Jensen, Michael D.

To determine the source(s) of blood and very low density lipoprotein (VLDL)-triglyceride glycerol during fasting, four men ingested2H2O from 14 to 20 h into a 60-h fast to achieve ∼0.5% body water enrichment. At 60 h of fasting, glycerol flux was measured using [2-14C]glycerol. Blood was taken for measurement of2H enrichment at carbon 6 of glucose and at carbon 3 of free glycerol and VLDL-triglyceride glycerol.2H enrichment of the 2 hydrogens bound to carbon 3 of VLDL-triglyceride glycerol was 105 ± 2% of the2H enrichment of the 2 hydrogens bound to carbon 6 of glucose, indicating isotopic equilibrium between hepatic glyceraldehyde 3- P and glycerol 3- P. The2H enrichment of the 2 hydrogens bound to carbon 3 of free glycerol was 17 ± 3% of VLDL-triglyceride glycerol, indicating that a significant percentage of free glycerol in blood originated from the hydrolysis of circulating VLDL-triglyceride or a pool of glycerol with similar2H enrichment. Glycerol flux was 6.3 ± 1.1 μmol · kg−1· min−1. Glycerol appearing from nonadipose tissue sources was then ∼1.1 μmol · kg−1· min−1. Seven other subjects were fasted for 12, 42, and 60 h. A small percentage of glycerol in the circulation after 12 h of fasting was enriched with2H. The enrichment of the 2 hydrogens bound to carbon 3 of free glycerol in the longer periods of fasting was ∼16% of the enrichment of the 2 hydrogens bound to carbon 6 of glucose. Therefore, as much as 15–20% of systemic glycerol turnover during fasting is not from lipolysis of adipose tissue triglyceride.

These studies were conducted to understand the relationship between measures of systemic free fatty acid (FFA) reesterification and regional FFA, glycerol, and triglyceride metabolism during fasting. Indirect calorimetry was used to measure fatty acid oxidation in six men after a 60-h fast. Systemic and regional (splanchnic, renal, and leg) FFA ([3H]palmitate) and glycerol ([3H]glycerol) kinetics, as well as splanchnic triglyceride release, were measured. The rate of systemic FFA reesterification was 366 ± 93 μmol/min, which was greater ( P < 0.05) than splanchnic triglyceride fatty acid output (64 ± 6 μmol/min), a measure of VLDL triglyceride fatty acid export. The majority of glycerol uptake occurred in the splanchnic and renal beds, although some leg glycerol uptake was detected. Systemic FFA release was approximately double that usually present in overnight postabsorptive men, yet the regional FFA release rates were of the same proportions previously observed in overnight postabsorptive men. In conclusion, FFA reesterification at rest during fasting far exceeds splanchnic triglyceride fatty acid output. This indicates that nonhepatic sites of FFA reesterification are important, and that peripheral reesterification of FFA exceeds the rate of simultaneous intracellular triglyceride fatty acid oxidation.

To determine whether type 2 diabetes mellitus alters systemic and regional free fatty acid ([3H]palmitate) metabolism, 14 nondiabetic (ND) and 14 type 2 diabetic (D) subjects underwent hyperinsulinemic-hyperglycemic (∼9.3 mM) clamps. The subjects were matched for age, body mass index, percent body fat, and fat-free mass. D subjects had more ( P < 0.05) visceral fat than ND. During somatostatin, replacement growth hormone, and glucagon infusions, insulin was infused to achieve moderate (∼75 pmol/l) and high (∼150 pmol/l) physiological insulin levels. D subjects had greater ( P < 0.02) systemic and regional (splanchnic and leg) palmitate release than ND subjects during both insulin infusion intervals. The relative contributions of splanchnic, leg, and nonsplanchnic upper body regions to systemic palmitate release did not differ between groups, although the last contributed the most (∼75%) to systemic palmitate release. Visceral fat area correlated with systemic palmitate flux ( r = 0.45, P < 0.03) during both insulin infusions. We conclude that type 2 diabetes is associated with a generalized impairment in insulin suppression of lipolysis compared with equally obese ND individuals.

