the reason for this was not using the concept of nutrition periodization and too high intensity of training. Recommendations were given to the athlete to lower carbohydrate intake to a moderate level and increase protein to a moderate level while maintaining fat intake. From a training perspective, the athlete was entering an off season where lower intensity was the primary goal for his training program. This supported the goal of altering substrate utilization through the manipulation of exercise intensity quite nicely.
Three weeks later, a significant substrate utilization shift was witnessed. In a very short time, this athlete was able to move from an energy flux where carbohydrate oxidation was the primary source of energy during training to a state where fat oxidation was the primary source of energy up to a certain intensity level.
After the second metabolic test, the athlete indicated that the exercise prescription provided to him, which included lower intensity and the manipulation of his macronutrients, was much easier to implement than he initially had thought. This is proof that even at an elite athlete level, it is possible to make significant metabolic changes with a nutrient and exercise program shift, and more importantly, benefits can be seen in a short amount of time.
As a fitness professional, it is important to assess your client’s goals as they relate to metabolic efficiency. I would be willing to bet that most people, athletes or non athletes included, would benefit from being more metabolically efficient in utilizing a predominantly higher level of fat for energy, both from a performance and health perspective.
- Brooks, G.A. & Mercier, J. (1994). Balance of carbohydrate and lipid utilization during exercise: the “crossover concept”. J. Appl. Physiol. 76(6): 2253-2261.
- Brooks, G.A. (1998). Mammalian fuel utilization during sustained exercise. Comparative Biochemistry and Physiology 120: 89-107.
- Brooks, G.A. (1997). Importance of the crossover concept in exercise metabolism. Clinical and Experimental Pharmacology and Physiology 24: 889-895.
- Tipton, C.M. (1997). Current issues in exercise metabolism: the crossover concept. Clinical and Experimental Pharmacology and Physiology 24: 887-888.
- Coggan, A.R. (1997. The glucose crossover concept is not an important new concept in exercise metabolism. Clinical and Experimental Pharmacology and Physiology 24: 896-900.