Relationship of intramyocellular lipid to insulin sensitivity may differ with ethnicity in healthy girls and women

Quantification of Extramyocellular Lipids and Intramuscular Fat from Muscle Echo Intensity in Lower Limb Muscles: A Comparison of Four Ultrasound Devices against Magnetic Resonance Spectroscopy

This study aimed to compare different ultrasound devices with magnetic resonance spectroscopy (MRS) to quantify muscle lipid content from echo intensity (EI). Four different ultrasound devices were used to measure muscle EI and subcutaneous fat thickness in four lower-limb muscles. Intramuscular fat (IMF), intramyocellular (IMCL) and extramyocellular lipids (EMCL) were measured using MRS. Linear regression was used to compare raw and subcutaneous fat thickness-corrected EI values to IMCL, EMCL and IMF. IMCL had a poor correlation with muscle EI (r = 0.17-0.32, NS), while EMCL (r = 0.41-0.84, p < 0.05-p < 0.001) and IMF (r = 0.49-0.84, p < 0.01-p < 0.001) had moderate to strong correlation with raw EI. All relationships were improved when considering the effect of subcutaneous fat thickness on muscle EI measurements. The slopes of the relationships were similar across devices, but there were some differences in the y-intercepts when raw EI values were used. These differences disappeared when subcutaneous fat thickness-corrected EI values were considered, allowing for the creation of generic prediction equations (r = 0.41-0.68, p < 0.001). These equations can be used to quantify IMF and EMCL within lower limb muscles from corrected-EI values in non-obese subjects, regardless of the ultrasound device used.

Importance of skeletal muscle lipid levels for muscle function and physical function in older individuals

The skeletal muscle contains lipids inside (intramyocellular lipids, IMCL) or outside (extramyocellular lipids, EMCL) its cells. The muscle lipid content increases with age; however, the characteristics of IMCL and EMCL in older individuals are not well known. We aimed to examine the characteristics of skeletal muscle lipids by investigating their relationship with muscle function and physical functions. Seven elderly men and 16 elderly women participated. The skeletal muscle lipid content, including IMCL and EMCL, was measured in the vastus lateralis by proton magnetic resonance spectroscopy. Isometric knee extension with maximal voluntary contraction (MVC) and time-to-task failure for knee extension with 50% MVC were measured as muscle functions. The participants performed six physical function tests: preferred gait speed, maximal gait speed, Timed Up and Go, chair sit-to-stand, handgrip strength, and stand from the floor. The time to knee extension task failure had a significant relationship with the IMCL (r_s = -0.43, P < 0.05), but not with the EMCL content. Significant relationships were confirmed in the EMCL content with the sit-to-stand (r_s = -0.48, P < 0.05) and stand-from-the-floor (r_s = 0.53, P < 0.05) tests. These findings indicated that muscle lipids are associated with muscle and physical functional performances in older individuals. Novelty: No relationship was confirmed between IMCL and EMCL in older individuals. Muscle endurance performance had a relationship with IMCL, but not with EMCL. Relationships between EMCL and physical functional tests (e.g., sit-to-stand and stand from the floor) were confirmed.

Mitochondrial Phenotype as a Driver of the Racial Dichotomy in Obesity and Insulin Resistance

African Americans (AA) are disproportionately burdened by metabolic diseases. While largely unexplored between Caucasian (C) and AA, differences in mitochondrial bioenergetics may provide crucial insight to mechanisms for increased susceptibility to metabolic diseases. AA display lower total energy expenditure and resting metabolic rate compared to C, but paradoxically have a higher amount of skeletal muscle mass, suggestive of inherent energetic efficiency differences between these races. Such adaptations would increase the chances of overnutrition in AA; however, these disparities would not explain the racial difference in insulin resistance (IR) in healthy subjects. Hallmarks associated with insulin resistance (IR), such as reduced mitochondrial oxidative capacity and metabolic inflexibility are present even in healthy AA without a metabolic disease. These adaptations might be influential of mitochondrial “substrate preference” and could play a role in disproportionate IR rates among races. A higher glycolytic flux and provision of shuttles transferring electrons from cytosol to mitochondrial matrix could be a contributing factor in development of IR via heightened reactive oxygen species (ROS) production. This review highlights the above concepts and provides suggestions for future studies that could help delineate molecular premises behind potential impairments in insulin signaling and metabolic disease susceptibility in AA.