Dual-energy X-ray absorptiometry (DEXA) is reported to be inferior to computed tomography (CT) to measure changes in appendicular soft tissue composition. We compared CT- and DEXA-measured thigh muscle and fat mass to evaluate the random and systematic discrepancies between these two methods. Thigh skeletal muscle area (single-slice CT) was suboptimally ( r2= 0.74, P < 0.0001) related to DEXA-measured thigh fat-free mass (FFM). In contrast, thigh muscle and adipose tissue volumes (multislice CT) were highly related to DEXA-measured thigh FFM and fat (both r2= 0.96, P < 0.0001). DEXA-measured leg fat was significantly less than multislice-CT-measured leg adipose tissue volume, whereas multislice-CT-measured leg muscle mass was less ( P < 0.0001) than DEXA-measured leg FFM. The systematic discrepancies between the two approaches were consistent with the 10–15% nonfat components of adipose tissue. In conclusion, CT and DEXA measures of appendicular soft tissue are highly related. Systematic differences between DEXA and CT likely relate to the underlying principles of the techniques.

During running exercise above the lactate threshold (LT), it is unknown whether free fatty acid (FFA) mobilization can meet the energy demands for fatty acid oxidation. This study was performed to determine whether FFA availability is reduced during running exercise above compared with below the LT and to assess whether the level of endurance training influences FFA mobilization. Twelve marathon runners and 12 moderately trained runners ran at a workload that was either above or below their individual LT. Fatty acid oxidation (indirect calorimetry) and FFA release ([1–14C]palmitate) were measured at baseline, throughout exercise, and at recovery. The plasma FFA rate of appearance increased during exercise in both groups; running above or below the LT, but the total FFA availability for 30 min of exercise was greater (P < 0.01) in the below LT group (marathon, 23 +/- 2 mmol; moderate, 21 +/- 2 mmol) than in the above LT group (18 +/- 3 and 13 +/- 3 mmol, respectively). Total fatty acid oxidation (indirect calorimetry) greatly exceeded circulating FFA availability, regardless of training or exercise group (P < 0.01). No statistically significant exercise intensity or training differences in fatty acid oxidation were found (above LT: marathon, 71 +/- 12, moderate, 64 +/- 17 mmol/30 min; below LT: marathon 91 +/- 12, moderate, 60 +/- 5 mmol/30 min). In conclusion, during exercise above or below LT, circulating FFA cannot meet the oxidative needs and intramuscular triglyceride stores must be utilized. Further marathon training does not enhance effective adipose tissue lipolysis during exercise compared with moderate endurance training.

To determine whether the sensitivity of adipose tissue lipolysis to catecholamines is increased in poorly controlled insulin-dependent diabetes, the lipolytic response to epinephrine was measured in seven nondiabetic volunteers and seven poorly controlled diabetic subjects with use of [1-(14)C]palmitate as a tracer. Subjects received sequential 1-h infusions of epinephrine, which produced epinephrine concentrations of approximately 1,000, approximately 1,750, approximately 3,500, and approximately 6,000 pmol/l. A pancreatic clamp was used to maintain constant plasma hormone levels. Concentration-response curves were constructed for each subject from the integrated lipolytic response during each epinephrine infusion. There was no difference in maximal lipolytic response (117 +/- 19 vs. 152 +/- 11 mumol.kg-1.h-1) or in maximally effective (3,171 +/- 267 vs. 3,357 +/- 349 pmol/l) or half-maximally effective (1,081 +/- 109 vs. 1,015 +/- 120 pmol/l) epinephrine concentrations between nondiabetic and diabetic subjects, respectively (all P = NS). In control subjects, maximum beta-hydroxybutyrate concentrations were achieved at lower epinephrine concentrations than those required for a maximum lipolytic effect. Thus, under pancreatic clamp conditions, the lipolytic response to epinephrine in nondiabetic and diabetic subjects was similar.

