Cystatin C and Cardiovascular Disease: A Mendelian Randomization Study

BACKGROUND

Epidemiological studies show that high circulating cystatin C is associated with risk of cardiovascular disease (CVD), independent of creatinine-based renal function measurements. It is unclear whether this relationship is causal, arises from residual confounding, and/or is a consequence of reverse causation.

OBJECTIVES

The aim of this study was to use Mendelian randomization to investigate whether cystatin C is causally related to CVD in the general population.

METHODS

We incorporated participant data from 16 prospective cohorts (n = 76,481) with 37,126 measures of cystatin C and added genetic data from 43 studies (n = 252,216) with 63,292 CVD events. We used the common variant rs911119 in CST3 as an instrumental variable to investigate the causal role of cystatin C in CVD, including coronary heart disease, ischemic stroke, and heart failure.

RESULTS

Cystatin C concentrations were associated with CVD risk after adjusting for age, sex, and traditional risk factors (relative risk: 1.82 per doubling of cystatin C; 95% confidence interval [CI]: 1.56 to 2.13; p = 2.12 × 10⁻¹⁴). The minor allele of rs911119 was associated with decreased serum cystatin C (6.13% per allele; 95% CI: 5.75 to 6.50; p = 5.95 × 10⁻²¹¹), explaining 2.8% of the observed variation in cystatin C. Mendelian randomization analysis did not provide evidence for a causal role of cystatin C, with a causal relative risk for CVD of 1.00 per doubling cystatin C (95% CI: 0.82 to 1.22; p = 0.994), which was statistically different from the observational estimate (p = 1.6 × 10⁻⁵). A causal effect of cystatin C was not detected for any individual component of CVD.

CONCLUSIONS

Mendelian randomization analyses did not support a causal role of cystatin C in the etiology of CVD. As such, therapeutics targeted at lowering circulating cystatin C are unlikely to be effective in preventing CVD.

Uric acid is associated with microalbuminuria and decreased glomerular filtration rate in the general population during 7 and 13 years of follow-up: The Tromsø Study

BACKGROUND

The role of uric acid in development of renal dysfunction (RD) remains controversial. Earlier studies have reported inconsistent results, possibly because of their varying ability to adjust for confounding. The impact of longitudinal change in uric acid on renal outcome has not been assessed previously. We aimed to study the impact of change in serum uric acid (SUA) as well as baseline SUA on the development of RD.

METHODS

In a prospective cohort study, we assessed the associations between change in SUA during follow-up, baseline SUA and RD (defined as albumin-creatinine-ratio (ACR) ≥1.13 mg albumin/mmol creatinine and/or eGFR < 60 ml/min/1.73 m(2)) in a large cohort from a general population participating in the Tromsø Study (n = 2637). Participants were stratified according to tertiles of change in SUA between baseline (1994/95) and follow-up 13 years later. (upper tertile: SUA increasing group, two lower tertiles: SUA non-increasing group). Logistic regression analysis was applied with RD and each component of RD after 7 and 13 years as the dependent variables. Adjustments were made for baseline eGFR, cardiovascular risk factors, and the use of antihypertensive drugs including diuretics.

RESULTS

After excluding participants with RD at baseline, SUA increasers, compared to SUA non-increasers, had a doubled risk of RD after 7 years (odds ratio 2.00, (95 % CI 1.45, 2.75)). Odds ratio for RD in SUA increasers after 13 years was 2.18 (95 % CI 1.71, 2.79). The risk of developing ACR ≥1.13 mg/mmol alone was not significantly increased after 7 years (odds ratio 1.30 (95 % CI 0.90, 1.89), but after 13 years (odds ratio 1.43 (95 % CI 1.09, 1.86)). An increase in baseline SUA of 59 μmol/L (1 mg/dL) gave an odds ratio for RD after 13 years of 1.16 (95 % CI 1.04, 1.29).

CONCLUSION

An increase in SUA during follow-up was associated with an increased risk of developing RD after 7 and 13 years.

