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A study published in March 2006 in the prestigious Journal of Sports Medicine and Physical Fitness found that athletes supplementing their diet with nucleotides had stronger immune system function and faster recovery.    The first claim is based on the finding of significantly higher levels of immunoglobin A (salivary immunoglobin, SIgA) in the supplemented group, compared to the control.  Salivary immunoglobin levels are closely associated with resistance to respiratory infections like colds and flu.  Endurance exercise has long been known to result in depressed immune system function and decreased SIgA levels have been suggested as the mechanism for this effect.    The second claim is based on the observation of lower levels of cortisol in the supplemented subjects following exercise.  Cortisol is a catabolic hormone released by the adrenal glands during high-intensity exercise.  One of the major effects of cortisol is to break down the protein in muscle tissue to provide the body with an additional source of energy.  To the extent cortisol levels are reduced, the muscle degrading effects of cortisol are also reduced and recovery is accelerated.