The sucrose space of human subcutaneous adipose tissue in obesity

Comparison of DXA and water measurements of body fat following gastric bypass surgery and a physiological model of body water, fat, and muscle composition

Measurement of body composition changes following bariatric surgery is complicated because of the difficulty of measuring body fat in highly obese individuals that have increased photon absorption and are too large for the standard dual-energy X-ray absorptiometry (DXA) table. We reproducibly measured body composition from half-body DXA scans and compared the values of total body fat estimated from total body water (TBW) and DXA measurements before and after Roux-en-Y gastric bypass surgery (RYGB). DXA, TBW (deuterium dilution), extracellular water (ECW; bromide dilution), and intracellular water (ICW) measurement (by subtraction) were made before surgery and at 2 wk, 6 wk, 6 mo, and 12 mo after surgery. Twenty individuals completed baseline and at least four follow-up visits. DXA appeared to underestimate the fat and bone mass in extreme obesity (before surgery), whereas at 6 and 12 mo after surgery, the DXA and TBW fat measurements were similar. The ECW-to-ICW ratio was increased in obese individuals and increased slightly more after surgery. We describe a new model that explains this abnormal water composition in terms of the normal physiological changes that occur in body composition in obesity and weight loss. This model is also used to predict the muscle mass loss following RYGB.

Physiological models of body composition and human obesity

Background

The body mass index (BMI) is the standard parameter for predicting body fat fraction and for classifying degrees of obesity. Currently available regression equations between BMI and fat are based on 2 or 3 parameter empirical fits and have not been validated for highly obese subjects. We attempt to develop regression relations that are based on realistic models of body composition changes in obesity. These models, if valid, can then be extrapolated to the high fat fraction of the morbidly obese.

Methods

The analysis was applied to 3 compartment (density and total body water) measurements of body fat. The data was collected at the New York Obesity Research Center, Body Composition Unit, as part of ongoing studies. A total of 1356 subjects were included, with a BMI range of 17 to 50 for males and 17 to 65 for females. The body composition model assumes that obese subjects can be represented by the sum of a standard lean reference subject plus an extra weight that has a constant adipose, bone and muscle fraction.

Results

There is marked age and sex dependence in the relationship between BMI and fat fraction. There was no significant difference among Caucasians, Blacks and Hispanics while Asians had significantly greater fat fraction for the same BMI. A linear relationship between BMI and fat fraction provides a good description for men but overestimates the fat fraction in morbidly obese women for whom a non-linear regression should be used. New regression relations for predicting body fat just from experimental measurements of body density are described that are more accurate then those currently used. From the fits to the experimental BMI and density data, a quantitative description of the bone, adipose and muscle body composition of lean and obese subjects is derived.

Conclusion

Physiologically realistic models of body composition provide both accurate regression relations and new insights about changes in body composition in obesity.

Lack of antilipolytic effect of lactate in subcutaneous abdominal adipose tissue during exercise

The purpose of our study was to evaluate the potential inhibition of adipose tissue mobilization by lactate. Eight male subjects (age, 26. 25 +/- 1.75 yr) in good physical condition (maximal oxygen uptake, 59.87 +/- 2.77 ml. kg-1. min-1; %body fat, 10.15 +/- 0.89%) participated in this study. For each subject, two microdialysis probes were inserted into abdominal subcutaneous tissue. Lactate (16 mM) was perfused via one of the probes while physiological saline only was perfused via the other, both at a flow rate of 2.5 microl/min. In both probes, ethanol was also perfused for adipose tissue blood flow estimation. Dialysates were collected every 10 min during rest (30 min), exercise at 50% maximal oxygen consumption (120 min), and recovery (30 min) for the measurement of glycerol concentration. During exercise, glycerol increased significantly in both probes. However, no differences in glycerol level and ethanol extraction were observed between the lactate and control probes. These findings suggest that lactate does not impair subcutaneous abdominal adipose tissue mobilization during exercise.

