White adipose tissue (WAT) expands under physiological conditions via an increase in adipocyte size (hypertrophy) and/or number (hyperplasia; adipogenesis), and the ability of WAT to expand to accommodate energy demands is a significant determinant of metabolic health status. Obesity is associated with impaired WAT expansion and remodeling, which results in the deposition of lipids to other non-adipose organs, leading to metabolic derangements. Although increased hyperplasia has been implicated as a cornerstone in promoting healthy WAT expansion, recent developments suggest that the role of adipogenesis as a contributing factor in the transition from impaired subcutaneous WAT expansion to impaired metabolic health remains up for debate. This mini-review will summarize recent developments and highlight emerging concepts on the features of WAT expansion and turnover, and the significance in obesity, health, and disease.

Exercise has beneficial effects on metabolism and health. Although the skeletal muscle has been a primary focus, exercise also mediates robust adaptations in white adipose tissue. To determine if exercise affects in vivo adipocyte formation, fifty-two, sixteen-week-old C57BL/6J mice were allowed access to unlocked running wheels [Exercise (EX) group; n = 13 males, n = 13 females] or to locked wheels [Sedentary (SED) group; n = 13 males, n = 13 females] for 4-weeks. In vivo adipocyte formation was assessed by the incorporation of deuterium (²H) into the DNA of newly formed adipocytes in the inguinal and gonadal adipose depots. A two-way ANOVA revealed that exercise significantly decreased new adipocyte formation in the adipose tissue of mice in the EX group relative to the SED group (activity effect; P = 0.02). This reduction was observed in male and female mice (activity effect; P = 0.03). Independent analysis of the depots showed a significant reduction in adipocyte formation in the inguinal (P = 0.05) but not in the gonadal (P = 0.18) of the EX group. We report for the first time that exercise significantly reduced in vivo adipocyte formation in the adipose tissue of EX mice using a physiologic metabolic ²H₂O-labeling protocol.