Genetic risk score prediction of leg fat and insulin sensitivity differs by race/ethnicity in early pubertal children

BACKGROUND

In the United States, the underlying reasons for racial/ethnic disparities in type 2 diabetes risk remain unclear. However, differences in genetic risk for insulin resistance and peripheral adipose tissue distribution may be contributing factors.

OBJECTIVE

To investigate racial/ethnic differences in associations of genetic risk for insulin resistance with leg fat and insulin sensitivity in a cohort of American children.

METHODS

Participants were healthy European-American (n = 83), African-American (n = 79) and Hispanic-American (n = 74) children aged 7-12 years. Genetic risk scores were derived from published variants associated with insulin resistance phenotypes in European adults. Body composition was assessed using dual-energy X-ray absorptiometry. Insulin sensitivity was determined from the frequently sampled intravenous glucose tolerance test and minimal modelling. Statistical models were adjusted for age, sex, pubertal stage and body composition.

RESULTS

In the combined cohort, risk score was inversely associated with insulin sensitivity (p = 0.033) but not leg fat (p = 0.170). Within Hispanic Americans, risk score was inversely associated with insulin sensitivity (p = 0.027) and leg fat (p = 0.005), while associations were non-significant in European and African Americans (p > 0.200).

CONCLUSIONS

The higher type 2 diabetes risk observed among Hispanic Americans may have a genetic basis related to an inability to store lipid in peripheral adipose tissue.

Sex and race contribute to variation in mitochondrial function and insulin sensitivity

OBJECTIVE

Insulin sensitivity is lower in African American (AA) versus Caucasian American (CA). We tested the hypothesis that lower insulin sensitivity in AA could be explained by mitochondrial respiratory rates, coupling efficiency, myofiber composition, or H2 O2 emission. A secondary aim was to determine whether sex affected the results.

METHODS

AA and CA men and women, 19-45 years, BMI 17-43 kg m2 , were assessed for insulin sensitivity (SIClamp ) using a euglycemic clamp at 120 mU/m2 /min, muscle mitochondrial function using high-resolution respirometry, H2 O2 emission using amplex red, and % myofiber composition.

RESULTS

SIClamp was greater in CA (p < 0.01) and women (p < 0.01). Proportion of type I myofibers was lower in AA (p < 0.01). Mitochondrial respiratory rates, coupling efficiency, and H2 O2 production did not differ with race. Mitochondrial function was positively associated with insulin sensitivity in women but not men. Statistical adjustment for mitochondrial function, H2 O2 production, or fiber composition did not eliminate the race difference in SIClamp .

CONCLUSION

Neither mitochondrial respiratory rates, coupling efficiency, myofiber composition, nor mitochondrial reactive oxygen species production explained lower SIClamp in AA compared to CA. The source of lower insulin sensitivity in AA may be due to other aspects of skeletal muscle that have yet to be identified.

Obesity in African-Americans: The role of physiology

The disproportionate obesity in African American (AA) women has a physiologic basis and can be explained by the interactive effects of insulin secretion, insulin clearance, insulin sensitivity and the glycaemic load of the diet. This review will present data supporting a physiologic basis for obesity propensity in obesity-prone AA women that resides in their unique metabolic/endocrine phenotype: high beta-cell responsiveness, low hepatic insulin extraction and relatively high insulin sensitivity, which together result in a high exposure of tissues and organs to insulin. When combined with a high-glycaemic (HG) diet (that stimulates insulin secretion), this underlying propensity to obesity becomes manifest, as ingested calories are diverted from energy production to storage. Our data indicate that both weight loss and weight loss maintenance are optimized with low-glycaemic (LG) vs HG diet in AA. Whether greater obesity in AA is mechanistically related to their greater prevalence of type 2 diabetes is debatable. This review provides data indicating that obesity is not strongly related to insulin resistance in AA. Rather, insulin resistance in AA is associated with relatively low adipose tissue in the leg, consistent with a genetic predisposition to impaired lipid storage. Greater bioenergetic efficiency has been reported in AA and, via resultant oxidative damage, could plausibly contribute to insulin resistance. In summary, it is proposed here that a subset of AA women are predisposed to obesity due to a specific metabolic/endocrine phenotype. However, greater diabetes risk in AA has an independent aetiology based on impaired lipid storage and mitochondrial efficiency/oxidative stress.

