Ectopic fat and aerobic fitness are key determinants of glucose homeostasis in nonobese Asians.

Background: The importance of ectopic fat deposition and physical fitness in the pathogenesis of insulin resistance and beta cell dysfunction in subjects from the nonobese Asians is not known. Materials and methods: We conducted a cross‐sectional study and measured insulin sensitivity (M value; 4‐hour hyperinsulinaemic‐euglycaemic clamp), insulin secretion rate (3‐hour mixed meal tolerance test with oral minimal modelling), percent body fat, visceral adipose tissue, intramyocellular and intrahepatic lipid contents (magnetic resonance imaging and spectroscopy), cardiorespiratory fitness (VO2max; graded exercise test) and habitual physical activity (short International Physical Activity Questionnaire) in 60 healthy nonobese Asian subjects (BMI = 21.9 ± 1.7 kg/m2, age = 41.8 ± 13.4 years). Results: M was inversely associated with percent body fat (r = −0.460, P < 0.001), visceral fat (r = −0.623, P < 0.001) and liver fat (r = −0.601, P < 0.001), whereas insulin secretion correlated positively with these adiposity indices (percent body fat: r = 0.303, P = 0.018; visceral fat: r = 0.409, P = 0.010; hepatic fat: r = 0.393, P = 0.002). VO2max correlated negatively with insulin secretion rate (r = −0.420, P < 0.001) and positively with M (r = 0.658, P < 0.001). The amount of vigorous physical activity was positively associated with VO2max (r = 0.682, P < 0.001). Multiple stepwise linear regression analyses indicated that VO2max, age, and IHTG or VAT were independent determinants of insulin sensitivity and secretion (adjusted R2 = 69% and 33%, respectively, P < 0.001). Conclusions: Increased ectopic fat deposition is associated with reduced insulin sensitivity and increased insulin secretion in healthy nonobese Asians. Poor cardiorespiratory fitness, likely due to inadequate participation in vigorous exercise, is strongly related to suboptimal metabolic function. Interventions to encourage engagement in physical activity may thus be important for improving metabolic health in nonobese Asians.

Keywords: diabetes; fitness; metabolic function; physical activity

The worldwide incidence of type 2 diabetes has increased by more than twofold over the past three decades,[1] with ~60% of diabetics in the world currently living in Asian countries.[2] Compared to other parts of the world, type 2 diabetes in Asia has developed in a shorter time, in younger ages, and in people with lower body mass index (BMI).[3] In Singapore, for instance, the prevalence of type 2 diabetes is comparable to that of the United States, even though the combined rate of overweight and obesity (BMI ≥ 25 kg/m2) is approximately half.[3] This implies that metabolic abnormalities involved in the pathogenesis of type 2 diabetes (insulin resistance and beta cell dysfunction),[4] are widespread among nonobese Asians. According to the prevailing line of thought, during the early stages of obesity‐related type 2 diabetes, peripheral insulin resistance is being countered by a compensatory increase in pancreatic insulin secretion, and the resulting hyperinsulinaemia maintains glucose homeostasis. Gradually, however, the beta cell function declines and insulin secretion diminishes, resulting in fasting and postprandial hyperglycaemia.[5] Several studies have reported that type 2 diabetes in Asians is characterised to a greater extent by beta cell dysfunction and inadequate insulin secretion, rather than insulin resistance.[[6]] It is therefore possible that the pathophysiology of type 2 diabetes linked to obesity differs from that occurring in nonobese people. Understanding the factors affecting metabolic function in nonobese subjects is therefore important, given that ~40% of newly diagnosed type 2 diabetes in Asians occurs in lean people with a BMI < 22 kg/m2.[8]

Substantial research has established the importance of ectopic fat deposition and physical inactivity in the pathogenesis of insulin resistance and beta cell dysfunction in United States and European subjects with a wide range of BMI and glycaemic control.[[9]] Nevertheless, the relevance of these factors to nonobese Asians is not clear,[12] because body fat distribution, physical activity patterns and metabolic function differ considerably between Western and Asian populations.[[13]] To this end, we conducted a cross‐sectional study and used state‐of‐the‐art imaging techniques and metabolic function tests to assess the relationships between insulin sensitivity, insulin secretion, body fat distribution and physical fitness in healthy nonobese Asian subjects.

Sixty nonobese and apparently healthy Asian subjects (BMI 19.0‐24.7 kg/m2; 32 men and 28 women; 55 Chinese and five Indian), aged 21‐65 years were recruited between May 2016 and April 2017. These volunteers were selected on the basis of having completed the required metabolic function tests, from a total of 78 subjects who had been screened for another project.[15] Participants had no prior history of underlying medical conditions, such as diabetes, hypertension, dyslipidaemia, significant organ system dysfunction or other metabolic diseases that require the use of drugs to treat. Volunteers who used tobacco products, consumed alcohol regularly (≥1 unit/d), used any medications known to affect metabolic function (including oral contraceptives and hormone replacement therapy), experienced recent weight loss or gain (≥5% over the past 6 months), and women who were pregnant or breastfeeding were excluded from the study. Individuals with any contraindications to exercise, including musculoskeletal, respiratory or circulatory disorders were also excluded. All participants provided their written informed consent prior to enrolment, and ethics approval was obtained from the Domain Specific Review Board of the National Healthcare Group in Singapore.

