Figure 3. (A) Calculated bioenergetic health index (BHI) from young and old CD4+ T cells. The BHI is derived from calculating a ratio of positive aspects of mitochondrial bioenergetic function (i.e. reserve capacity and ATP-linked respiration) to potentially deleterious aspects of mitochondrial bioenergetic function (i.e. non-mitochondrial oxygen consumption and proton leak). Cellular mitochondrial function was determined using high-resolution respirometry with oligomycin, FCCP, rotenone, and antimycin A. For BHI, one outlier of 14 participants was noted and a comparison of the old and young BHI with this outlier was not significantly different (p=0.19). (B) In further analysis, FACS showed the outlier subject had a higher percentage of total memory CD4+ T cells (59% compared to a range 26-48%) than other participants. Adjusting for the average percentage of memory CD4+ T cells, the calculated the BHI was significantly higher for younger compared to older participants (*p = 0.036). (C) Nonmitochondrial respiration was found to be significantly higher in CD4+ T cells from older compared to younger individuals (*p = 0.049). (D) Reserve capacity was significantly higher in cells from young compared to older participants (*p = 0.045). (A–D) P-values were calculated by Welch’s t-test, a variation of the Student’s t-test that does not make the assumption of equal variance in the two compared samples . Error bars reflect the standard error of the mean (±SEM). N = 7 young, 7 old donors.