N-Acetyl-β-D-Glucosaminidase Does Not Enhance Prediction of Cardiovascular or All-Cause Mortality by Albuminuria in a Low-Risk Population

Albuminuria is a well known risk factor for cardiovascular disease and mortality, but focus on renal tubular dysfunction as a potential risk factor is growing also. The association between the urinary activity of N-acetyl-β-D-glucosaminidase (NAG) and cardiovascular risk has been assessed mostly in cross-sectional studies. We studied the cross-sectional associations between urinary NAG and cardiovascular risk factors and the longitudinal associations between NAG, cardiovascular disease, and all-cause mortality in a general population. Urinary NAG/creatinine ratio (NAG ratio) and albumin/creatinine ratio (ACR) were measured in 6834 participants of the Tromsø Study in 1994-1995. During the median 17.5 years of follow-up, 958 myocardial infarctions, 726 ischemic strokes, and 2358 deaths were registered. In multivariable analyses adjusted for albuminuria and cardiovascular risk factors, a baseline NAG ratio in the highest quartile was associated with an increased risk of myocardial infarction (hazard ratio [HR], 1.43; 95% confidence interval [95% CI], 1.16 to 1.76), ischemic stroke (HR, 1.41; 95% CI, 1.10 to 1.80), and all-cause mortality (HR, 1.60; 95% CI, 1.39 to 1.84). Combined, ACR and NAG ratio above median associated with a 48%-80% increased risk for the three end points. However, the NAG ratio did not add significantly to the baseline risk-prediction models when assessed by area under the receiver operating characteristics curve or net reclassification improvement. In conclusion, the nonsignificant improvement of risk prediction does not support the clinical use of NAG ratio in cardiovascular risk assessment in a low-risk group.

Cross-sectional analysis of nutrition and serum uric acid in two Caucasian cohorts: the AusDiab Study and the Tromsø study

Background

Hyperuricemia can lead to gout, and may be a risk factor for cardiovascular events, hypertension, diabetes and renal disease. There is well-known link between gout and habitual intake of meat and seafood, however the association between hyperuricemia and micro-and macro-nutrient intake has not been established.

Methods

We studied associations between intakes of food categories, macro-and micronutrients and serum uric acid (SUA) levels in two cross-sectional surveys of Caucasian adults deriving from different food traditions: Australian Diabetes, Obesity and Lifestyle Study 1999/00 (n=9734, age 25–91) and Tromsø Study 4 1994/95 (n = 3031, age 25–69). Dietary intake was calculated from self-administered Food Frequency Questionnaires. In some analyses we stratified according to abdominal obesity status and gender.

Results

In both cohorts, lower levels of SUA were found in subjects with higher consumption of carbohydrates, calcium and vitamin B2, while higher fat intake was associated with higher SUA, after adjustment for age, body mass index, estimated glomerular filtration rate, physical activity, total energy intake, use of diuretics, presence of hypertension, diabetes and gout. Among individual food items, high consumption of dairy products, high-fibre bread, cereals and fruits were associated with lower SUA in most subject groups while consumption of meat, eggs, beer and spirits, but not wine, with elevated levels.

Conclusions

Healthy food choices with high intake of carbohydrates, dairy products, fiber and micronutrient-rich foods, and limited intake of fat, beer and spirits, might be recommended to prevent high SUA. Dietary factors seem to have qualitatively similar impact on SUA in obese and non-obese men and women from Australia and Norway.

Electronic supplementary material

The online version of this article (doi:10.1186/s12937-015-0032-1) contains supplementary material, which is available to authorized users.

Sex differences in risk factors for retinopathy in non-diabetic men and women: the Tromsø Eye Study

PURPOSE

  To determine the prevalence and risk factors for retinopathy in a nondiabetic population.

METHODS

  The study population included 5869 participants without diabetes aged 38-87 years from the Tromsø Eye Study, a substudy of the population-based Tromsø Study in Norway. Retinal images from both eyes were graded for retinopathy. We collected data on risk factors from self-report questionnaires, clinical examinations, laboratory measurements and case note reviews. The cross-sectional relationship between potential risk factors and retinopathy was assessed using logistic regression analysis.

RESULTS

  The overall prevalence of retinopathy was 14.8%. Men had a higher prevalence of retinopathy compared with women (15.9% versus 14.0%, p=0.04). In men, retinopathy was associated with hypertension (odds ratio [OR], 1.59; 95% confidence interval [CI], 1.24-2.04) and HbA1c (OR per %, 1.41; 95% CI, 1.01-1.96). In women, retinopathy was associated with age (OR per 10 years, 1.32; 95% CI, 1.14-1.52), log-transformed urinary albumin excretion (OR per log unit, 1.46; 95% CI, 1.14-1.87) and hypertension (OR, 1.36; 95% CI, 1.08-1.71). In women, retinopathy was associated with very low levels of urinary albumin excretion (urinary albumin/creatinine ratio >0.43 mg/mmol).

CONCLUSION

  This study confirms results from previous studies on the strong association between blood pressure and retinopathy. A novel finding is the sex differences in risk factors for retinopathy, suggesting a sex difference in the pathogenesis leading to retinopathy.