Body fat was measured by dual‐energy X‐ray absorptiometry (DXA), abdominal visceral adipose tissue (VAT) was measured by magnetic resonance imaging (MRI), and intrahepatic triglyceride (IHTG) and intramyocellular lipid (IMCL; right soleus) contents were measured by magnetic resonance spectroscopy (MRS), as previously described.[[15]] Liver fat was expressed as the ratio of fat to fat plus water (%), and muscle fat was expressed as the ratio of IMCL to creatine (arbitrary units, AU).

Participants were asked to abstain from strenuous exercise, alcohol and caffeine on the previous day. Aerobic fitness was determined during a graded exercise test on a cycloergometer (Monark 839E Digital Ergomedic, Monark Exercise AB, Vansbro, Sweden). Subjects were fitted with an airtight face mask for the analysis of expired air and a heart rate monitor (T31 coded transmitter, Polar Electro, Kempele, Finland) on the anterior chest wall for continuous heart rate monitoring. Breath‐by‐breath analysis of O2 consumption and CO2 production was conducted by using the CareFusion Vmax Encore metabolic cart (Becton, Dickinson and Company, NJ, USA), which was calibrated against an internal calibrator syringe and gases of known composition. Maximal oxygen uptake (VO2max, in mL/kg/min) was determined after confirming at least two of the following criteria were met: (a) visible plateau in VO2; (b) heart rate ≥90% of age‐predicted maximum; and (c) respiratory quotient ≥1.0 during the last test stage.

Subjective measures of habitual physical activity were obtained by using the short version of the International Physical Activity Questionnaire (IPAQ). The self‐administered questionnaires were analysed to provide an estimate of exercise participation in terms of Metabolic Equivalent Task minutes per week (MET‐min/wk).[17] Definition of exercise intensity in the IPAQ is subjective, for example "vigorous physical activities refer to activities that take hard physical effort and make you breathe much harder than normal."

Metabolic tests were conducted at the Clinical Nutrition Research Centre (CNRC), National University of Singapore (NUS), on two additional occasions, scheduled ~1 week apart. These procedures have been described previously in detail.[[15]] Briefly, after an overnight fast, subjects underwent a 4‐hour hyperinsulinaemic‐euglycaemic clamp (insulin infusion of 40 mU/m2/min) to determine insulin sensitivity as the insulin‐mediated whole‐body glucose uptake, and a 3‐hour mixed meal challenge (600 kcal) to determine glucose tolerance and pancreatic insulin secretion by using oral minimal modelling.

Plasma glucose concentration was measured using an automated glucose analyser (YSI 2300 Stat Plus; YSI Life Sciences, Yellow Spring, OH, USA). Plasma insulin and C‐peptide concentrations were determined using commercially available electrochemiluminescence assays (Roche/Hitachi cobas e411 immunochemistry analyser; Roche Diagnostics, IN, USA).

Insulin‐mediated whole‐body glucose disposal (M value, in µmol of glucose per kg body weight per minute) was calculated as the average rate of dextrose infusion during the final 30 minutes of insulin infusion (mean group CV during steady state of the clamp = 2.5%).

The total insulin secretion rate (ISR, in pmol/L/min) after ingestion of the mixed meal was assessed by using oral minimal model analysis of plasma C‐peptide and glucose concentrations (Simulation, Analysis and Modeling Software, SAAM II version 2.3, The Epsilon Group, Charlottesville, VA).[18] Cumulative total postprandial ISR (in pmol/L) was computed as the corresponding area under the curve (AUC) over 180 minutes after ingestion of the mixed meal.

Reporting of the study conforms to the STROBE statement and the broader EQUATOR guidelines.[19] All analyses were performed on available datasets (ie, no missing data imputation was done) and were carried out with SPSS version 24 (IBM SPSS, Chicago, IL). The Shapiro‐Wilk test was used to evaluate the distribution of data. Descriptive characteristics are shown as means with standard deviations for normally distributed data or medians with quartiles for non‐normally distributed data. Associations between metabolic function (insulin sensitivity and insulin secretion) and variables of interest were assessed by using Pearson's or Spearman's correlation for normally or non‐normally distributed data, respectively. These relationships were also evaluated separately in men and women; adjusting for sex did not affect their significance or strength. Multiple stepwise regression analysis was used to determine independent determinants of insulin sensitivity and insulin secretion. Statistical significance was set at P < 0.05.

