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Recent studies have shown that endurance overtraining could result from successive and cumulative alterations in metabolism, which become chronic during training. The onset of this process is a biochemical alteration in carbohydrate (saccharide) metabolism. During endurance exercises, the amount of saccharide chains from two blood glycoproteins (alpha(2)-macroglobulin and alpha(1)-acid glycoprotein) was found to have decreased, i.e. concentrations of these proteins remained unchanged but their quality changed. These saccharide chains were probably used for burning liver glycogen stores during exercise. This step was followed by alterations in lipid metabolism. The most relevant aspect of this step was that the mean chain length of blood fatty acids decreased, i.e. the same amount of fatty acids were found within the blood, but overtrained individuals presented shorter fatty acids than well-trained individuals. This suggests that alterations appeared in the liver synthesis of long-chain fatty acids or that higher peroxidation of blood lipoparticles occurred. For the final step of this overtraining process, it was found that these dysfunctions in carbohydrate/lipid metabolism led to the higher use of amino acids, which probably resulted from protein catabolism. The evolution of three protein concentrations (alpha(1)-acid glycoprotein, alpha(2)-macroglobulin and IgG(3)) correlated with this amino acid concentration increase, suggesting a specific catabolism of these proteins. At this time only, overtraining was clinically diagnosed through conventional symptoms. Therefore, this process described successive alterations in exercise metabolism that shifted from the main energetic stores of exercise (carbohydrates and lipids) towards molecular pools (proteins) normally not substantially used for the energetic supply of skeletal muscles. Now, a general biochemical model of the overtraining process may be proposed which includes most of the previously identified metabolic hypotheses.

Top-level performances in endurance sports require several years of hard training loads. A major objective of this endurance training is to reach the most elevated metabolic adaptations the athlete will be able to support. As a consequence, overtraining is a recurrent problem that highly-trained athletes may experience during their career. Many studies have revealed that overtraining could be highlighted by various biochemical markers but a principal discrepancy in the diagnosis of overtraining stems from the fact that none of these markers may be considered as universal. In endurance sports, the metabolic aspects of training fatigue appear to be the most relevant parameters that may characterise overtraining when recovery is not sufficient, or when dietary habits do not allow an optimal replenishment of substrate stores. From the skeletal muscle functions to the overall energetic substrate availability during exercise, six metabolic schemes have been studied in relation to overtraining, each one related to a central parameter, i.e. carbohydrates, branched-chain amino acids, glutamine, polyunsaturated fatty acids, leptin, and proteins. We summarise the current knowledge on these metabolic hypotheses regarding the occurrence of overtraining in endurance sports.

Doping prevention is mainly directed to providing information on the dangers of doping to young athletes and to every profession concerned with athletic performance. Unfortunately, repression is also necessary in the fight against doping. Measurement of performance-enhancing drugs is complex, partly because of the large number of prohibited substances. A number of sophisticated analytical techniques are increasingly being used to provide the maximum detection time window. However, the effectiveness of methods to separate exogenous from endogenous biological molecules and the cost of antidoping analyses makes controls invalid or impossible. Moreover, most athletes, because of the metabolic and psychological stresses caused, legitimately refuse blood testing. It is becoming crucial to introduce new methods in the form of longitudinal health monitoring, since this is probably the most effective tool to prevent the use of doping agents when athletes become overtrained and/or overstressed. This paper describes new methods using Fourier-transform infrared spectroscopy to analyse serum from 50 microl samples of capillary blood. This technique has been shown to allow determination of the concentration of a wide range of biological molecules in a single microsample with clinically useful accuracy, and to provide a 'discriminatory biomolecular profile' to differentiate individuals on the basis of their physiological status. A specific application of this methodology is to perform longitudinal health monitoring in athletes, allowing prevention of overtraining. It is proposed to apply such methods in longitudinal studies for health monitoring and prevention of doping.