Intramyocellular Lipids, Insulin Resistance, and Functional Performance in Patients with Severe Obstructive Sleep Apnea

PURPOSE

An increasing number of studies have linked the severity of obstructive sleep apnea (OSA) with metabolic dysfunction. However, little is known about the lipid compartments (intramyocellular [IMCL] and extramyocellular [EMCL] lipids) inside the musculature in these patients. The present study was designed to investigate the IMCL and EMCL, biochemical data, and functional performance in patients with severe OSA, and to examine the correlations between intramuscular lipid contents and test variables.

PARTICIPANTS AND METHODS

Twenty patients with severe OSA (apnea-hypopnea index [AHI]: ≥30/h; body mass index [BMI]: 26.05±2.92) and 20 age- and BMI-matched controls (AHI <5/h) were enrolled. Proton magnetic resonance spectroscopy was used to measure the IMCL and EMCL of the right vastus lateralis muscle. Biochemical data, including levels of fasting plasma glucose, insulin, lipid profiles, and high-sensitivity C-reactive protein (hsCRP), were measured. Insulin resistance index (IR) was calculated using the homeostasis model assessment method. Performance tests included a cardiopulmonary exercise test and knee extension strength and endurance measurements.

RESULTS

Patients with severe OSA had significantly (P<0.05) lower values of IMCL (14.1±5.4 AU) and EMCL (10.3±5.8 AU) compared to the control group (25.2±17.6 AU and 14.3±11.1 AU, respectively). Patients with severe OSA had significantly higher hsCRP, IR, and dyslipidemia compared with controls (all P<0.05). Furthermore, IMCL was negatively correlated with AHI, cumulative time with nocturnal pulse oximetric saturation lower than 90% (TSpO₂<90%) (ρ=-0.35, P<0.05), IR (ρ=-0.40, P<0.05), glucose (ρ=-0.33, P<0.05), and insulin (ρ=-0.36, P<0.05), and positively correlated with lowest oximetric saturation (ρ=0.33, P<0.01).

CONCLUSION

Skeletal muscle dysfunction and metabolic abnormalities were observed in patients with OSA that did not have obesity. IMCL was positively correlated with aerobic capacity and muscular performance, but negatively correlated with AHI and IR. Large-scale clinical trials are required to explore the complicated mechanism among OSA, intramuscular metabolism, and insulin action.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00813852.

Ethnic distinctions in the pathophysiology of type 2 diabetes: a focus on black African-Caribbean populations

Type 2 diabetes (T2D) is a global public health priority, particularly for populations of black African-Caribbean ethnicity, who suffer disproportionately high rates of the disease. While the mechanisms underlying the development of T2D are well documented, there is growing evidence describing distinctions among black African-Caribbean populations. In the present paper, we review the evidence describing the impact of black African-Caribbean ethnicity on T2D pathophysiology. Ethnic differences were first recognised through evidence that metabolic syndrome diagnostic criteria fail to detect T2D risk in black populations due to less central obesity and dyslipidaemia. Subsequently more detailed investigations have recognised other mechanistic differences, particularly lower visceral and hepatic fat accumulation and a distinctly hyperinsulinaemic response to glucose stimulation. While epidemiological studies have reported exaggerated insulin resistance in black populations, more detailed and direct measures of insulin sensitivity have provided evidence that insulin sensitivity is not markedly different to other ethnic groups and does not explain the hyperinsulinaemia that is exhibited. These findings lead us to hypothesise that ectopic fat does not play a pivotal role in driving insulin resistance in black populations. Furthermore, we hypothesise that hyperinsulinaemia is driven by lower rates of hepatic insulin clearance rather than heightened insulin resistance and is a primary defect rather than occurring in compensation for insulin resistance. These hypotheses are being investigated in our ongoing South London Diabetes and Ethnicity Phenotyping study, which will enable a more detailed understanding of ethnic distinctions in the pathophysiology of T2D between men of black African and white European ethnicity.

