An association between visceral or subcutaneous fat accumulation and diabetes mellitus among Japanese subjects

Background The impact of subcutaneous fat accumulation remains controversial. This study assessed the association between visceral or subcutaneous fat area (VFA and SFA, respectively) and diabetes mellitus (DM) among Japanese subjects. Methods This was a cross-sectional study involving 1907 eligible participants (men, 1050; women, 857) who participated in a voluntary health check-up conducted at Juntendo University Hospital from January 2017 to December 2018, in Tokyo, Japan. Associations between VFA or SFA quartiles and DM were identified using adjusted odds ratios (AORs) and 95% confidence intervals (CIs) with multivariable logistic regression analysis adjusted for confounders. Receiver operating characteristic (ROC) curve analysis was used to assess appropriate cut-off values of VFA or SFA. Results Multivariate analyses showed that Q4 (≥ 125 cm2) of VFA was significantly positively associated with DM compared to Q1 (< 65 cm2) (AOR = 1.94, 95% CI 1.02–3.71), whereas there was no association between SFA and DM in men. Among women, Q4 (≥ 85 cm2) of VFA was significantly positively associated with DM compared to Q1 (< 30 cm2) (Q4, AOR = 6.15, 95% CI 1.65–22.99). Also, Q3 and Q4 (≥ 135 cm2) of SFA were significantly positively associated with DM compared to Q1 (< 90 cm2) (Q3, AOR = 5.64, 95% CI 1.21–26.25; Q4, AOR = 7.81, 95% CI 1.71–35.65). The appropriate cut-off value of VFA in men was 101.5 cm2. Those of VFA and SFA in women were 72.5 cm2 and 165.3 cm2, respectively. Conclusions Our results suggest the importance of considering SFA as well as VFA, especially in women, for primary and secondary prevention of DM.


Introduction
Type 2 diabetes mellitus (DM) has become a highly prevalent disease worldwide as changing lifestyles have led to reduced physical activity and increased obesity, and it is now recognized as major public health burden. The prevalence of both type 2 DM and obesity has increased worldwide over the last century, not only in developed countries but also developing countries, sometimes coexisting with undernutrition [1]. It was estimated that in 2017 there were 451 million (age 18-99 years) people with diabetes worldwide. These figures are expected to increase to 693 million by 2045, and it is estimated that almost half of all people (49.7%) living with diabetes are undiagnosed. In 2017, approximately 5 million deaths

Open Access
Diabetology & Metabolic Syndrome *Correspondence: yokokawa@pa3.so-net.ne.jp 2 Department of General Medicine, Juntendo University School of Medicine, Hongo 2-1-1, Bunkyo-ku, Tokyo 113-8421, Japan Full list of author information is available at the end of the article Page 2 of 10 Yokokawa et al. Diabetol Metab Syndr (2021) 13:44 worldwide in persons in the 20-99 years age range were attributed to diabetes [1]. Obesity, especially abdominal obesity, is a well-known underlying risk factor for the development of diabetes [2]. Visceral fat accumulation, which is a key feature of abdominal obesity, is in an upstream position in the pathogenesis and development of metabolic syndrome (Mets) with clustering of DM, dyslipidemia, and hypertension [2]. In particular, several epidemiologic studies have shown that excess visceral fat area (VFA) is a wellknown risk factor for the development of DM and onset of cardiovascular disorders [2,3]. Therefore, measurement of visceral fat accumulation is important to assess subjects with high risk of DM and other cardiovascular disorders [2][3][4]. As for VFA cut-off values for Mets, there are several differences associated with age distribution, the number of study participants, degree of obesity, and ethnicity [2][3][4][5]. In Japan, visceral fat accumulation is generally recognized as a waist circumference (WC) of over 85 cm in men and over 90 cm in women, which correspond to a VFA of 100 cm 2 in an abdominal computed tomography (CT) scan at the umbilical level [3].
However, the impact of subcutaneous fat accumulation remains controversial. Several previous reports indicated a protective effect for glucose metabolism [6,7]. In contrast, there is some evidence indicating a positive association between subcutaneous fat accumulation and adverse cardiometabolic risk factors, including diabetes [8,9]. There are differences between subcutaneous adipose tissue (SCAT) and visceral adipose tissue (VAT) present in the abdominal cavity [10], including anatomic, cellular, molecular, physiologic, clinical, and prognostic differences. Anatomically, VAT is present mainly in the mesentery and omentum and drains directly through the portal circulation to the liver. Compared with SCAT, VAT is more cellular, vascular, and innervated, contains a larger number of inflammatory and immune cells, has less preadipocyte differentiating capacity, and has a greater percentage of large adipocytes [10].
The present study assessed the association between DM and VFA and subcutaneous fat area (SFA) estimated by CT scans in Japanese subjects.

Subjects and methods
This was a cross-sectional study that screened 2885 Japanese adults who participated in a voluntary health checkup conducted at Juntendo University Hospital from January 2017 to December 2018, in Tokyo, Japan. A total of 978 subjects were excluded due to missing data, and 1 subject was excluded due to a duplicate case. Ultimately, 1907 participants were included in the present study (men, 1050; women, 857).

