Athletes with heavy training loads are prone to infectious illnesses, suggesting that their training may suppress immune function. This study sought to determine whether supplementation with the amino acid glutamine, which supports immune health, alters immune function in athletes during heavy load training. 24 athletes were randomly assigned to either an experimental group (n = 12) or a control group (n = 12). Athletes exercised using heavy training loads for 6 weeks. Athletes in the experimental group took 10 g glutamine orally once a day beginning 3 weeks after initial testing, while athletes in the control group were given a placebo. Immune function was assessed by measuring the following immunity markers: CD4⁺ and CD8⁺ T cell counts, serum IgA, IgG, and IgM levels, and natural killer (NK) cell activity both before and after the completion of training. The percentages of circulating CD8⁺ T cells were significantly different before (39.13 ± 5.87%) and after (26.63 ± 3.95%) training in the experimental group (p < 0.05). Although CD8⁺ T cell percentages in the control group were similar before (38.57 ± 5.79%) and after (37.21 ± 5.58%) training, the post-training CD8⁺ T cell percentages were significantly different between the two groups (p < 0.05). The ratios of CD4⁺/CD8⁺ cells in the experimental group were significantly different before (0.91 ± 0.14) and after (1.39 ± 0.19) training (p < 0.05). The CD4⁺/CD8⁺ ratios in the control group were similar before (0.93 Â ± 0.15) and after (0.83 ± 0.11) training, but the post-training CD4⁺T/CD8⁺ T cell ratio was higher in the experimental group than in the control group (p < 0.05). NK cell activity was also significantly different between the two groups after training (experimental, 25.21 ± 3.12 vs. control, 20.21 ± 2.59; p < 0.05). However, no differences were observed in serum IgA, IgG, or IgM levels. Thus, glutamine supplementation may be able to restore immune function and reduce the immunosuppressive effects of heavy-load training.