Effects of a High-Protein Diet Including Whole Eggs on Muscle Composition and Indices of Cardiometabolic Health and Systemic Inflammation in Older Adults with Overweight or Obesity: A Randomized Controlled Trial

Age-related increases in intermuscular adipose tissue (IMAT) impair muscle quality, decrease functional capacity, and promote several cardiometabolic and inflammatory disorders. Whether these age-related alterations in muscle composition improve by consuming a high-protein (HP) diet with whole eggs are unclear. This parallel-design, randomized-controlled trial assessed the effects of a 12-week eucaloric HP diet with three whole eggs per day (1.4 g protein kg⁻¹ day⁻¹) versus a normal-protein diet void of eggs (NP, 0.8 g protein kg⁻¹ day⁻¹) on muscle composition (IMAT), cardiometabolic health, and systemic inflammation in older adults with overweight or obesity (12 men and 10 women; age 70 ± 5 years, BMI 31.3 ± 3.2 kg/m², mean ± SD). No changes in muscle composition were observed over time, independent of protein intake. Total body weight was reduced in both groups (−3.3 ± 1.2%) and lean mass was preserved only with the HP diet. LDL concentration and hip circumference decreased only with the NP diet, while MCP-1 and HsCRP concentrations increased over time in both groups. A HP diet with whole eggs promotes lean mass retention with modest weight loss, but does not positively influence muscle composition, cardiometabolic health or systemic inflammation, compared to a NP diet void of eggs.

Multiple Healthful Dietary Patterns and Type 2 Diabetes in the Women's Health Initiative

The relationship between various diet quality indices and risk of type 2 diabetes (T2D) remains unsettled. We compared associations of 4 diet quality indices--the Alternate Mediterranean Diet Index, Healthy Eating Index 2010, Alternate Healthy Eating Index 2010, and the Dietary Approaches to Stop Hypertension (DASH) Index--with reported T2D in the Women's Health Initiative, overall, by race/ethnicity, and with/without adjustment for overweight/obesity at enrollment (a potential mediator). This cohort (n = 101,504) included postmenopausal women without T2D who completed a baseline food frequency questionnaire from which the 4 diet quality index scores were derived. Higher scores on the indices indicated a better diet. Cox regression was used to estimate multivariate hazard ratios for T2D. Pearson coefficients for correlation among the indices ranged from 0.55 to 0.74. Follow-up took place from 1993 to 2013. During a median 15 years of follow-up, 10,815 incident cases of T2D occurred. For each diet quality index, a 1-standard-deviation higher score was associated with 10%-14% lower T2D risk (P < 0.001). Adjusting for overweight/obesity at enrollment attenuated but did not eliminate associations to 5%-10% lower risk per 1-standard-deviation higher score (P < 0.001). For all 4 dietary indices examined, higher scores were inversely associated with T2D overall and across racial/ethnic groups. Multiple forms of a healthful diet were inversely associated with T2D in these postmenopausal women.

Intramuscular adipose tissue determined by T1-weighted MRI at 3T primarily reflects extramyocellular lipids

PURPOSE

The purpose of this study was to assess relationships between intramuscular adipose tissue (IntraMAT) content determined by MRI and intramyocellular lipids (IMCL) and extramyocellular lipids (EMCL) determined by (1)H magnetic resonance spectroscopy ((1)H MRS) or echo intensity determined by B-mode ultrasonography of human skeletal muscles.

METHODS

Thirty young and elderly men and women were included. T1-weighted MRI was taken from the right mid-thigh to measure IntraMAT content of the vastus lateralis (VL) and biceps femoris (BF) using a histogram shape-based thresholding technique. IMCL and EMCL were measured from the VL and BF at the right mid-thigh using (1)H MRS. Ultrasonographic images were taken from the VL and BF of the right mid-thigh to measure echo intensity based on gray-scale level for quantitative analysis.

RESULTS

There was a significant correlation between IntraMAT content by MRI and EMCL of the VL and BF (VL, r=0.506, P<0.01; BF, r=0.591, P<0.001) and between echo intensity and EMCL of the VL and BF (VL, r=0.485, P<0.05; BF, r=0.648, P<0.01). IntraMAT content was also significantly correlated with echo intensity of the VL and BF (VL, r=0.404, P<0.05; BF, r=0.493, P<0.01).

CONCLUSION

Our study suggests that IntraMAT content determined by T1-weighted MRI at 3T primarily reflects extramyocellular lipids, not intramyocellular lipids, in human skeletal muscles.

