L’adiposité viscérale et le contenu en lipides du foie : des médiateurs importants de
l’association entre la condition cardiorespiratoire l’homéostasie du glucose et de
l’insuline plasmatique
L’article composant ce chapitre est intitulé :
Visceral adiposity and liver fat as mediators of the association between
cardiorespiratory fitness and plasma glucose-insulin homeostasis
(Chartrand DJ, Larose E, Poirier P, et al. Visceral adiposity and liver fat as mediators of the
association between cardiorespiratory fitness and plasma glucose-insulin homeostasis. Am J Physiol
Résumé
La condition cardiorespiratoire est associée positivement à la sensibilité à l’insuline, alors que des quantités excessives de tissu adipeux viscéral et de lipides au foie sont toutes deux associés à la résistance à l’insuline et à une perturbation de l’homéostasie du glucose et de l’insuline. La contribution de l’adiposité viscérale et du contenu en lipides du foie à la relation entre la condition cardiorespiratoire et les indicateurs de l’homéostasie du glucose et de l’insuline est méconnue. Notre objectif était d’explorer les interactions entre la condition cardiorespiratoire, l’adiposité viscérale, le contenu en lipides du foie, la tolérance au glucose et l’insulinémie chez des individus sans maladie cardiovasculaire connue. La condition cardiorespiratoire a été mesurée chez 135 femmes et 177 hommes à l’aide d’une épreuve d’effort maximal sur tapis roulant. Les indicateurs de l’homéostasie du glucose et de l’insuline ont été obtenus à partir d’un test d’hyperglycémie orale provoquée à jeun. L’adiposité viscérale et le contenu en lipides du foie ont été mesurés par résonance magnétique incluant une spectroscopie du foie. Pour une condition cardiorespiratoire donnée, les femmes présentaient significativement moins d’adiposité viscérale et de lipides au foie que les hommes, ainsi qu’une résistance à l’insuline calculée par le modèle hométostatique (HOMA-IR), une glycémie et une insulinémie moindres par rapport aux hommes. Chez les deux sexes, on observait des corrélations négatives significatives entre la condition cardiorespiratoire et l’HOMA-IR, la glycémie et l’insulinémie. La glycémie et l’insulinémie corrélaient positivement avec l’adiposité viscérale et le contenu en lipides du foie. Chez les deux sexes, le meilleur tiers de condition cardiorespiratoire était associé à peu d’adiposité viscérale et de lipides au foie. Des analyses de régressions multivariables ont démontré que l’adiposité viscérale et le contenu en lipides du foie étaient largement responsables de l’effet favorable de la condition cardiorespiratoire sur les indicateurs de l’homéostasie du glucose et de l’insuline.
Title:
Visceral Adiposity and Liver Fat as Mediators of the Association Between Cardiorespiratory Fitness and Plasma Glucose/Insulin Homeostasis
Authors:
Dominic J. Chartrand1,2, Eric Larose1,3, Paul Poirier1,4, Patrick Mathieu1,5, Natalie
Alméras1,2, Philippe Pibarot1,3, Benoît Lamarche6,7, Caroline Rhéaume1,8, Jean-Pierre
Després1,2,9
Author contributions:
D.J.C. analyzed data; D.J.C., E.L., P.P., P.M., N.A., P.P., B.L., C.R., J.-P.D. interpreted results of experiments; D.J.C. prepared figures; D.J.C. drafted manuscript; N.A., J.-P.D. edited and revised manuscript; D.J.C., E.L., P.P., P.M., N.A., P.P., B.L., C.R., J.-P.D. reviewed and approved final version of the manuscript; E.L., P.P., N.A., B.L, J.-P.D. conceived and designed research.
