Importance of the 'crossover' concept in exercise metabolism.
Level 5 - mechanism / opinion, no new human data
Narrative review and theoretical framework without primary experimental data.
PubMed 9363377 · doi:10.1111/j.1440-1681.1997.tb02712.x
What was done
This paper describes the theoretical framework of the 'crossover' concept, a physiological model predicting patterns of substrate supply and utilization (carbohydrates versus lipids) at rest and during varying exercise intensities.
What was found
The author outlines the following model predictions without presenting new empirical trial numbers: - Lipids constitute the major fuel (approximately 60%) for non-contracting skeletal muscle and the whole body at rest. - Exercise intensity and energy flux serve as the primary drivers of substrate balance; moderate to high intensities stimulate glycogenolysis, glycolysis, fast-twitch fiber recruitment, and sympathetic nervous system activity while down-regulating mitochondrial fatty acid uptake. - Carbohydrate (glycogen and glucose) utilization scales exponentially with relative exercise power output, with glycogen use showing a greater gain than glucose at high power. - Free fatty acid flux follows an inverted hyperbolic relationship with power output, reducing lipid oxidation at high intensities. - Factors like endurance training, dietary manipulation, and prior exercise act as secondary modulators. - No primary experimental dataset, sample size, or statistical measures were reported in the abstract.
Why it matters
The paper outlines a central paradigm in exercise physiology explaining how relative intensity dictates fuel selection and why metabolic comparisons across cohorts require normalization to total energy flux.
Limits
The abstract presents a theoretical model rather than primary experimental data. No sample sizes, participant demographics, control groups, or empirical measurements are provided to directly evaluate the claims.
Cited by
- supports During progressive exercise, energy substrate contribution reaches an equal 50% split between carbohydrates and fats at approximately 60% to 70% of maximum heart rate.