Dietary Protein
An impairment of antigen aggression has been observed
with diet protein imbalance (i.e., when intake differs markedly
from consumption). A protein imbalance is particularly harmful
to the T-cell system [21,32]. In a crossover study, a highprotein
diet (versus a control diet) was evaluated with regard to
leukocyte trafficking and URTI symptoms after 7 days of high-intensity training in cyclists. High-intensity exercise
attenuated leukocyte response, but protein consumption
restored the impaired trafficking observed during training and
was also associated with fewer URTI symptoms [33]. Protein
supplementation with bovine colostrum that contains high levels
of antimicrobial proteins (e.g., lactoferrin, lysozyme,
S-IgA) could ameliorate susceptibility to infection. Bovine
colostrum improves salivary bacterial load during the winter
months in individuals who performed 3 hours of vigorous
endurance exercise per week as well as number of self-reported
URTI days during a 12-week period. These results indicate that
immuno-protection may be performed by nutritional interventions
that support innate and adaptive mucosal immunity [34].
Glutamine is used as a fuel at a high rate by lymphocytes and
macrophages and for the synthesis of DNA and RNA [35].
Decreases in plasma glutamine levels have been observed in
response to prolonged exercise, but glutamine supplementation
in marathon runners did not influence exercise-induced immunological
changes. The link between an exercise-induced
decrease in plasma glutamine and impaired immunity is not
clear, and many studies have not supported this [36,37].
Branched-chain amino acid (BCAA) supplementation may
promote such immune responses in skeletal muscle [38]. In
particular, Bassit et al. have shown positive effects on plasma
glutamine concentration and cytochine response in professional
triathletes and marathon runners involved in a long-distance
race [39]. In a recent review, it was reported that the
administration of BCAA could exert an anti-inflammatory role
or indirectly modulate the inflammatory status and balance of
muscle cells in order to favor the biological response and tissue
adaptation during training conditions. The mechanisms
involved the suppression of skeletal muscle proteolysis and the
stimulation of protein synthesis by BCAA supplementation
[40]. More recently, other findings have underlined a possible
effect of b-hydroxy-b-methylbutyrate (HMB) supplementation
on the immunomodulation of the muscle damage in response
to 12 weeks of resistance exercise. However, HMB’s immune
effects on muscle adaptations have not yet been fully elucidated
and future investigations should clarify whether HMB
could act directly on inflammatory cytokines to resolve muscle
damage [41].