¹Department of Internal Medicine, Chibaken Saiseikai Narashino Hospital, Narashino, Japan

²Department of Radiology, Chibaken Saiseikai Narashino Hospital, Narashino, Japan

Correspondence to: J Kobayashi, 1-1-1 Izumicho, Narashino City, Chiba Prefecture 275-8580 Japan. E-mail: koba231@attglobal.net

OBJECTIVES: We present a novel method of scanning for intra-abdominal fat volume by helical computed tomography (CT), and describe the clinical significance of measuring the volumes of intra-abdominal visceral fat (V_vol) and subcutaneous fat (S_vol) vs these respective areas determined by conventional slice-by-slice CT at the umbilical level.

METHOD: Subjects with obesity or hyperlipidemia (79 men, 74 women) were recruited for this study. We obtained helical CT scans with a tube current of 150 mA, voltage of 120 kV and 2:1 pitch (table speed in relation to slice thickness), starting at the upper edge of the liver and continuing until the pelvis. The intra-abdominal visceral fat volume was measured by drawing a line within the muscle wall surrounding the abdominal cavity. The abdominal subcutaneous fat volume was calculated by subtracting the visceral fat volume from the total abdominal fat volume. By comparison, the intra-abdominal visceral and subcutaneous fat areas were determined at the umbilical level by the established slice-by-slice CT scanning technique.

RESULTS: V_vol was correlated positively with visceral fat area (V_area) measured by conventional CT in both genders (in men (n=79) V_vol vs V_area, r=0.81 P<0.0001; in women (n=74) V_vol vs V_area, r=0.85, P<0.0001). S_vol also showed a positive correlation with subcutaneous fat area (S_area) in both genders (in men (n=78) S_vol vs S_area, r=0.95, P<0.0001; in women (n=74) S_vol vs S_area, r=0.92, P<0.0001).

CONCLUSION: We have reported a novel method for measuring intra-abdominal fat volume by the use of helical CT.

International Journal of Obesity (2002) 26, 398-402. DOI: 10.1038/sj/ijo/0801921

intra-abdominal fat; helical computed tomography; subcutaneous fat; visceral fat; leptin

Introduction

Helical computed tomography (CT) is a new volume acquisition mode in X-ray CT which was introduced in the late 1980s. This helical data set is obtained by moving the table on which the patient lies at constant speed, while the X-ray tube continuously rotates and a sustained exposure is made.^1,2 Anatomical ranges are scanned continuously within the scanned volume. Helical CT is known to have a number of important potential advantages for examining the abdomen, compared with standard CT.^3,4 In comparison to conventional CT, helical CT is less sensitive to motion artifacts and, because of the volume acquisition during breathing suspension, there is no misregistration due to different breathing levels. Until recently, however, there had been no study that applied this technique to the measurement of body fat mass. Rogalla et al⁵ reported with low-dose helical CT in 24 post-gastrectomy patients that percentage fat and percentage subcutaneous fat were significantly larger in women, as were percentage intra-abdominal and mediastinal fat in men.

It has been demonstrated that individuals with upper body fat accumulation are more susceptible to progression of atherosclerosis than those with lower body fat accumulation.⁶ In the 1980s the method of evaluating intra-abdominal visceral fat area using CT was reported, and this fat was suggested to play an important role in the development of diabetes mellitus, hypertension and hyperlipidemia.^7,8,9,10 However, it remains to be clarified in what manner and to what extent individual fat tissue contributes to abdominal metabolism. In this study, therefore, we conducted helical CT to measure the precise value of fat accumulation and fat volume. We also investigated the relationship between visceral fat mass vs serum lipids by utilizing V_vol as an indicator of visceral fat accumulation as we presume that V_vol might be a more reliable indicator of metabolic disorders than V_area.

In recent years, adipocytes have been found to be responsible for the secretion of various kinds of hormones and cytokines,¹¹ in contrast to the previous notion. Leptin, a cytokine identified and cloned by Zhang et al,¹² which is produced and secreted mainly in adipocytes,¹³ is known to be involved in the neuroendocrine regulation of adiposity and its metabolic sequelae.^14,15,16 Its plasma concentrations have been reported to be associated with body mass index (BMI) or percentage fat,^17,18 or total body fat mass by dual energy X-ray absorptiometry.¹⁹ However, controversy still exists as to which fat mass is strongly associated with plasma leptin level, visceral or subcutaneous fat.^20,21 As the importance of these various findings has become apparent, the requirement for improved techniques that can reliably measure intra-abdominal fat mass has become equally evident.

