Among its many roles in the body, ascorbic acid functions as a cofactor in carnitine and catecholamine synthesis, metabolites involved in fat oxidation and mood regulation, respectively. Given that fat oxidation and mood affect one's feelings of vigor, I hypothesized that those with lower levels of plasma ascorbic acid would be less likely to exercise at high levels than individuals with adequate or high levels of vitamin C. To test this, I conducted a double-blind, placebo-controlled intervention. A group of healthy, non-smoking males between the ages of 18 and 40 were put on a vitamin C-restricted diet for two weeks and then randomized to a control group that received placebo capsules for six weeks or an intervention group that received 500 mg of vitamin C daily for six weeks. The men were restricted from eating foods high in vitamin C, instructed to wear a pedometer daily and to record their step counts, and to take a pill daily (either the placebo or vitamin C supplement). Unexpectedly, the subjects receiving the intervention had lower step counts than the control group; the control group, rather than the vitamin C group, significantly (p=0.017) increased their steps at week 8 compared to week 2. However, I also estimated daily Metabolic Equivalent Tasks (METs), and subjects receiving the placebo had lower MET outputs than subjects receiving vitamin C at the end of the trial, in spite of having higher step counts. This means the intensity of their activity was higher, based on METs expenditure. Additionally, depression scores (POMS-D) as measured by the Profile of Mood States (POMS) questionnaire were significantly higher (p=0.041) among subjects receiving the placebo at the end of the study. These latter results are consistent with my expectations that subjects with higher levels of plasma vitamin C would have improved mood and higher energy output than subjects with low levels of vitamin C.