A comparison of indices of glucose metabolism in five black populations: data from modeling the epidemiologic transition study (METS)

Background

Globally, Africans and African Americans experience a disproportionate burden of type 2 diabetes, compared to other race and ethnic groups. The aim of the study was to examine the association of plasma glucose with indices of glucose metabolism in young adults of African origin from 5 different countries.

Methods

We identified participants from the Modeling the Epidemiologic Transition Study, an international study of weight change and cardiovascular disease (CVD) risk in five populations of African origin: USA (US), Jamaica, Ghana, South Africa, and Seychelles. For the current study, we included 667 participants (34.8 ± 6.3 years), with measures of plasma glucose, insulin, leptin, and adiponectin, as well as moderate and vigorous physical activity (MVPA, minutes/day [min/day]), daily sedentary time (min/day), anthropometrics, and body composition.

Results

Among the 282 men, body mass index (BMI) ranged from 22.1 to 29.6 kg/m² in men and from 25.8 to 34.8 kg/m² in 385 women. MVPA ranged from 26.2 to 47.1 min/day in men, and from 14.3 to 27.3 min/day in women and correlated with adiposity (BMI, waist size, and % body fat) only among US males after controlling for age. Plasma glucose ranged from 4.6 ± 0.8 mmol/L in the South African men to 5.8 mmol/L US men, while the overall prevalence for diabetes was very low, except in the US men and women (6.7 and 12 %, respectively). Using multivariate linear regression, glucose was associated with BMI, age, sex, smoking hypertension, daily sedentary time but not daily MVPA.

Conclusion

Obesity, metabolic risk, and other potential determinants vary significantly between populations at differing stages of the epidemiologic transition, requiring tailored public health policies to address local population characteristics.

Race and the insulin resistance syndrome

Type 2 diabetes remains an important cause of morbidity and mortality. The metabolic syndrome affects 25% of the adult US population based on the Third Report of the Expert Panel on Detection, Evaluation and Treatment of High Blood Cholesterol in Adults from the National Cholesterol Education Program. Knowledge on the impact of obesity on metabolic health parameters has increased greatly over the past decade. This review discusses the limitations of the National Cholesterol Education Program metabolic syndrome definition and the racial disparities in the clinical presentation of the insulin resistance syndrome. We also examine the current literature with particular emphasis on albuminuria, nonalcoholic fatty liver disease, and intramyocellular lipid content. This review explores potential environmental and genetic reasons for differences in the manifestation of insulin resistance across racial/ethnic groups and highlights several promising areas for further study.

Race/ethnicity disparities in dysglycemia among U.S. women of childbearing age found mainly in the nonoverweight/nonobese

OBJECTIVE

To describe the burden of dysglycemia-abnormal glucose metabolism indicative of diabetes or high risk for diabetes-among U.S. women of childbearing age, focusing on differences by race/ethnicity.

RESEARCH DESIGN AND METHODS

Using U.S. National Health and Nutrition Examination Survey data (1999-2008), we calculated the burden of dysglycemia (i.e., prediabetes or diabetes from measures of fasting glucose, A1C, and self-report) in nonpregnant women of childbearing age (15-49 years) by race/ethnicity status. We estimated prevalence risk ratios (PRRs) for dysglycemia in subpopulations stratified by BMI (measured as kilograms divided by the square of height in meters), using predicted marginal estimates and adjusting for age, waist circumference, C-reactive protein, and socioeconomic factors.

RESULTS

Based on data from 7,162 nonpregnant women, representing>59,000,000 women nationwide, 19% (95% CI 17.2-20.9) had some level of dysglycemia, with higher crude prevalence among non-Hispanic blacks and Mexican Americans vs. non-Hispanic whites (26.3% [95% CI 22.3-30.8] and 23.8% [19.5-28.7] vs. 16.8% [14.4-19.6], respectively). In women with BMI<25 kg/m2, dysglycemia prevalence was roughly twice as high in both non-Hispanic blacks and Mexican Americans vs. non-Hispanic whites. This relative increase persisted in adjusted models (PRRadj 1.86 [1.16-2.98] and 2.23 [1.38-3.60] for non-Hispanic blacks and Mexican Americans, respectively). For women with BMI 25-29.99 kg/m2, only non-Hispanic blacks showed increased prevalence vs. non-Hispanic whites (PRRadj 1.55 [1.03-2.34] and 1.28 [0.73-2.26] for non-Hispanic blacks and Mexican Americans, respectively). In women with BMI>30 kg/m2, there was no significant increase in prevalence of dysglycemia by race/ethnicity category.

