This study will evaluate HIV infected patients who are enrolled in ACTG A5202, starting their first nucleoside reverse transcriptase inhibitor (NRTI)-containing regimen. This regimen will include either tenofovir disoproxil fumarate/emtricitabine (TDF/FTC) or abacavir/lamivudine (ABC/3TC), which are thymidine-sparing regimens. The changes in mitochondrial activity and fat cell death will be compared between participants taking thymidine-sparing regimens (TDF/FTC or ABC/3TC) and thymidine-containing regimens.