Uric acid has a J-shaped association with cardiovascular and all-cause mortality in kidney transplant recipients

The association between serum uric acid and kidney graft and recipient survival is uncertain. During 2000-2011, we measured serum uric acid at week 10 after transplantation. Of 2748 transplanted patients, 2200 (80.1%) attended this visit. After a median follow-up of 7.4 yr, 378 patients had died, 143 from a cardiovascular cause, and 185 patients lost their graft. The third quintile of uric acid levels (357-405 μM) had the lowest mortality risk and was used as reference group. In Cox proportional hazard models adjusting for graft and patient characteristics, the fifth quintile of uric acid levels (>474 μM) was independently associated with cardiovascular mortality (hazard ratio [HR] = 2.87 [1.55-5.32], p = 0.001) and all-cause mortality (HR = 1.57 [1.09-2.25], p = 0.02). Also, the lowest quintile of uric acid levels (<309 μM) showed a trend toward increased risk of cardiovascular mortality (HR = 1.79 [0.90-3.58], p = 0.10) and all-cause mortality (HR = 1.31 [0.89-1.93], p = 0.18). The increased risk at low uric acid levels was confined to diabetic recipients. Uric acid was not associated with death-censored graft loss. In conclusion, uric acid has a J-shaped association with cardiovascular and all-cause mortality in kidney transplant recipients.

Uric acid is a risk factor for ischemic stroke and all-cause mortality in the general population: a gender specific analysis from The Tromsø Study

BACKGROUND

The role of serum uric acid as an independent predictor of cardiovascular disease and death is uncertain in the general population. Adjustments for additional cardiovascular risk factors have not been consistent. We examined the association of serum uric acid with all-cause mortality, ischemic stroke and myocardial infarction in a prospective population based study, with several traditional and non-traditional risk factors for cardiovascular disease included in the model.

METHODS

A population-based prospective cohort study was performed among 2696 men and 3004 women. Endpoints were all-cause mortality after 15 years, and fatal or non-fatal myocardial infarction (MI) and ischemic stroke after 12 years.

RESULTS

1433 deaths, 659 MIs and 430 ischemic strokes occurred during follow-up. Fully adjusted Cox regression analyses showed that per 1 SD (87 μmol/L) increase in serum uric acid level, the risk of all-cause mortality increased in both genders (hazard ratios, HR men; 1.11, 95% CI 1.02-1.20, women; 1.16, 1.05-1.29). HRs and 95% CI for stroke were 1.31, 1.14-1.50 in men, 1.13, 0.94-1.36 in women, and 1.22 (1.09, 1.35) in the overall population. No independent associations were observed with MI.

CONCLUSION

Serum uric acid was associated with all-cause mortality in men and women, even after adjustment for blood pressure, estimated GFR, urinary albumin/creatinine ratio, drug intake and traditional cardiovascular risk factors. After the same adjustments, serum uric acid was associated with 31% increased risk of stroke in men.

Tromsø eye study: prevalence and risk factors of diabetic retinopathy

PURPOSE

To determine the prevalence of visual impairment, retinopathy and macular oedema, and assess risk factors for retinopathy in persons with diabetes.

METHODS

The present study included 514 participants with diabetes aged 46-87 years from the Tromsø Eye Study, a sub-study of the population-based Tromsø Study in Norway. Visual acuity was measured using an auto-refractor. Retinal images from both eyes were graded for retinopathy and macular oedema. We collected data on risk factor exposure from self-report questionnaires, clinical examinations, laboratory measurements and case note reviews. Regression models assessed the cross-sectional relationship between potential risk factors and diabetic retinopathy.

RESULTS

The prevalence of visual impairment (corrected Snellen visual acuity <20/60 in the better-seeing eye) was 1.6%. The prevalence of diabetic retinopathy was 26.8% and macular oedema 3.9%. In a multivariable logistic regression model, retinopathy was associated with longer diabetes duration (odds ratio, OR 1.07, 95% CI 1.03-1.11), insulin use (OR 2.14, 95% CI 1.19-3.85), nonfasting glucose (OR 1.07, 95% CI 1.00-1.15) and microalbuminuria (OR 1.89, 95% CI 1.28-2.81). Sub-group analyses showed association between retinopathy and even low levels of microalbuminuria (1.16 mg/mmol).

CONCLUSION

The findings suggest that low levels of microalbuminuria may be a useful risk predictor for identifying individuals with diabetes at high risk of retinopathy. The study confirms previous findings that insulin use, longer diabetes duration and higher levels of blood glucose are associated with retinopathy in persons with diabetes. The prevalence of diabetic retinopathy was similar as reported in other studies.

