Purpose of reviewChanges in body composition accompany and predict medical and surgical risk; interventions may be considered. Aging affects body composition, particularly in reduction of the body cell mass.Recent findingsThe molecular biology of aging has become an active frontier of research in the past 30 years, with acceleration in the past decade. A review of the toxicities of reactive oxygen and nitrogen species, especially at the mitochondria, can now be studied at the molecular, genetic, and proteomic scales, in individual cell components and systems, with major implications for patient management, for planning therapeutic interventions, and for predicting future age spans.SummaryThe intersections between the clinical fields of endocrinology, nuclear medicine/radiology, and geriatrics, with the more fundamental fields of physiology, molecular biology, genetics, and proteomics are indicated.