The anthropometric and metabolic characteristics of the study participants are shown in Table ; all subjects had normal body weight (based on BMI) and nondiabetic fasting blood glucose.

Anthropometric and metabolic characteristics of healthy nonobese Asians

1 Data are mean ± SD for normally distributed variables or median (quartiles) for non‐normally distributed variables for n = 60, except for VO2max (n = 59) and physical activity (n = 41).

2 AUC, area under the curve; IHTG, intrahepatic triglyceride; IMCL, intramyocellular lipid; M value, insulin‐mediated glucose uptake; MET, metabolic equivalent; VAT, visceral adipose tissue; VO2max, maximal oxygen uptake.

Insulin sensitivity (M value) and insulin secretion (total ISR AUC) varied ~threefold among subjects and were associated in a nonlinear, hyperbolic fashion (Figure).

M correlated inversely with indices of total body adiposity and ectopic fat deposition, such as BMI, percent body fat, VAT volume and IHTG content (Figure). Total ISR AUC was not associated with BMI, but correlated positively with percent body fat, VAT and IHTG (Figure). IMCL content tended to be negatively related to insulin sensitivity (r = −0.239, P = 0.066) and positively to insulin secretion (r = 0.241, P = 0.064).

Cardiorespiratory fitness (VO2max) was inversely associated with percent body fat, VAT and IHTG, but not IMCL (Figure). VO2max correlated positively with M and negatively with total ISR AUC (Figure).

IPAQ data were available in a subset of participants (n = 41). The time spent in vigorous physical activity (in MET‐min/wk) was associated positively with both VO2max (r = 0.682, P < 0.001) and handgrip strength (r = 0.436, P = 0.004); no such relationships were observed for nonvigorous physical activity and sitting time.

Multiple stepwise linear regression analyses with M value and total ISR AUC as the dependent variables; and age, sex, BMI, percent body fat, VAT, IHTG, IMCL and VO2max as putative predictors revealed that VO2max (positive), female sex (positive), BMI (negative), age (positive) and IHTG (negative) were independent determinants of M (adjusted R2 = 69%, P < 0.001), and VO2max (negative), age (negative) and VAT (positive) were independent determinants of total ISR AUC (adjusted R2 = 33%, P < 0.001).

In this study, we observed that visceral adipose tissue and intrahepatic fat (but not intramuscular fat) are associated with reduced peripheral insulin sensitivity and greater pancreatic insulin secretion in healthy nonobese Asians. Our findings also indicate that poor cardiorespiratory fitness, likely due to insufficient participation in vigorous physical activity, is strongly associated with ectopic fat deposition and metabolic dysfunction. To our knowledge, this is the first study to evaluate the relationships among robust assessments of metabolic function, body composition and fat distribution, and objective measures of physical fitness in healthy nonobese Asians. In contrast, previous investigations have been conducted in individuals who were either overweight or obese,[20] had clinical features of metabolic dysfunction, such as hyperglycaemia, diabetes, dyslipidaemia and hypertension,[[21]] or—most commonly—were of Western descent.[[9], [11], [23]] Our findings suggest there is a need to develop effective strategies to encourage participation in vigorous exercise to improve aerobic fitness, body composition and metabolic function in nonobese Asians who could potentially be at risk of diabetes.

We observed a hyperbolic relationship between insulin sensitivity and insulin secretion, in line with the classical description of the disposition index, whereby the product of insulin sensitivity and insulin secretion is constant for a given degree of glucose tolerance. Glucose homeostasis can be maintained despite worsening insulin sensitivity because pancreatic insulin secretion is adequately upregulated to offset insulin resistance. This relationship has been described in the initial stages during the progression from normal glucose tolerance to obesity‐related prediabetes and diabetes,[24] and our study confirms it holds true in the absence of obesity as well. Therefore, even though our study is not a longitudinal one, our observations in nonobese Asians are concordant with the pathophysiology of obesity‐related type 2 diabetes in Western populations.