Here we present the basic principles and clinical significance of a novel method of scanning for intra-abdominal fat volume using helical CT. We also analyzed the relationship between intra-abdominal visceral (or subcutaneous) fat volume measured by this method (or these respective areas) vs serum leptin levels, as well as vs serum lipids.

Materials and methods

Subjects (Table 1)

Subjects with obesity or hyperlipidemia (79 men aged 24-72, 74 women aged 26-73) were recruited for this study. Obesity was defined as individuals having a BMI greater than 25 kg/m², according to the definition established by the Japan Society for Study of Obesity in 1999. Of the study subjects, five men and six women were on glibenclamide. However, none of the subjects received insulin therapy or insulin sensitizing agents such as biguanide or thiazolidinediones. Individuals with poorly controlled diabetes mellitus, kidney disease, liver disease and endocrinological disorders were excluded from this study. Of the study subjects, four men had received statins, two had probucol, five had fibrates, one had niacin and one had resin, while 10 women had received statins, two probucol, three fibrates and one niacin, but these drugs were discontinued at least 6 weeks before this study. Venous blood was drawn from the forearm after 12 h fasting for assay of serum lipid and lipoprotein levels. Serum total cholesterol (TC) and triglyceride (TG) concentrations were measured enzymatically. High-density lipoprotein cholesterol (HDL-C) was measured after precipitation of whole plasma with heparin calcium.

Scanning for intra-abdominal fat volume in spiral CT

The subjects were examined in a supine position with both arms stretched above the head using Toshiba X-Vision GX (Tokyo, Japan). We obtained helical CT scans with a tube current of 150 mA, voltage of 120 kV and 2:1 pitch (table speed in relation to slice thickness), starting at the upper edge of the liver and continuing to the pelvis. The intra-abdominal visceral fat volume was measured by drawing a line within the muscle wall surrounding the abdominal cavity. The abdominal subcutaneous fat volume was calculated by subtracting the visceral fat volume from the total abdominal fat volume. The intra-abdominal visceral and subcutaneous fat areas were determined at the umbilical level by the previously reported CT scanning technique.^7,8

Measurement of serum leptin

Measurement of serum leptin was carried out by a radioimmunoassay as described by Zhongmin et al.²² This assay measures the total (free plus bound) leptin in the blood. Sensitivity was 0.5 ng/ml and the intra-assay and interassay coefficients of variation were less than 3 and 8%, respectively.

Statistical analysis

StatView-J 5.0 software was used for all statistical analysis in this study. Values are shown as mean±s.d. Simple linear correlations, Pearson's correlation coefficients and partial correlation analysis were carried out. A difference with a P-value <0.05 was considered to be statistically significant.

Results

Intra-adbdominal visceral fat volume (V_vol), measured by helical CT, was found to be positively correlated with visceral fat area (V_area) measured by conventional slice-by-slice CT in both genders (Figure 1). Intra-abdominal subcutaneous fat volume (S_vol) also showed a positive correlation with subcutaneous fat area (S_area) in both genders (Figure 2). We then investigated the respective correlations of BMI vs V_vol, V_area, S_vol or S_area in both genders. Subcutaneous fat mass correlated with BMI more strongly than visceral fat mass both in terms of area and volume. V_vol was found to have a stronger correlation with BMI than V_area in both genders (Table 2).

We then analyzed the correlation between intra-abdominal fat mass vs serum lipids (Table 3). V_area and V_vol showed a positive correlation with serum TG levels in men, whereas in women this association was not observed. In both genders, subcutaneous fat showed a weak inverse correlation with serum HDL-C level. However, serum TC did not show correlation with any of intra-abdominal fat mass.

