Votruba, Susanne B.

Objectives: To determine whether meal fatty acid (FA) metabolism differs by sex and obesity phenotypes and to examine how weight loss (WtL) through diet and exercise affects meal FA trafficking. Methods: Before WtL (study 1) 13 obese men (ObM), 9 upper body obese (UbO) and 8 lower body obese (LbO) women consumed a standard meal containing [ 3 H]triolein followed by subcutaneous abdominal and gluteal fat biopsies. Indirect calorimetry was measured by 5 h substrate oxidation. Whole/regional fat distribution was measured using DXA and CT scan. Participants then enrolled in a structured WtL program, after which 23 returned for study 2. Results: There were no differences (p=0.32) in meal FA oxidation between groups. Abdominal uptake of meal FA in LOb was greater than in UOb (p=0.026) and ObM (p<0.001). LOb took up more meal FA in the lower body than ObM (p=0.001). Participants who returned for study 2 lost 14 ± 1.1 kg. After WtL, uptake of meal FA in upper body subcutaneous fat increased (p=0.028) in UOb and ObM (p=0.046) and decreased in lower body fat (p=0.025) in ObM. WtL led to reduced 5 h postprandial total fat oxidation and increased 24 h meal FA oxidation. Conclusion: Changes in uptake of meal FA after WtL followed a pattern indicative of body fat distribution maintenance. These results highlight the potential importance of meal fat uptake in modulating body fat distribution in obesity. Funding: NIH RO1 DK45343, DK50456 and R00585.

An upper body (UB) fat distribution is correlated with insulin resistance (IR). It is not known whether IR is caused by an UB fat distribution or if IR predisposes to UB fat gain. We recruited 25 lean volunteers to gain ~2 kg of body fat over ~8 weeks. We hypothesized that IR individuals would gain more UB fat than insulin sensitive individuals. At baseline, insulin sensitivity was assessed by 24‐hour insulin AUC over a meal controlled inpatient stay. Meal fat disposal was measured by the meal fat tracer/fat biopsy approach. Participants were grouped into tertiles based on their AUC insulin: least insulin sensitive tertile 1 (T1) = 37,449±2600; middle tertile 2 (T2) = 22,11±3002; most insulin sensitive tertile 3 (T3)= 13,233±2452 uU/ml/24h (mean±sem). At baseline, volunteers in the different tertiles had significantly different % body fat (T1=29±2%, T2=22±2%, and T3=18±2%; P<0.005). The T1 had more upper body subcutaneous (UBSQ) fat at baseline (8.3±0.5 kg) than the T3 (6.1±0.5 kg; P<0.05). Visceral and lower body subcutaneous (LBSQ) fat at baseline did not differ significantly between tertiles. Baseline meal fat storage in LBSQ fat was greater in T1 than T3 (20.9±3.4% vs. 9.0±1.9%, P < 0.05). Changes in regional fat gain as a percentage of total fat gain was not different by tertiles (53.3±2.7%, 8.3±2.0%, and 37.8±1.8%, for UBSQ, visceral, and LBSQ). Baseline insulin sensitivity was not predictive of the proportion of fat gain in the visceral or UBSQ regions. This argues against the hypothesis that insulin resistance results in UB fat gain. The alternative explanation is that those who gain UB fat eventually develop IR. T3 stored the least meal fat in the LBSQ region, suggesting that regional fat depots have differing responses to the prevailing insulinemia, perhaps through regulation of LPL activity.