Affiliations:
1 Centre de recherche de l’Institut universitaire de cardiologie et de pneumologie de
Québec – Université Laval, Québec, QC, Canada
2 Department of Kinesiology, Faculty of Medicine, Université Laval, Québec, QC, Canada 3 Department of Medicine, Faculty of Medicine, Université Laval, Québec, QC, Canada 4 Faculty of Pharmacy, Université Laval, Québec, QC, Canada
5 Department of Surgery, Faculty of Medicine, Université Laval, Québec, QC, Canada 6 School of Nutrition, Université Laval, Québec, QC, Canada
7 Centre Nutrition, santé et société (NUTRISS), Institut sur la nutrition et les aliments
fonctionnels (INAF), Université Laval, Québec, QC, Canada
8 Department of Family Medicine and Emergency Medicine, Faculty of Medicine,
9 Vitam – Centre de recherche en santé durable, Québec, QC, Canada
Running Head:
CRF, Visceral/Liver Fat and Glucose/Insulin Homeostasis
Address for Correspondence:
Jean-Pierre Després, C.Q., Ph.D., FAHA, FIAS Director of Research
Vitam – Centre de recherche en santé durable CIUSSS de la Capitale-Nationale
2525, chemin de la Canardière Pavillon Landry-Poulin, A-2419 Québec QC G1J 0A4
Canada
Tel.: +1 418 821 4125
E-mail: Jean-Pierre.Despres.ciussscn@ssss.gouv.qc.ca
Keywords: cardiorespiratory fitness, insulin resistance, liver fat, glucose/insulin
Abstract
Cardiorespiratory fitness (CRF) is positively associated with insulin sensitivity, whereas excessive levels of visceral adipose tissue (AT) and liver fat (LF) are both associated with insulin resistance and impaired plasma glucose/insulin homeostasis. To what extent levels of visceral AT and LF content contribute to the relationship between CRF and indices of plasma glucose/insulin homeostasis is uncertain. Our objective was to explore the interactions between CRF, visceral AT and LF with glucose tolerance/insulin levels in asymptomatic and apparently healthy individuals. CRF was measured in 135 women and 177 men with a maximal treadmill graded exercise test. Indices of plasma glucose/insulin homeostasis were derived from a 3-hour oral glucose tolerance test (OGTT) performed in the morning after a 12-hour fast. Visceral AT levels and LF content were measured using magnetic resonance imaging and spectroscopy. For any given CRF level, women presented significantly lower visceral AT and LF than men as well as lower HOMA-IR and plasma glucose/insulin levels during the OGTT compared to men. In both sexes, there were significant negative correlations between CRF and HOMA-IR as well as glucose and insulin levels measured during the OGTT. Both glucose and insulin levels during the OGTT correlated positively with visceral AT and LF. In women and men, being in the top CRF tertile was associated with low levels of visceral AT and LF. Multivariable linear regression analyses suggested that visceral AT and LF were plausible mediators of the association between CRF and indices of plasma glucose/insulin homeostasis.
Introduction
It is now widely accepted that regular physical activity/exercise improves insulin sensitivity, contributing to lower circulating insulin levels and potentially improve glucose tolerance (4, 24), thereby reducing the risk of developing type 2 diabetes (19). Regular physical activity/exercise can also alter favorably body composition even without weight loss (42). Recent computed tomography and magnetic resonance imaging (MRI) studies have also revealed that changes in body composition induced by regular physical activity and exercise involve lowering visceral adiposity and liver fat (LF) content, two powerful risk factors for the development of type 2 diabetes (3, 6, 13, 37).
As the level of physical activity reported through questionnaires is subject to an obvious misclassification bias, Blair and colleagues (2) were the first to suggest that cardiorespiratory fitness (CRF), an objective and reliable physiological measurement of exercise capacity, could be used to identify inactive and poorly fit individuals at increased risk for various chronic conditions including type 2 diabetes as well as cardiovascular disease. Although CRF is multifactorial and has a genetic component, a large body of literature has shown that physically inactive, poorly fit individuals can significantly improve their CRF levels by regular physical activity/exercise (28, 30). Thus, the identification of individuals with low levels of CRF in clinical practice should be encouraged as a simple approach to assess the risk associated with physical inactivity and sedentary behaviors leading to deconditioning and chronic diseases. In this regard, we have previously shown that visceral adipose tissue (AT) is an important mediator of the link between CRF and indices of plasma glucose/insulin homeostasis (1).
