Direct Free Fatty Acid Storage in Different Sized Adipocytes from the Same Depot

Objective—Human adipocytes take up free fatty acids (FFA) directly from the circulation, even at times of high lipolytic activity. Whether these processes occurs simultaneously within the same cells or are partitioned between different cells, for example large and small cells, is unknown. Design and Methods—We measured direct FFA storage in subcutaneous fat in 13 adults using a continuous infusion of [U-13C]palmitate and a bolus of [1-14C]palmitate followed 30 min later by abdominal and femoral adipose biopsies. The adipocytes were isolated by digestion procedures and separated into small, medium and large populations by differential floatation. Results—We were able to isolate populations of adipocytes that were statistically and clinically (~3 fold different) in size. Adipocyte lipid SA was not different between small, medium and large cells, therefore, FFA storage per unit lipid was not different. However, FFA storage rates were significantly (2-4 times) greater per cell in large than small cells (P < 0.005). In summary, relative to lipid content, FFA storage rates are not different in large and small adipocytes, however, large cells have greater storage rates per cell. Conclusions—This suggests that the processes of FFA release and storage are taking place simultaneously in adipocytes. Keywords adipose biopsy; free fatty acid isotopic tracers; femoral adipose tissue; abdominal adipose tissu