Women differ from men by the larger lower‐body subcutaneous fat mass and the tendency to gain more fat in that depot if obesity develops. We hypothesized that these differences in fat distribution may depend on preadipocyte availability. We identified the proportion of preadipocytes from the stromovascular (SV) cells in adipose tissue by immunofluorescent staining for aP2 (the adipocyte fatty acid trafficking protein) and CD68 (a macrophage marker). Macrophages are the other cell type, aside from preadipocytes, in the SV fraction of adipose tissue that express aP2. We determined the percent preadipocytes in the SV fraction of subcutaneous abdominal and femoral adipose tissue of 17 premenopausal lean women and 10 age‐matched lean men. Body fat compartments were measured by DEXA and CT scans of the abdomen. Compared to men, women had less visceral fat and more subcutaneous fat mass, predominantly in the lower‐body compartment: 7.0 ± 0.3 kg vs. 5.2 ± 0.4 kg, respectively (p < 0.01). Femoral adipocytes were significantly larger than abdominal adipocytes in both groups (lipid per fat cell in women and men: abdomen, 0.46 ± 0.03 and 0.44 ± 0.06 μg/cell; thigh, 0.66 ± 0.04 and 0.55 ± 0.08 μg/cell, respectively). The percent preadipocytes in the SV fraction from femoral subcutaneous adipose tissue was greater in women than in men (29.7 ± 2.6% vs. 19.3 ± 2.0%, p < 0.02) but similar when isolated from the abdominal depot (29.9 ± 3.3% vs. 27.2 ± 1.9%). Our findings suggest that the fraction of femoral stromovascular cells as preadipocytes is a potential factor for sex‐dependent differences in regional fat distribution. The study was funded by RO1 DK 45343‐13S to M. Jensen.

Although body fat distribution strongly predicts metabolic health outcomes related to excess weight, little is known about the factors an individual might exhibit that predict a particular fat distribution pattern. We utilized the meal fatty acid tracer-adipose biopsy technique to assess upper and lower body subcutaneous (UBSQ and LBSQ, respectively) meal fat storage in lean volunteers who then were overfed to gain weight. Meal fatty acid storage in UBSQ and LBSQ adipose tissue, as well as daytime substrate oxidation (indirect calorimetry), was measured in 28 nonobese volunteers [ n = 15 men, body mass index = 22.1 ± 2.5 (SD)] before and after an ∼8-wk period of supervised overfeeding (weight gain = 4.6 ± 2.2 kg, fat gain = 3.8 ± 1.7 kg). Meal fat storage (mg/g adipose tissue lipid) in UBSQ ( visit 1: 0.78 ± 0.34 and 1.04 ± 0.71 for women and men, respectively, P = 0.22; visit 2: 0.71 ± 0.24 and 0.90 ± 0.37 for women and men, respectively, P = 0.08) and LBSQ ( visit 1: 0.60 ± 0.23 and 0.48 ± 0.29 for women and men, respectively, P = 0.25; visit 2: 0.62 ± 0.24 and 0.65 ± 0.23 for women and men, respectively, P = 0.67) adipose tissue did not differ between men and women at either visit. Fractional meal fatty acid storage in UBSQ (0.31 ± 0.15) or LBSQ (0.19 ± 0.13) adipose tissue at visit 1 did not predict the percent change in regional body fat in response to overfeeding. These data indicate that meal fat uptake trafficking in the short term (24 h) is not predictive of body fat distribution patterns. In general, UBSQ adipose tissue appears to be a favored depot for meal fat deposition in both sexes, and redistribution of meal fatty acids likely takes place at later time periods.

Body fat distribution is an important predictor of the metabolic consequences of obesity, but the cellular mechanisms regulating regional fat accumulation are unknown. We assessed the changes in adipocyte size (photomicrographs) and number in response to overfeeding in upper- and lower-body s.c. fat depots of 28 healthy, normal weight adults (15 men) age 29 ± 2 y. We analyzed how these changes relate to regional fat gain (dual energy X-ray absorptiometry and computed tomography) and baseline preadipocyte proliferation, differentiation [peroxisome proliferator-activated receptor-γ2 (PPARγ2) and CCAAT/enhancer binding protein-α (C/EBPα) mRNA]), and apoptotic response to TNF-α. Fat mass increased by 1.9 ± 0.2 kg in the upper body and 1.6 ± 0.1 kg in the lower body. Average abdominal s.c. adipocyte size increased by 0.16 ± 0.06 μg lipid per cell and correlated with relative upper-body fat gain ( r = 0.74, P < 0.0001). However, lower-body fat responded to overfeeding by fat-cell hyperplasia, with adipocyte number increasing by 2.6 ± 0.9 × 10 9 cells ( P < 0.01). We found no depot-differences in preadipocyte replication or apoptosis that would explain lower-body adipocyte hyperplasia and abdominal s.c. adipocyte hypertrophy. However, baseline PPARγ2 and C/EBPα mRNA were higher in abdominal than femoral s.c. preadipocytes ( P < 0.005 and P < 0.03, respectively), consistent with the ability of abdominal s.c. adipocytes to achieve a larger size. Inherent differences in preadipocyte cell dynamics may contribute to the distinct responses of different fat depots to overfeeding, and fat-cell number increases in certain depots in adults after only 8 wk of increased food intake.

