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Am J Physiol Endocrinol Metab 279: E875-E885, 2000;
0193-1849/00 $5.00
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Vol. 279, Issue 4, E875-E885, October 2000

Fat oxidation, lipolysis, and free fatty acid cycling in obesity-prone and obesity-resistant rats

S. Renee Commerford1,2,3, Michael J. Pagliassotti1,3, Christopher L. Melby2, Yuren Wei1, Ellis C. Gayles1, and James O. Hill1

1 Department of Pediatrics, University of Colorado Health Sciences Center, Denver 80262; 2 Department of Food Science and Human Nutrition, Colorado State University, Fort Collins, Colorado 80525; and 3 Department of Exercise Science and Physical Education, Arizona State University, Tempe, Arizona 85287

Defects in fat metabolism may contribute to the development of obesity, but what these defects are and where they occur in the feeding/fasting cycle are unknown. In the present study, basal fat metabolism was characterized using a high-fat diet (HFD)-induced model of obesity development. Male rats consumed a HFD (45% fat, 35% carbohydrate) ad libitum for either 1 or 5 wk (HFD1 or HFD5). After 1 wk on the HFD, rats were separated on the basis of body weight gain into obesity-prone (OP, >= 48 g) or obesity-resistant (OR, <= 40 g) groups. Twenty-four-hour-fasted rats were studied either at this time (OP1, OR1) or after 5 wk (OP5, OR5). Fat pad weight (sum of epididymal, retroperitoneal, and mesenteric fat pads) at HFD1 was 26% greater and at HFD5 was 43% greater (P<= 0.05) in OP vs. OR. Free fatty acid rates of appearance (FFA Ra) and oxidation were not significantly different between OP and OR at 1 or 5 wk. Glycerol Ra, when expressed in absolute terms (µmol/min), increased from 1 to 5 wk of HFD feeding in both OP and OR, but significantly so only in OP. Likewise, increased rates of intracellular FFA cycling [estimated as (3 × glycerol Ra) - FFA Ra] were observed in both OP and OR rats from 1 to 5 wk of HFD feeding, but significantly so in OP rats only. When expressed relative to fat cell volume (µmol · pl-1 · min-1), neither lipolysis nor intracellular cycling was significantly different between OP and OR, regardless of time on HFD. These data suggest that 1) if low rates of fat oxidation contribute to obesity development in OP rats, the contribution does not occur at times when fat oxidation is at or near maximum rates (i.e., 24-h fasted conditions), and 2) between 1 and 5 wk of HFD feeding, basal lipolysis and reesterification may work to expand fat cell volume and increase fat pad weight in both OP and OR rats, although more so in OP rats.

fatty acid metabolism; reesterification; triglyceride cycling; high-fat diet


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