We previously found that epinephrine, a mixed beta- and alpha-adrenoreceptor agonist, stimulates systemic and nonsplanchnic upper body free fatty acid (FFA) release but not lower body FFA release in healthy nonobese women. To evaluate the role of beta-adrenergic-mediated effects on this regional difference in lipolysis, we measured systemic, leg, and splanchnic FFA kinetics ([3H]palmitate) in seven healthy nonobese women before and during an intravenous isoproterenol infusion. Isoproterenol increased systemic palmitate flux (87 +/- 12 vs. 100 +/- 10 mumol/min, P < 0.05) but failed to affect leg [10.8 +/- 1.2 vs. 11.4 +/- 2.3 mumol/min, P = not significant (NS)] or splanchnic (10.8 +/- 3.2 vs. 10.0 +/- 1.8 mumol/min, P = NS) palmitate release. Upper body nonsplanchnic palmitate release increased from 56 +/- 14 to 71 +/- 10 mumol/min. Systemic O2 consumption increased (227 +/- 11 to 241 +/- 10 ml/min, P = 0.006) during isoproterenol infusion, as did leg (318 +/- 42 vs. 404 +/- 53 ml/min, P < 0.01) and splanchnic (827 +/- 104 vs. 970 +/- 108 ml/min, P < 0.05) plasma flow. These results suggest that lower body adipose tissue lipolysis in women is less sensitive or responsive than nonsplanchnic upper body adipose tissue to beta-adrenergic stimulation and that regional differences in alpha 2-adrenergic-receptor responses were not responsible for the similar regional differences we observed previously with epinephrine.

These studies were conducted to determine whether there are gender-specific regional differences in meal triglyceride fatty acid uptake. Systemic and regional oleate ([3H]oleate) kinetics were measured in nine nonobese men and eight nonobese women before and at the end of a 6-h meal, administered as small frequent feedings to achieve steady-state chylomicronemia. Chylomicron uptake in the splanchnic bed accounted for 71 +/- 15% of meal triglyceride disappearance in men and 20 +/- 7% in women (P < 0.01), whereas leg chylomicron uptake could only account for 12 +/- 2 and 8 +/- 4% (P not significant in men vs. women) of meal triglyceride disappearance. Meal ingestion suppressed (P < 0.05) systemic and regional free fatty acid release in both men and women. Splanchnic nonchylomicron triglyceride release and leg nonchylomicron triglyceride uptake were not significantly different in men and women. In summary, the largest quantitative difference between men and women in fatty acid kinetics during meal ingestion is a substantially greater splanchnic uptake of meal triglyceride fatty acids in men. This could represent greater meal fatty acid storage in visceral adipose tissue.

Upper-body and lower-body adipocytes respond differently to physiological catecholamines in vitro. It is not known whether this is true in vivo or whether gender differences exist in the regional adipose tissue responses to epinephrine. These studies were therefore conducted to examine free fatty acid (FFA) release ([3H]palmitate) from lower-body (leg), splanchnic, and upper-body adipose tissue in normal-weight adult men (n = 8) and women (n = 7). In response to intravenous epinephrine (10 ng.kg-1.min-1), palmitate release increased (P < 0.01) in both men (168 +/- 10 to 221 +/- 15 mumol/min) and women (177 +/- 12 to 234 +/- 18 mumol/min). Basal leg palmitate release was similar in women and men (16.8 +/- 2.9 and 12.4 +/- 1.3 mumol/min, P = not significant) but doubled (P < 0.01) in response to epinephrine in men and was virtually unchanged in women. Splanchnic palmitate release increased (P < 0.05) in men (n = 6) but not in women (n = 6), whereas nonsplanchnic upper-body palmitate release increased more in women than in men. Upper-body (splanchnic and nonsplanchnic) palmitate release increased (P < 0.05) in both men and women in response to epinephrine. In summary, lower-body adipose tissue FFA release increased in response to epinephrine in men but not women, whereas upper-body palmitate release increased in both groups. These findings are consistent with some in vitro findings and suggest that catecholamine action may play a role in determining gender-based differences in body fat distribution.

We tested for gender differences in dietary fatty acid metabolism in 12 nonobese men and 12 nonobese women using the meal fatty acid tracer/adipose tissue biopsy study design. In addition to determining body composition, measurements of regional adipose tissue lipoprotein lipase activity, blood flow, and fat cell size were performed to place the meal fatty acid kinetic studies in perspective. Twenty-four hours after ingesting the test meal, the concentration of meal fatty acids was greater ( P < 0.05) in abdominal subcutaneous than in thigh adipose tissue in both men (0.61 ± 0.12 vs. 0.45 ± 0.09 mg/g) and women (0.59 ± 0.10 vs. 0.43 ± 0.05) but was not different between men and women. A greater percentage of dietary fat was stored in subcutaneous adipose tissue in women than in men (38 ± 3 vs. 24 ± 3%, respectively, P < 0.05), and a greater portion of meal fatty acid disposal was unaccounted for in men. Significant gender differences in regional adipose tissue blood flow after meal ingestion were noted; the differences were in the direction that could support greater nutrient storage in lower body fat in women.