To determine how intra-abdominal fat mass affects serum leptin levels, we analyzed the correlations of BMI, subcutaneous or visceral fat mass vs serum leptin levels in a subset of these subjects (men (n=26), age 48±15 y, BMI 30±5.0 kg/m², leptin 6.5±3.4 ng/ml; women (n=26), age 56±12 y, BMI 29±3.8 kg/m², leptin 15±7.8 ng/ml). In both genders, BMI correlated positively with serum leptin levels (men, r=0.69, P=0.0004; women, r=0.67, P=0.0002). We then investigated the relationship between subcutaneous fat mass and serum leptin levels. Subcutaneous fat mass, either volume or area, correlated with serum leptin more strongly than BMI in both genders. In both genders, the correlation coefficient was higher in S_vol vs leptin than S_area vs leptin (men, r=0.73, P<0.0001 for S_area vs leptin; r=0.74, P<0.0001 for S_vol vs leptin; women, r=0.73, P<0.0001 for S_area vs leptin; r=0.75, P<0.0001 for S_vol vs leptin). Next we investigated the relationship between visceral fat mass and serum leptin levels. In men neither V_vol nor V_area showed significant correlation with serum leptin levels (r=0.33, P=0.10 for V_vol vs leptin; r=0.20, P=0.31 for V_area vs leptin), whereas in women, both V_vol and, to a lesser degree, V_area had a positive correlation with serum leptin (r=0.58, P=0.002 for V_vol vs leptin; r=0.41, P=0.035 for V_area vs leptin). The correlation between V_area vs serum leptin in women did not persist after adjustments for V_vol, while V_vol was still positively correlated with serum leptin after adjustments for V_area (data not shown).

Discussion

In the present study we have described a novel method of assessing intra-abdominal fat mass using helical CT, which made it possible to measure fat volume. The main findings of the present study were as follows: (1) there were strong positive associations between intra-abdominal V_vol and S_vol measured by helical CT vs respective fat areas assessed by conventional slice-by-slice CT in both genders; (2) V_vol showed a stronger correlation with BMI than V_area in both genders; (3) in men, V_area and V_vol showed a positive correlation with plasma TG levels; (4) in both genders, intra-abdominal visceral fat mass showed a weaker correlation with serum leptin levels than did subcutaneous fat mass.

Helical CT is a rather new, advanced technique used for diagnosing several disorders.^1,3,4 However, until recently, there had been no report describing its application to the assessment of intra-abdominal body fat distribution. In 1998, it was clarified by using low-dose helical CT technique in 24 post-gastrectomy subjects comprising nine women and 15 men with a mean age of 60 y, that percentage fat and percentage subcutaneous fat were significantly greater in women than was percentage intra-abdominal fat in men.⁵ However, they did not study the correlation between fat volumes by helical CT vs those respective areas measured using conventional methods.

One of the great advantages of measuring intra-abdominal body fat volumes over measuring those respective areas is assumed to be that measuring fat volumes reflects body fat mass more precisely than measuring the respective areas. Indeed, in our study, visceral fat volume showed a stronger correlation with BMI than visceral fat area in both genders (Table 2).

Another method of measuring intra-abdominal fat volume distribution has been by magnetic resonance imaging (MRI). Total body MRI studies have demonstrated that intra-abdominal visceral fat volume is relatively small, 0.5-8.5 1 in men and 0.9-5.5 1 in obese women, and accounts for only 6-20% of total adipose tissue volume in obese individuals.^23,24 The results of the present study, showing that visceral and subcutaneous fat accounts for 47 and 53%, respectively, of intra-abdominal total fat volume in men, whereas in women they account for 32 and 68%, respectively (Table 1), also indicated that visceral fat volume accounted for less than subcutaneous fat volume.

Considering the fact that the method of evaluating intra-abdominal visceral fat area using conventional CT clarified that this fat plays an important role in the development of hyperlipidemia,^7,8,9,10 we also analyzed in this study the relationship between fat mass measured by helical CT vs serum lipids. The result that in men visceral fat mass showed a positive correlation with plasma TG levels is consistent with the previous findings.^8,9 This was not the case in women, which may be relevant to the fact that the present study included both pre- and post-menopausal women.

There are conflicting reports as to whether plasma leptin is associated with total body, subcutaneous or visceral adipose tissue.^25,26,27 Our present findings indicate that in both genders subcutaneous fat mass shows a stronger association with serum leptin than BMI or visceral fat mass.

In conclusion, we have reported a new technique for measuring intra-abdominal fat volume by the use of helical CT, which potentially reflects intra-abdominal fat mass more precisely than conventional slice-by-slice CT at the umbilical level.

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Figure 1 Correlation between visceral fat area measured by conventional CT, and visceral fat volume measured by helical CT in men (A) and women (B).

Figure 2 Correlation between subcutaneous fat area measured by conventional CT, and subcutaneous fat volume measured by helical CT in men (A) and women (B).

Table 1 Clinical profile of the study subjects

Table 2 Correlations of several adipose tissue distribution variables to body mass index

Table 3 Correlation between abdominal fat mass vs serum lipids