Variables
Height, weight, body mass index (BMI), and WC were measured with participants in the standing position. BMI was calculated by dividing body weight (kg) by height squared (m 2 ). Mean systolic blood pressure (SBP) and diastolic blood pressure (DBP) were calculated from the means of two upper-arm blood pressure measurements taken from participants who had been seated for at least 5 min. Serum levels of total cholesterol (mg/dl; TC), high-density-lipoprotein cholesterol (mg/dl; HDL-C), low-density-lipoprotein cholesterol (mg/dl; LDL-C), and triglycerides (mg/dl; TGs) were also measured. LDL-C was estimated using the Friedewald equation [(TC) -(HDL-C) -(TG/5)] [11]. Hemoglobin A1c (HbA1c) levels were determined by high-performance liquid chromatography using an automated analyzer. Serum uric acid (mg/ dl) and high-sensitivity C-reactive protein (mg/dl) were also measured.
Participants were asked to complete a self-administered questionnaire that addressed healthy lifestyle characteristics based on Breslow's seven health practices [12]. These characteristics can be used to assess lifestyle health, and strong associations have been found between healthy lifestyle practices and successful blood pressure control among patients with hypertension [13]. Healthy lifestyle items in the questionnaire included non-daily alcohol consumption, non-smoker status, exercise frequency of two or more times per week, BMI of 18.5-24.9 kg/m 2 , adequate sleep duration, daily breakfast consumption, and no snacking between meals [12,13].
From the self-administered questionnaire, we also collected information on present medical history of comorbidities, such as hypertension, DM, dyslipidemia, hyperuricemia, cardiovascular disease, and cerebrovascular disease. If participants answered as having these comorbidities, we registered the participants with a medical history of these comorbidities (present).

CT measurement of abdominal adipose tissue
Abdominal fat area, including VFA and SFA, was measured from CT scans taken at the level of the umbilicus while in the supine position and during late expiration according to Japanese guidelines for obesity treatment [14]. An Aquilion ONE/GENESIS Edition CT scanner (Canon Medical Systems Corp., Tokyo, Japan) was used to obtain CT scans. We manually traced the inner aspect of the whole trunk, muscular layer, and the abdominal wall. In the computerized method using commercial software designed for quantification of VFA and SFA (Canon Medical Systems Corp.), fat was defined as any tissue with a threshold of − 150 to − 70 Hounsfield units. Abdominal VFA was defined as the fat area enclosed by the inner aspect of the abdominal wall, and SFA was defined as the fat area enclosed by the outer aspect of the abdominal wall [15,16]. The method is widely used and a previous study indicated that CT and magnetic-resonance imaging (MRI) may yield different absolute values of fat areas (especially visceral fat) but that the ranking of individuals on the basis of their fat areas will be similar by both methods [17,18].

Statistical analysis
Results are presented as mean ± standard deviation ( Associations between VFA or SFA quartiles and DM were identified using adjusted odds ratios (AORs) and 95% confidence intervals (CIs) with multivariable logistic regression analysis adjusted for age (years), dyslipidemia (yes), hypertension (yes), hyperuricemia (yes), alcohol consumption (non-daily drinker), and smoking (non-smoker). Receiver operating characteristic (ROC) curve analysis was used to assess appropriate cut-off values of VFA and SFA, and we estimated the area under the curve (AUC) and measured the sensitivity and specificity for DM in both sexes. All statistical analyses were performed using the Statistical Package for Social Sciences, version 22 (SPSS Inc., Chicago, IL, USA).
The research protocol was reviewed and approved by the Ethics Committee of the Juntendo University Hospital (no. 18-297), and written informed consent was obtained from all participants.

Results
The mean age (SD) of non-DM and DM was 59.9 (12.3) and 65.7 (8.9) years in men and 60.2 (12.9) and 66.5 (9.4) years in women, respectively (Table 1). Participants with DM had significantly higher mean BMI, WC, and VFA compared to non-DM participants. The mean SFA of DM participants was significantly higher than that of non-DM participants among women, whereas no statistically significant difference was observed among men.