CONCLUSIONS

Our findings show that dysglycemia affects a significant portion of U.S. women of childbearing age and that disparities by race/ethnicity are most prominent in the nonoverweight/nonobese.

Effects of an overnight intravenous lipid infusion on intramyocellular lipid content and insulin sensitivity in African-American versus Caucasian adolescents

OBJECTIVE

To explain the predisposition for insulin resistance among African American (AA) adolescents, this study aimed to: 1) examine changes in intramyocellular lipid content (IMCL), and insulin sensitivity with intralipid (IL) infusion; and 2) determine whether the increase in IMCL is comparable between AA and Caucasian adolescents.

MATERIALS AND METHODS

Thirteen AA and 15 Caucasian normal-weight adolescents (BMI <85th) underwent a 3-h hyperinsulinemic-euglycemic clamp, on two occasions in random order, after an overnight 12-h infusion of: 1) 20% IL and 2) normal saline (NS). IMCL was quantified by (1)H magnetic resonance spectroscopy in tibialis anterior muscle before and after IL infusion.

RESULTS

During IL infusion, plasma TG, glycerol, FFA and fat oxidation increased significantly, with no race differences. Hepatic insulin sensitivity decreased with IL infusion with no difference between the groups. IL infusion was associated with a significant increase in IMCL, which was comparable between AA (Δ 105%; NS: 1.9±0.8 vs. IL: 3.9±1.6 mmol/kg wet weight) and Caucasian (Δ 86%; NS: 2.8±2.1 vs. IL: 5.2±2.4 mmol/kg wet weight), with similar reductions (P<0.01) in insulin sensitivity between the groups (Δ -44%: NS: 9.1±3.3 vs. IL: 5.1±1.8 mg/kg/min per μU/ml in AA) and (Δ -39%: NS: 12.9±6.0 vs. IL: 7.9±3.8 mg/kg/min per μU/ml in Caucasian) adolescents.

CONCLUSIONS

In healthy adolescents, an acute elevation in plasma FFA with IL infusion is accompanied by significant increases in IMCL and reductions in insulin sensitivity with no race differential. Our findings suggest that AA normal-weight adolescents are not more susceptible than Caucasians to FFA-induced IMCL accumulation and insulin resistance.

The role of adipose tissue in insulin resistance in women of African ancestry

Women of African ancestry, particularly those living in industrialized countries, experience a disproportionately higher prevalence of type 2 diabetes (T2D) compared to their white counterparts. Similarly, obesity and insulin resistance, which are major risk factors for T2D, are greater in black compared to white women. The exact mechanisms underlying these phenomena are not known. This paper will focus on the role of adipose tissue biology. Firstly, the characteristic body fat distribution of women of African ancestry will be discussed, followed by the depot-specific associations with insulin resistance. Factors involved in adipose tissue biology and their relation to insulin sensitivity will then be explored, including the role of sex hormones, glucocorticoid metabolism, lipolysis and adipogenesis, and their consequent effects on adipose tissue hypoxia, oxidative stress, and inflammation. Finally the role of ectopic fat deposition will be discussed. The paper proposes directions for future research, in particular highlighting the need for longitudinal and/or intervention studies to better understand the mechanisms underlying the high prevalence of insulin resistance and T2D in women of African ancestry.

Skeletal muscle-specific CPT1 deficiency elevates lipotoxic intermediates but preserves insulin sensitivity

OBJECTIVE

By specific knockout of carnitine palmitoyl transferase 1b (CPT1b) in skeletal muscles, we explored the effect of CPT1b deficiency on lipids and insulin sensitivity.

METHODS

Mice with specific knockout of CPT1b in skeletal muscles (CPT1b M-/-) were used for the experiment group, with littermate C57BL/6 as controls (CPT1b). General and metabolic profiles were measured and compared between groups. mRNA expression and CPT1 activity were measured in skeletal muscle tissues and compared between groups. Mitochondrial fatty acid oxidation (FAO), triglycerides (TAGs), diglycerides (DAGs), and ceramides were examined in skeletal muscles in two groups. Phosphorylated AKT (pAkt) and glucose transporter 4 (Glut4) were determined with real-time polymerase chain reaction (RT-PCR). Insulin tolerance test, glucose tolerance test, and pyruvate oxidation were performed in both groups.