Moreover, with the development of imaging techniques such as magnetic
measure LF content (29, 44, 46). Several MRS studies have shown that LF content is a powerful correlate of glucose tolerance (10). However, as both visceral AT and LF are important drivers of glucose tolerance and insulin levels, to what extent the relationships between CRF and these indices of plasma glucose/insulin homeostasis are mediated by either visceral AT vs. LF (or both) is not known. In addition, whether these relationships differ between women and men remains undetermined. Therefore, the aim of the present study was to examine the respective contributions of visceral AT and LF content
(assessed by MRI and MRS) to the well-established relationship between CRF and indices of plasma glucose/insulin homeostasis in women and men.
Materials and methods Study subjects
The sample includes 312 asymptomatic volunteers aged between 35 and 66 years recruited from the Québec City metropolitan area who completed the baseline evaluation from the Visceral obesity/ectopic fat and non-invasive markers of atherosclerosis: a
cardiometabolic-cardiovascular imaging study (CMCV imaging study). Inclusion criteria
were body mass index (BMI) <40 kg/m2 and non-smoker for ≥12 months. Participants
presenting symptoms or being treated for chronic or inflammatory diseases including cardiovascular disease, dyslipidemia, hypertension and diabetes were excluded. Subjects undergoing hormonal or corticosteroid therapy, presenting a cancer that is not in remission or an absolute MRI or CRF testing contraindication, having participated in a research project involving medication in the past 3 months, as well as women who were less than 12 months post menopause were also excluded. The local Institutional Review Board approved the study (2012-1747, 20769) and participants provided their informed consent.
Anthropometry and body composition
According to standardized procedures, height, weight (15) and waist circumference (32) were measured. BMI was calculated. Body composition was assessed by Dual-
Energy X-ray Absorptiometry (DEXA) using a Lunar Prodigy (GE Healthcare, Madison, WI, USA).
Magnetic resonance imaging and spectroscopy and image analysis
Magnetic resonance measurements were performed on a 1.5T system (first 230 participants) and then a 3.0T system following an upgrade at the research center (both systems by Philips Healthcare, Best, the Netherlands). Validation of continuity was
performed by repeating the studies on both systems in a sample of 10 study participants. Agreement was strong (Lin’s concordance rho 0.84-0.90, P < 0.001 for the different imaging variables). Subcutaneous and visceral AT volumes were measured by axial T1- weighted (TW1) spin echo sequence (slice thickness=5 mm; repetition time (TR)=750 ms; echo time (TE)=6-8 ms; resolution=0.78x0.78 mm) performed at L4-L5 intervertebral space. Acquisitions were accomplished with and without fat saturation to facilitate
identification of AT when required. LF fraction was determined with spectroscopy by single breath-hold STEAM sequence. Voxels were located in liver tissue while avoiding bile ducts and vessels. B0 field was optimized through shimming by pencil-beam method, while a long repetition time minimized T1 bias, and the use of 4 echoes corrected for T2 effects.
Images were analyzed offline in a dedicated and standardized core laboratory by trained technicians blinded to patient data. For AT volumes, semi-automated methodology performed with dedicated software (Qmass MR, Medis, Leiden, the Netherlands) was developed to maximize reproducibility. Briefly, on fat-enhanced images, the subcutaneous AT contour was easily identified and manually delineated. To minimize observer bias and therefore maximize reproducibility of visceral AT quantification, a standardized region of interest was positioned in the homogeneous AT, and all pixels with similar signal
intensities were automatically classified as visceral AT by the software. Abdominal fat volumes were measured and reported for a standardized length of 5 mm. Analysis of liver MRS was performed by AMARES algorithm via multi- frequency pattern using jMRUI software (49). Each peak was estimated through gaussian modelling. Matlab algorithms were applied to the fat peaks in order to 1) correct for T2 effects determined from multiple collected echoes, and 2) correct for fat peaks hidden within the water peak. Fat fractions obtained through these corrected measurements are reliable throughout the spectrum of fat content within the liver (from fat-free liver to fatty liver).
Oral glucose tolerance test
An oral glucose tolerance test (OGTT) was performed in the morning following a 12-hour fast while subjects were in a semi-reclined position. Two blood samples were obtained within 15 minutes of each other for fasting plasma glucose and insulin
determinations. Subjects then ingested a 300 ml solution containing 75 g of glucose in £3 minutes. Blood samples were taken at 15¢, 30¢, 45¢, 60¢, 90¢, 120¢, 150¢, and 180¢ after glucose ingestion for assessment of plasma glucose and insulin levels. Plasma glucose was measured using the enzymatic reference method with hexokinase whereas plasma insulin was determined by electrochemiluminescence immunoassay (ECLIA).
