From the article:

In summary, runners completing the 90-km Comrades Ultramarathon experienced strong increases in concentrations of serum cortisol and plasma IL-6, IL-10, IL-1RA, and IL-8. These increases were attenuated in runners ingesting 1500 mg but not 500 mg vitamin C supplements for 1 week prior to the race and on race day.

Vitamin C may have a special role in reducing stress-induced cortisol release:

In contrast, others have shown that vitamin C infusion enhances adrenal glucocorticoid production or has no effect on the serum cortisol response to ACTH. Our data suggest that within the context of prolonged and intensive exertion, high vitamin C intake significantly lowers serum cortisol levels.