RESULTS

CPT1b M-/- model was successfully established, with impaired muscle CPT1 activity. Compared with CPT1b mice, CPT1b M-/- mice had similar food intake but lower body weight or fat mass and higher lipids but similar glucose or insulin levels. Their mitochondrial FAO of skeletal muscles was impaired. There were lipids accumulations (TAGs, DAGs, and ceramides) in skeletal muscle. However, pAkt and Glut4, insulin sensitivity, glucose tolerance, and pyruvate oxidation were preserved.

CONCLUSION

Skeletal muscle-specific CPT1 deficiency elevates lipotoxic intermediates but preserves insulin sensitivity.

Ethnic differences in glucose disposal, hepatic insulin sensitivity, and endogenous glucose production among African American and European American women

Intravenous glucose tolerance tests have demonstrated lower whole-body insulin sensitivity (S(I)) among African Americans (AA) compared with European Americans (EA). Whole-body S(I) represents both insulin-stimulated glucose disposal, primarily by skeletal muscle, and insulin's suppression of endogenous glucose production (EGP) by liver. A mathematical model was recently introduced that allows for distinction between disposal and hepatic S(I). The purpose of this study was to examine specific indexes of S(I) among AA and EA women to determine whether lower whole-body S(I) in AA may be attributed to insulin action at muscle, liver, or both. Participants were 53 nondiabetic, premenopausal AA and EA women. Profiles of EGP and indexes of Disposal S(I) and Hepatic S(I) were calculated by mathematical modeling and incorporation of a stable isotope tracer ([6,6-(2)H(2)]glucose) into the intravenous glucose tolerance test. Body composition was assessed by dual-energy x-ray absorptiometry. After adjustment for percentage fat, both Disposal S(I) and Hepatic S(I) were lower among AA (P = .009 for both). Time profiles for serum insulin and EGP revealed higher peak insulin response and corresponding lower EGP among AA women compared with EA. Indexes from a recently introduced mathematical model suggest that lower whole-body S(I) among nondiabetic AA women is due to both hepatic and peripheral components. Despite lower Hepatic S(I), AA displayed lower EGP, resulting from higher postchallenge insulin levels. Future research is needed to determine the physiological basis of lower insulin sensitivity among AA and its implications for type 2 diabetes mellitus risk.

Postprandial endothelial function does not differ in women by race: an insulin resistance paradox?

Insulin resistance is associated with endothelial dysfunction. Because African-American women are more insulin-resistant than white women, it is assumed that African-American women have impaired endothelial function. However, racial differences in postprandial endothelial function have not been examined. In this study, we test the hypothesis that African-American women have impaired postprandial endothelial function compared with white women. Postprandial endothelial function following a breakfast (20% protein, 40% fat, and 40% carbohydrate) was evaluated in 36 (18 African-American women, 18 white women) age- and body mass index (BMI)-matched (age: 37 ± 11 yr; BMI: 30 ± 6 kg/m(2)) women. Endothelial function, defined by percent change in brachial artery flow-mediated dilation (FMD), was measured at 0, 2, 4, and 6 h following a meal. There were no significant differences between the groups in baseline FMD, total body fat, abdominal visceral fat, and fasting levels of glucose, insulin, total cholesterol, low-density lipoprotein cholesterol, or serum estradiol. Although African-American women were less insulin-sensitive [insulin sensitivity index (mean ± SD): 3.6 ± 1.5 vs. 5.2 ± 2.6, P = 0.02], both fasting triglyceride (TG: 56 ± 37 vs. 97 ± 49 mg/dl, P = 0.007) and incremental TG area under the curve (AUC(0-6hr): 279 ± 190 vs. 492 ± 255 mg·dl(-1)·min(-1)·10(-2), P = 0.008) were lower in African-American than white women. Breakfast was associated with a significant increase in FMD in whites and African-Americans, and there was no significant difference in postprandial FMD between the groups (P > 0.1 for group × time interactions). Despite being insulin-resistant, postprandial endothelial function in African-American women was comparable to white women. These results imply that insulin sensitivity may not be an important determinant of racial differences in endothelial function.