Discussion
The present cross-sectional study results showed that VFA ≥ 125 cm 2 was significantly positively associated with DM compared to VFA < 65 cm 2 in men, and VFA ≥ 85 cm 2 was significantly positively associated with DM compared to < 30 cm 2 in women after adjusting for confounders. SFA ≥ 135 cm 2 was significantly positively associated with DM compared to SFA < 90 cm 2 in women, whereas no association was observed in men. VFA was closely and positively associated with DM in both sexes, and appropriate estimated cut-off points might be 101.5 cm 2 in men and 72.5 cm 2 in women for DM, respectively. SFA was also associated with DM only in women, suggesting a cut-off value of 165.3 cm 2 . To the best of our knowledge, analyses of the association between DM and VFA and SFA are limited.
Visceral fat accumulation is widely regarded as a risk factor for cardiovascular diseases, including DM. Mets is a metabolic condition that predicts individuals who are likely to be affected by cardiovascular disorders via insulin resistance [3]. One major feature of Mets is visceral fat accumulation, which is closely related to insulin resistance. Visceral fat accumulation is generally recognized as a WC ≥ 85 cm 2 in men and ≥ 90 cm 2 in women, which correspond to a VFA of 100 cm 2 in an abdominal    13:44 CT scan at the umbilical level [3]. Visceral fat accumulation is also known to be an independent risk factor for type 2 diabetes. A longitudinal study that determined the optimal cut-off value of VFA for predicting type 2 diabetes among 13,004 Koreans reported values of 118.8 cm 2 in men and 82.6 cm 2 in women [4]. Another longitudinal survey that followed Japanese Americans for 10 years reported a baseline intra-abdominal fat area (IFA) of 102.7 cm 2 in the incident diabetes group and 74.3 cm 2 in those without incident diabetes, respectively. Also, an increase of 1 SD in IFA was associated with a 1.65-fold increase in the odds of diabetes over 10 years (OR = 1.65, 95% CI 1.21-2.25) after adjusting for the above covariates [19]. These previous study results are closely similar to our results. Thus, sex-specific reference values for visceral fat accumulation such as that men with a VFA ≥ 100 cm 2 and women with a VFA ≥ 80 cm 2 should be considered to prevent and manage type 2 diabetes. Our results showed that SFA was significantly positively associated with DM in women, whereas no association was observed in men. The role of subcutaneous fat in cardiovascular risk remains controversial. The Shanghai Nicheng Cohort Study, which was conducted among 12,137 Chinese adults aged 45-70 years, reported multivariable-adjusted ORs and 95% CIs of newly diagnosed diabetes per 1-standard deviation increase in SFA and VFA of 1.29 (1.19-1.39) and 1.61 (1.49-1.74) in men and 1.10 (1.03-1.18) and 1.56 (1.45-1.67) in women, respectively [6]. However, the association between SFA and newly diagnosed diabetes disappeared in men and Table 2 Odds ratios and 95% confidence intervals for diabetes mellitus among men (logistic regression analysis) (n = 1050) OR: odds ratio; AOR: adjusted odds ratio; CI: confidence interval Multivariable analysis was adjusted for visceral/subcutaneous fat area quartiles, age (years), dyslipidemia (yes), hypertension (yes), hyperuricemia (yes), alcohol consumption (non-daily drinker), and smoking (non-smoker)   [9]. The possible mechanism of the association between diabetes and SAT as well as VAT is insulin resistance. To date, numerous studies have assessed the association between excess visceral fat accumulation and insulin resistance. Regarding SAT, several previous surveys indicated a positive association between excess subcutaneous fat accumulation and insulin resistance. A Japanese study that surveyed 912 non-diabetic participants reported that subjects in higher tertiles of SAT and VAT had significantly higher HOMA-IR and lower Matsuda ISI levels (P < 0.001) [20]. Excess SAT accumulation may cause insulin resistance and contribute to glucose intolerance as well as VAT. Therefore, it is necessary to consider adiposity, including SAT and VAT, to better maintain body composition. In regard to the impact of SAT, a sex difference was observed. There is little evidence available to explain the difference. The Jackson Heart Study, which involved 2,799 African Americans, reported a direct association between SAT and adiponectin (β = 0.18; P = 0.002) that persisted when controlling for BMI and WC among men, whereas the significance was borderline among women (β = 0.05; P = 0.05) [21]. Although the evidence is limited to explain the sex difference, it is possible that adiponectin may contribute. Further analyses are required to assess the sex difference.
Our study has several limitations. First, it was susceptible to selection bias, as the participants consisted of those who received voluntary medical check-ups at a single medical institution. As such, these participants may be inherently more aware of their health behaviors relative to the general population. In addition, 978 among 2885 screened participants were excluded due to missing data (33.9%). It is necessity to minimize the exclusion rate. Second, this was a cross-sectional observational study, thus limiting consideration of the causal relationship between SFA/VFA and DM. Further analyses that include data from a more diverse cohort are thus needed. Third, some key data regarding items such as details of diabetes medications, eating behaviors, and nutritional status were not collected. Future prospective studies including these data are also needed.
In conclusion, the results of the present crosssectional study indicate that VFA ≥ 125 cm 2 in men is significantly positively associated with DM compared to VFA < 65 cm 2 , and VFA ≥ 85 cm 2 in women is significantly positively associated with DM compared to VFA < 30 cm 2 after adjusting for confounders. SFA ≥ 135 cm 2 in women is significantly positively associated with DM compared to SFA < 90 cm 2 , but no association was observed in men. Appropriate estimated VFA cut-off points for DM are 101.5 cm 2 in men and 72.5 cm 2 in women, respectively. As SFA was associated with DM only in women, the appropriate estimated cut-off is 165.3 cm 2 . Our results suggest that it is important to consider both SFA and VFA, especially in women, for primary and secondary prevention of DM.