Brooks and Mercier (1994) present the crossover concept as "…power output at which energy from carbohydrate (CHO)-derived fuels predominate over energy from lipids, with further increases in relative power eliciting an increment in CHO utilization and a decrement in lipid oxidation". According to Brooks (1997), this model is important because it provides a unifying conceptual framework for interpreting observed metabolic responses to exercise, as evidenced in contemporary scholarly work (Roberts, Weber, Hoppeler, Weibel, & Taylor, 1996). Specifically, during moderate to high-intensity exercise ( ~ 50% VO2max) the role of lipid oxidation decreases while glucose and glycogen use increases exponentially, independent of training status. These trends are more profound as one proceeds to exclusively high-intensity exercise ( ~ 65% VO2max).

Proponents of the crossover concept contend that endurance training, energy supply, and prior exercise, play ancillary roles in determining substrate utilization during relative high-intensity exercise. If validated, this model would provide novel insight into energy transformation during relative high-intensity exercise where energy flux would be the most important predictor of substrate utilization during exercise. It is important to note that opponents of the concept agree that CHO is the primary substrate during intense exercise for all subjects.

The primary contention for opponents, that trained subjects are able to utilize greater fat oxidation during relative high-intensity exercise, is based on decades of data. Attempting to debunk current understanding, Brooks (1997) offers contradictory evidence in the form of unpublished data (Brooks & Bergman 1996), and a collection of studies reporting RER ranges of .93 - .95 in endurance trained subjects performing

relative high-intensity work. Brooks also questions the validity and presentation of contradictory findings by Coggan Raguso, Gastaldelli, Sidossis, & Wolfe (1995) that suggests FFA flux is higher in trained subjects at 80% of VO2max. Validity concerns are founded on the legitimacy of making a cross-sectional comparison without considering possible genetic differences between subjects. In terms of interpretation, Brooks suggests that a refusal to even consider the crossover concept led the authors to disregard the fact that CHO metabolism accounted for at least 80% of energy production in all subjects. Analyzing the same data, Brooks finds support for his hypothesis, that the pattern of substrate utilization at relative high work is similar for all subjects, independent of training status.

Coggan, a leading opponent of the crossover concept, presents several categories of evidence suggesting that training can alter substrate utilization during relative high-intensity exercise (1997). First, numerous studies have found that training can decrease CHO utilization and increase fat oxidation during intense exercise, as measured by respiratory exchange ratio (Hagberg, Seals, & Yerg et al. 1988). Second, Jansson & Kaijser (1987) found that when compared to untrained cyclists, trained subjects used significantly less glycogen at 65% of VO2max. Further, the authors also established that glucose uptake was lower in trained cyclists. A finding confirmed by advocates of the crossover concept themselves (Friedlander, Casazza, Horning, Huie, & Brooks, 1997). These results are at odds with the prediction that plasma-borne glucose will be greater, or even equal, in trained subjects during high-intensity exercise. Lastly, Coggan et al. (1995) found that the rate of glycerol Ra and FFA Rd in the plasma, is greater in trained subjects during relative high-intensity exercise. Notably, the difference in FFA flux accounted for only half of the difference in fat oxidation between trained and untrained subjects. This led the authors to speculate on the role of intramuscular triglycerides during intense exercise in trained athletes. Scientific acceptance of this potentially interesting idea, like the crossover concept, awaits further empirical support.

The idea that substrate utilization is influenced by exercise intensity has been put forth, tested, and supported for several decades (Edwards, Margaria, & Dill, 1934). Proponents of the crossover concept suggest that training status does not interact with exercise intensity to alter the relative contribution of lipid oxidation during relative high-intensity exercise ( ~ 65% VO2max). A claim of this magnitude requires more than one comparative refereed publication from various laboratories (Roberts et al.1996), to be considered a serious contender against decades of contradictory experimental data. Thus, at this time, I believe proponents of the crossover concept have provided insufficient evidence when considering the vast literature supporting the theory that trained subjects rely less on CHO fuel during high-intensity exercise.