OBJECTIVE— We assessed the direct (VLDL-triglycerides [VLDL-TG] independent) storage of circulating free fatty acids (FFAs) in visceral and subcutaneous fat in postabsorptive women. RESEARCH DESIGN AND METHODS— Twelve women (BMI 29.6 ± 6.6 kg/m2) received an identical, intravenous bolus dose of [1-14C]oleate followed by timed subcutaneous fat biopsies (abdominal and femoral) and then omental fat biopsy during tubal ligation surgery. Regional fat masses were assessed by combining dual-energy X-ray absorptiometry and computed tomography scanning. Separately, we assessed the fraction of FFA tracer entering VLDL-TG over the time representing the delay in collecting omental fat. RESULTS— Site-specific fat specific activity (SA) (dpm/g lipid) decreased as a function of fat mass in both upper-body subcutaneous (UBSQ) and visceral fat depots. These patterns are consistent with dilution of a relatively fixed amount of FFA tracer within progressively greater amounts of fat. Interestingly, femoral SA did not vary as a function of lower-body subcutaneous (LBSQ) fat mass. [1-14C]oleate storage per million LBSQ adipocytes was positively associated with LBSQ fat mass, but no significant relationships were observed in UBSQ or visceral fat depot. The fraction of [1-14C]oleate stored in UBSQ, LBSQ, and visceral fat was 6.7 ± 3.2, 4.9 ± 3.4, and 1.0 ± 0.3%, respectively. Only ∼4% of the tracer traversed VLDL-TG over 9.5 h. CONCLUSIONS— The increase in FFA tracer storage per adipocyte as a function of LBSQ fat mass implies that LBSQ adipocytes, in contrast to UBSQ and omental adipocytes, store more FFA in women with greater adiposity. The direct FFA storage pathway might play a role in favoring lower-body fat accumulation in women.

OBJECTIVE—Differential meal fat uptake into adipose tissue depots may be a determinant of body fat distribution. RESEARCH DESIGN AND METHODS—We used the meal fat tracer/adipose tissue biopsy approach to compare the effects of meal fat content on the fat uptake into visceral and upper and lower body subcutaneous fat depots in 21 premenopausal women. [3H]triolein was used to trace the fate of fatty acids from a normal-fat or high-fat meal. RESULTS—The proportion of dietary fat uptake into the three depots did not differ between meals; visceral fat accounted for only ∼5% of meal fat disposal irrespective of visceral fat mass. For the women consuming the normal-fat meal, the uptake of meal fatty acid into femoral fat (milligrams meal fat per gram lipid) increased as a function of leg fat mass (r = 0.68, P &lt; 0.05), which we interpret as increased efficiency of uptake. The opposite pattern was seen in omental fat with the normal-fat meal and in all depots after the high-fat meal. For both meals, ∼40% of meal fat was oxidized (3H2O production) after 24 h. CONCLUSIONS—We conclude that greater thigh adipose tissue in women is associated with greater efficiency of meal fat storage under conditions of energy balance, whereas the opposite is seen with visceral fat. These findings imply that different mechanisms may regulate fatty acid uptake in different depots, which may in turn impact on body fat distribution.

Humans have a large variability in body fat distribution, which has tremendous implications for metabolic health. Obese individuals with an upper-body-fat distribution have increased health complications such as dyslipidemia, hypertension, insulin resistance, and type 2 diabetes in comparison with lower-body-obese individuals. Additionally, females have more body fat, a greater proportion of fat in their lower body, and much less visceral fat than do lean males at the same body mass index. The reasons for these differences in body fat distribution have not been clearly identified but could be important. Herein we review what has been learned about regional differences in triglyceride storage capacity and lipolysis as they relate to the causes and consequences of regional fat accumulation. Both sex and site differences in regional fat storage have been described. In contrast, with the exception of variations between men and women in the contribution of visceral adipose tissue to hepatic FFA delivery, most studies have failed to show important sex differences in regional lipolysis in vivo.