Comparison of two methods to assess the tissue/blood partition coefficient for xenon in subcutaneous adipose tissue in man

A new method to calculate the tissue/blood partition coefficient (lambda) for xenon in studies on the subcutaneous adipose tissue blood flow was compared with a previously reported method based on local skinfold thickness (lambda LST). The former method included needle biopsies from the abdominal and femoral subcutaneous adipose tissue, and the mean fat cell diameter was measured (lambda ECT). The extracellular tissue fraction in subcutaneous tissue was then estimated from a diagram. The tissue lipid content was approximated to equal the relative intracellular volume and Ostwald's solubility coefficients for 133Xe, based on the distribution of xenon in lipid, albumin and 0.9% saline were applied. Estimated lambda-values based on needle biopsies from the abdominal site were: 8.6 +/- 0.1 versus 9.9 +/- 0.4 ml g-1 (mean +/- SE) (P < 0.05) and from the femoral site: 9.1 +/- 0.1 versus 9.6 +/- 0.2 in lean (n = 10) and obese subjects (n = 10), respectively. The corresponding lambda-values obtained from skinfold measurements were: 6.2 +/- 0.5 versus 11.0 +/- 0.4 (P < 0.001) and 6.9 +/- 0.3 versus 11.4 +/- 0.4 (P < 0.001) in lean and obese subjects, respectively. Pooled lambda LST-values correlated positively with estimated adipose tissue blood flow (ATBF) (r: 0.34, P < 0.05, n = 40) whereas no such correlation was found for lambda ECT-values. In conclusion, a new method is presented which may allow an accurate determination of, and which may lead to reliable data on, subcutaneous ATBF in both lean and obese subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of weight reduction on plasma lipoproteins and adipose tissue metabolism in obese subjects

The relationship between obesity and alterations in adipose tissue metabolism and lipid transport was studied in fourteen obese subjects before and after a weight reduction of 4-22 kg. Blood glucose and plasma insulin patterns after peroral glucose intake improved significantly, and plasma glucagon levels decreased markedly after treatment. Plasma triglyceride and total cholesterol levels were not altered, but there was a 20% (P less than 0.05) increase in HDL concentrations. Plasma free fatty acid and glycerol concentrations decreased, in parallel to a decrease in lipolysis rate in vitro. Lipoprotein lipase and hepatic lipase activities in postheparin plasma, as well as the intravenous fat tolerance test, were normal and did not change significantly after weight loss. Lipoprotein lipase activity in adipose tissue, expressed per cell, was elevated and did not change after weight reduction. Also, the enzyme activity did not increase after glucose intake before or after treatment. The lack of effect on lipoprotein lipase activity and regulation in combination with significant improvements of other aspects of lipid and glucose transport is consistent with the view that alterations in LPL activity and regulation may represent an early and possibly primary defect in the development of obesity.

The interstitial space of adipose tissue as determined by single injection and equilibration techniques

The interstitial 14C-sucrose space was determined in the subsutaneous adipose tissue of the dog using both a single injection-indicator dilution technique and an equilibration-tissue sampling method. 131I-albumin and 51Cr-labelled erythrocytes served as intravascular indicators. The conventional extrapolation method for single injection curves yielded space values several times lower than those measured by the equilibration technique, although sampling was continued until the venous outflow samples contained less than 1% of the peak 14C-sucrose activity. This discrepancy may be due to long transit times, for which the conventional extrapolation procedure does not account. An extrapolation procedure is proposed based on the assumption that the area under the normalized extra- and intravascular curves should be equal. By this procedure as well as by the equilibration technique the interstitial 14C-sucrose space in adipose tissue measures approximately 10 mlX100 g-1. The single injection technique does not appear applicable for space determinations in adipose tissue during sympathetic nerve stimulation (3-7 Hz), possibly due to "trapping" of the tracers within the tissue.