Homeostasis model assessment of insulin resistance (HOMA-IR) was calculated from fasting glucose and insulin concentrations (27). Areas under the curve (AUC) of glucose and insulin levels during the OGTT were calculated by the trapezoid method.
Hemodynamic profile
Three sitting blood pressure and heart rate measurements were taken 3 minutes apart on the nondominant arm using an appropriate cuff size with a Tango+ stress test blood pressure monitor (SunTech Medical, Morrisville, USA) and a 12-lead
electrocardiogram was also taken. The mean values of the 3 measurements were retained (36). Blood pressure was measured after the patient had been resting in the sitting position for at least 5 minutes.
Liver enzymes
Plasma alanine transaminase and aspartate transaminase were measured
in the clinical biochemistry laboratory of our healthcare center by standardized and
automated photometric methods (Roche Diagnostics, Indianapolis, USA).
Dietary patterns
Foods and drinks consumed were assessed using a validated 3-day dietary record including two week days and one weekend day (47). Specific questions were administered to assess average weekly alcohol consumption (wine, spirits, mixed drinks and beer). Nutrients were calculated using a computerized version of the Canadian Nutrient File (18).
Cardiorespiratory fitness, resting blood pressure and electrocardiogram
CRF was evaluated using a maximal symptom-limited treadmill graded exercise test on a Q65 treadmill linked to a Q4000 monitor (Quinton Instruments Co., Seattle, WA, USA) with indirect calorimetry using a QuarkB2 calorimeter (COSMED, Rome, Italy). The maximal exercise test was performed using an incremental protocol consisting of 30- second steps until peak oxygen uptake (VO2peak) was reached followed by a 5-minute
active recuperation. The maximal duration of the exercise was about 10-12 minutes. During the test, blood pressure was measured every 2 minutes and heart rate was monitored continuously. Tests were performed by exercise physiology specialists.
Statistical analyses
Participants who did not complete the MRI evaluation or the exercise tolerance test were excluded from the analyses. Student’s t-tests were used to compare the
cardiometabolic risk profile between sexes. Pearson’s partial correlation coefficients (adjusted for age) were also computed by sex. Fisher’s Z-transformation was used to compare correlation coefficients between women and men. To test the effect of sex on the relationships, linear regression analyses for each dependent variable were performed, and the slopes and intercepts of the regression lines were compared. Women and men were also classified into sex-specific CRF tertiles based on their respective CRF values. Age- adjusted ANOVAs were performed to compare visceral AT and LF across sex-specific CRF tertiles with Tukey-Kramer’s post hoc corrections for multiple comparisons. Top CRF
tertiles included participants with a peak VO2 expressed in ml/kg/min >33.6 in women and
>44.1 in men, while lowest CRF tertiles included participants with VO2peak ≤27.9 in women
and ≤37.1 ml/kg/min in men. Pearson’s partial correlation coefficients (adjusted for age) were calculated by sex to explore the contribution of potential confounding factors such as alcohol, fructose and saturated fat consumption. Age-adjusted multivariable stepwise regression analyses were computed to quantify the independent contribution of visceral AT, LF and CRF to the variance of AUC glucose, AUC insulin and HOMA-IR in both women and men. Models with the lowest AIC were chosen. Log-transformed values for LF were used for statistical analyses. Statistical analyses were performed using SAS Studio 3.71 (SAS Institute Inc., Cary, NC, USA).