Intramyocellular lipid content and insulin sensitivity are increased following a short-term low-glycemic index diet and exercise intervention

The relationship between intramyocellular (IMCL) and extramyocellular lipid (EMCL) accumulation and skeletal muscle insulin resistance is complex and dynamic. We examined the effect of a short-term (7-day) low-glycemic index (LGI) diet and aerobic exercise training intervention (EX) on IMCL and insulin sensitivity in older, insulin-resistant humans. Participants (66 ± 1 yr, BMI 33 ± 1 kg/m(2)) were randomly assigned to a parallel, controlled feeding trial [either an LGI (LGI/EX, n = 7) or high GI (HGI/EX, n = 8) eucaloric diet] combined with supervised exercise (60 min/day, 85% HR(max)). Insulin sensitivity was determined via 40 mU·m(-2)·min(-1) hyperinsulinemic euglycemic clamp and soleus IMCL and EMCL content was assessed by (1)H-MR spectroscopy with correction for fiber orientation. BMI decreased (kg/m(2) -0.6 ± 0.2, LGI/EX; -0.7 ± 0.2, HGI/EX P < 0.0004) after both interventions with no interaction effect of diet composition. Clamp-derived insulin sensitivity increased by 0.91 ± 0.21 (LGI/EX) and 0.17 ± 0.55 mg·kg(-1)·min(-1) (HGI/EX), P = 0.04 (effect of time). HOMA-IR was reduced by -1.1 ± 0.4 (LGI/EX) and -0.1 ± 0.2 (HGI/EX), P = 0.007 (effect of time), P = 0.02 (time × trial). Although both interventions increased IMCL content, (Δ: 2.3 ± 1.3, LGI/EX; 1.4 ± 0.9, HGI/EX, P = 0.03), diet composition did not significantly effect the increase. However, the LGI/EX group showed a robust increase in the [IMCL]/[EMCL] ratio compared with the HGI/EX group (Δ: 0.5 ± 0.2 LGI/EX vs. 0.07 ± 0.1, P = 0.03). The LGI/EX group also demonstrated greater reductions in [EMCL] than the HGI/EX group (Δ: -5.8 ± 3.4, LGI/EX; 2.3 ± 1.1, HGI/EX, P = 0.03). Changes in muscle lipids and insulin sensitivity were not correlated; however, the change in [IMCL]/[EMCL] was negatively associated with the change in FPI (r = -0.78, P = 0.002) and HOMA-IR (r = -0.61, P = 0.03). These data suggest that increases in the IMCL pool following a low glycemic diet and exercise intervention may represent lipid repartitioning from EMCL. The lower systemic glucose levels that prevail while eating a low glycemic diet may promote redistribution of lipid stores in the muscle.

Body composition phenotypes in pathways to obesity and the metabolic syndrome

Dynamic changes in body weight have long been recognized as important indicators of risk for debilitating diseases. While weight loss or impaired growth can lead to muscle wastage, as well as to susceptibility to infections and organ dysfunctions, the development of excess fat predisposes to type 2 diabetes and cardiovascular diseases, with insulin resistance as a central feature of the disease entities of the metabolic syndrome. Although widely used as the phenotypic expression of adiposity in population and gene-search studies, body mass index (BMI), that is, weight/height(2) (H(2)), which was developed as an operational definition for classifying both obesity and malnutrition, has considerable limitations in delineating fat mass (FM) from fat-free mass (FFM), in particular at the individual level. After an examination of these limitations within the constraints of the BMI-FM% relationship, this paper reviews recent advances in concepts about health risks related to body composition phenotypes, which center upon (i) the partitioning of BMI into an FM index (FM/H(2)) and an FFM index (FFM/H(2)), (ii) the partitioning of FFM into organ mass and skeletal muscle mass, (iii) the anatomical partitioning of FM into hazardous fat and protective fat and (iv) the interplay between adipose tissue expandability and ectopic fat deposition within or around organs/tissues that constitute the lean body mass. These concepts about body composition phenotypes and health risks are reviewed in the light of race/ethnic variability in metabolic susceptibility to obesity and the metabolic syndrome.