The obesocentric view of type 2 diabetes has been challenged with the identification of many individuals of normal body weight, who nevertheless exhibit a cluster of obesity‐related metabolic abnormalities such as hyperinsulinaemia, insulin resistance, hypertriglyceridaemia, hypertension, etc.[25] These normal‐weight but metabolically unhealthy individuals have several‐fold greater risk for developing cardiometabolic disease, not only compared to metabolically healthy lean subjects, but also when compared to metabolically healthy obese subjects.[[26]] This underscores the importance of metabolic dysfunction independent of excess body weight and whole‐body adiposity. Accordingly, we observed that ectopic fat deposition, particularly in the intra‐abdominal area and the liver (ie, VAT and IHTG, respectively), is related with metabolic abnormalities in nonobese Asian subjects to a greater extent than indices of whole‐body adiposity such as BMI and percent body fat. Our observations are in agreement with previous studies that found excess VAT accumulation had a significant negative impact on glycaemic control[28] and was associated with greater prevalence of insulin resistance when individuals were followed up prospectively (no relationships were detected for subcutaneous abdominal adipose tissue).[29] However, when variations in IHTG content were assessed in normal‐weight and moderately obese men, individuals with elevated IHTG presented with features of hepatic insulin resistance and hyperinsulinaemia compared to those with low IHTG, without differences in VAT volume.[30] Kusters et al[9] on the other hand, identified mutually independent contributions of VAT and IHTG on whole‐body glucose disposal in healthy lean men. Nevertheless, it was found that only a reduction in IHTG content contributed to improved insulin sensitivity following weight loss in obese men.[9] It is thus possible that inherent differences exist between lean and obese individuals with regards to the importance of ectopic fat depots in affecting metabolic function. Unfortunately, VAT volume correlates directly with IHTG content,[31] which makes it difficult to dissect the independent contribution of each to metabolic dysfunction. Clearly, however, our results indicate that excess fat deposition in the intra‐abdominal area and the liver is strongly associated with metabolic dysfunction in nonobese Asians.

Interestingly, we did not detect any associations between IMCL and measures of metabolic function. Contrary to our findings, previous studies in nonobese Caucasian subjects found an inverse correlation between IMCL and whole‐body insulin sensitivity.[[32]] Although the relationship between IMCL and metabolic function may depend on the training status of the subjects,[34] the correlations of IMCL with insulin sensitivity and insulin secretion in our study remained not significant even after adjusting for VO2max (r = −0.191, P = 0.152 and r = 0.208, P = 0.117; respectively). As our subjects were of Asian descent (predominantly Chinese), our findings are consistent with the results of Khoo et al[35] who reported weak associations between IMCL content and insulin sensitivity in all Asian ethnic groups, along with a reduced tendency for Chinese subjects to accumulate IMCL with increasing levels of adiposity. In addition, there are differences in IMCL content between different muscles,[36] thus we cannot rule out the possibility that the soleus is not a representative skeletal muscle with respect to the relationship between IMCL and whole‐body insulin sensitivity.

Cardiorespiratory fitness is a key determinant of body fat distribution and metabolic function. Hall et al[37] demonstrated that South Asian subjects have reduced cardiorespiratory fitness, impaired oxidative capacity during exercise, and attenuated capacity for fatty acid utilisation compared to European counterparts, which likely mediate the insulin resistant phenotype of South Asians. Moreover, physical fitness may result in a favourable metabolic profile by preventing abdominal obesity.[38] Studies conducted in overweight and obese women with comparable total body fat, but mismatched for abdominal obesity, showed that greater cardiorespiratory fitness was associated with better insulin sensitivity. Collectively, the authors inferred that VAT may be an important modulator of this relationship.[[39]] Together with our findings, these observations underscore the importance of aerobic fitness for metabolic function and cardiometabolic risk, particularly in nonobese people, and suggest part of this relationship could be due to the beneficial effects of regular exercise on ectopic fat deposition.

The strengths of the present study include the recruitment of a reasonably large number of volunteers comprising a relatively homogeneous group of healthy nonobese Asians. Additionally, we incorporated assessments of total body fat and adipose tissue depots, metabolic function and aerobic fitness using state‐of‐the‐art techniques. By contrast, prior studies have used waist circumference as a surrogate marker of VAT, the Homeostasis Model Assessment score as a surrogate marker of insulin action, and self‐reported activity as a proxy of fitness. However, the cross‐sectional nature of our study precludes drawing any conclusions concerning causality. Also, VO2max is not merely a function of regular physical activity habits, which we assessed with a self‐reported questionnaire rather than more objective methods (eg accelerometry or heart rate monitoring).

In summary, our results indicate that excess fat deposition in the intra‐abdominal area and the liver is associated with reduced peripheral insulin sensitivity and increased pancreatic insulin secretion in apparently healthy nonobese Asians. Furthermore, poor aerobic fitness, likely because of low engagement in vigorous physical activity, is strongly related to ectopic fat deposition and abnormalities in the mechanisms regulating glucose homeostasis. Taken together, our findings suggest that interventions and public health policies to promote participation in exercise for improving aerobic fitness and body composition may be useful to protect against metabolic abnormalities involved in the pathogenesis of type 2 diabetes in nonobese Asians.

The authors would like to thank the study volunteers for their participation.

The authors have no conflicts of interest relevant to the content of this article.

ZC, CD, SAS, NM, SSV, MKSL and FM analysed the data. ZC, CD, JC, YCC, SS and AC were involved in data collection. JC and MKSL provided medical supervision. ZC and FM wrote the manuscript. All authors were involved in manuscript review and approved the version submitted for publication.