Results
Participants’ characteristics are presented in Table 1. The sample included 135 women [51.7 (8.5) years] and 177 men [49.2 (8.5) years]. Men showed a higher BMI than women (P < 0.005) with values ranging between 18.4 and 39.1 kg/m2 in women and
between 19.8 and 36.1 kg/m2 in men. Men were also characterized by a lower body fat
mass (P < 0.0001) and percent body fat (P < 0.0001) compared to women. Men displayed a larger waist circumference (P <0.0001) and more visceral AT (P < 0.0005) and LF (P < 0.0001) in comparison to women who had more subcutaneous AT than men (P <0.005). Sex differences were also noted in CRF, with women presenting lower VO2peak values as
opposed to men (P <0.0001). Although no differences were observed between women and men in glucose levels at 120 minutes and AUC glucose and AUC insulin, men were
characterized by higher fasting glucose (P < 0.0001) and insulin (P < 0.005) levels as well as by a higher HOMA-IR than women (P < 0.005). Women presented lower resting blood pressure (P < 0.0001) and higher heart rate (P < 0.0005) than men. Although mean liver enzymes were within the normal range, men had higher levels than women (P < 0.0001). Correlations were observed between LF and alanine transaminase in women (r=0.29, P < 0.001) and men (r=0.50, P < 0.0001). A relationship was also observed between LF and aspartate transaminase in men (r=0.27, P < 0.0005).
Figure 1 shows the relationship between CRF and indices of glucose/insulin
homeostasis derived from the 75 g OGTT in both sexes. Significant correlations were observed between CRF and AUC glucose, AUC insulin and HOMA-IR (P < 0.001). The correlation coefficient value for the relationship between CRF and AUC insulin was higher in men than in women (P < 0.05), while correlation coefficient values between CRF and both AUC glucose and HOMA-IR were not statistically different between sexes (P = NS). Moreover, although the relationship between CRF and indices of glucose/insulin
homeostasis was similar between women and men (slope: P = NS), AUC glucose, AUC insulin and HOMA-IR at any CRF level were all lower in women compared to men (intercept: P < 0.0001).
Figure 2 shows that both visceral AT and LF were negatively correlated with CRF
in women and men (P < 0.0001). Correlation coefficient values were not different between sexes (P = NS). In addition, this figure illustrates the substantial individual variation in visceral AT and LF for any given CRF level, such variation being particularly obvious for LF at any given CRF level. Finally, in women, a given CRF level was associated with lower levels of both visceral AT and LF compared to men (intercept: P < 0.0001).
Results of Figure 3 indicate that both visceral AT and LF were positively correlated with AUC glucose and AUC insulin (P < 0.0001) derived from the OGTT. Moreover, there were significant relationships between visceral AT (women: r = 0.62, P < 0.0001; men: r = 0.61, P < 0.0001) and LF (women and men: r = 0.53, P < 0.0001) with HOMA-IR.
Correlation coefficient values were not different between women and men (P = NS). Furthermore, there was no sex difference in these relationships (slope: P = NS; intercept:
P = NS), indicating that women and men with similar visceral AT and LF essentially had
comparable AUC glucose, AUC insulin and HOMA-IR values.
In order to evaluate the contribution of CRF to the variation of visceral AT and LF, mean levels of both variables are displayed across sex-specific tertiles of CRF (Figure 4). In men, participants with higher CRF presented substantially lower levels of visceral AT (P < 0.0001 across all CRF tertiles) and LF (P < 0.0001 between the low and top CRF tertiles,
P < 0.005 between the middle and top CRF tertiles). In women, participants with higher
and top CRF tertiles, P < 0.05 between the low and middle CRF tertiles and P < 0.005 between the middle and top CRF tertiles) and LF (P < 0.005 between the low and top CRF tertiles and P < 0.05 between the middle and top CRF tertiles).
Age-adjusted correlations between potential confounding factors and visceral AT or LF were computed. In our cohort, alcohol consumption was not associated with visceral AT in either women or men (P = NS). However, alcohol consumption was positively associated with LF in men only (r = 0.18, P < 0.05). In both sexes, fructose and saturated fat consumption were not associated with visceral AT or LF (P = NS).
Finally, we performed multivariable stepwise regression analyses to quantify the respective and independent contribution of visceral AT, LF and CRF to the variation in indices of plasma glucose/insulin homeostasis after statistical adjustment for age (Table
2). In women, LF explained the majority of the variance in AUC glucose while visceral AT
explained most of the variance in AUC insulin and HOMA-IR. In men, visceral AT was the independent variable explaining the largest variation in AUC glucose and HOMA-IR while LF explained most of the variance in AUC insulin. Although of lower magnitude, CRF made a significant contribution to the variance in AUC insulin and HOMA-IR in men. Finally, as alcohol consumption was associated with LF content in men, this confounding