Physical exercise, fasting glucose, and renal hyperfiltration in the general population: the Renal Iohexol Clearance Survey in Tromsø 6 (RENIS-T6)

BACKGROUND AND OBJECTIVES

Abnormally elevated GFR, or hyperfiltration, is a proposed mechanism for kidney injury in diabetes, prediabetes, and obesity. This study investigated whether lack of physical exercise is associated with hyperfiltration and whether exercise modifies the positive association between fasting glucose and measured GFR.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The Renal Iohexol Clearance Survey in Tromsø 6 measured GFR as single-sample plasma iohexol clearance in 1506 members of the general population (age 50-62 years) without diabetes, cardiovascular disease, or kidney disease. Leisure-time physical exercise was assessed by a self-administered questionnaire. Hyperfiltration was defined as GFR above the 90th percentile after adjustment for sex, age, weight, height, and use of renin-angiotensin system inhibitors.

RESULTS

High-intensity exercise was associated with lower adjusted odds of hyperfiltration in men (odds ratio [OR], 0.47; 95% confidence interval [CI], 0.28-0.80) but not in women (OR, 1.02; 95% CI, 0.60-1.72). In both sexes, high-intensity exercise modified the association between fasting glucose and GFR. A fasting glucose level 1 mmol/L higher was associated with a GFR that was 7.3 (95% CI, 4.0-10.6) and 6.2 (95% CI, 3.4-9.0) ml/min per 1.73 m(2) higher in men and women who never exercised or exercised with low intensity. There was no association between fasting glucose and GFR in men and women who exercised with high intensity (interaction, P<0.001).

CONCLUSIONS

High-intensity exercise was associated with lower odds of hyperfiltration in men and modified the association between glucose and GFR of both sexes in a population without diabetes.

The role of cystatin C in improving GFR estimation in the general population

BACKGROUND

The equations used to estimate glomerular filtration rate (GFR) based on serum creatinine level are limited by their dependence on muscle mass. Although cystatin C level predicts clinical outcomes better than creatinine level in the general population, its role in estimating GFR in the reference range is unclear. Cystatin C level is not influenced by muscle mass, but by several other non-GFR determinants. We investigated whether regression models using cystatin C level alone or in combination with creatinine level in principle would improve GFR estimation in the general population compared with models using creatinine level alone.

STUDY DESIGN

Study of diagnostic accuracy.

SETTING & PARTICIPANTS

A representative sample (n = 1,621; aged 50-62 years) of the general population in Tromsø, Norway, without coronary heart disease, stroke, diabetes mellitus, or kidney disease. Individuals had participated in the Renal Iohexol Clearance Survey (RENIS-T6), part of the sixth Tromsø Study.

INDEX TEST

Performance of multiple linear and fractional polynomial regression models with plasma creatinine and/or cystatin C levels as independent variables and measured GFR as a dependent variable.

REFERENCE TEST

Plasma iohexol clearance.

OTHER MEASUREMENTS

Creatinine measured with an enzymatic method. Cystatin C measured with a particle-enhanced turbidimetric immunoassay.

RESULTS

In internal validation of models with cystatin C, creatinine, or both levels, percentages of GFR estimates within 10% of measured GFR were 61% (95% CI, 58%-63%), 62% (95% CI, 59%-64%), and 68% (95% CI, 65%-70%), respectively. Models with either cystatin C or creatinine level had very similar precision and ability to detect GFR <90 mL/min/1.73 m(2), whereas models based on both markers performed better.

LIMITATIONS

Only middle-aged individuals of European ancestry were investigated. Lack of standardization between cystatin C assays. No external validation of regression models.

CONCLUSIONS

Models based on cystatin C alone are not superior to those based on creatinine, but models based on both markers can improve GFR estimation in the reference range.

GFR normalized to total body water allows comparisons across genders and body sizes

The normalization of GFR to a standardized body-surface area of 1.73 m(2) impedes comparison of GFR across individuals of different genders, heights, or weights. Ideally, GFR should be normalized to a parameter that best explains variation in GFR. Here, we measured true GFR by iohexol clearance in a representative sample of 1627 individuals from the general population who did not have diabetes, cardiovascular disease, or kidney disease. We also estimated total body water (TBW), extracellular fluid volume, lean body mass, liver volume, metabolic rate, and body-surface area. We compared two methods of normalizing GFR to these physiologic variables: (1) the conventional method of scaling GFR to each physiologic variable by simple division and (2) a method based on regression of the GFR on each variable. TBW explained a higher proportion of the variation in GFR than the other physiologic variables. GFR adjusted for TBW by the regression method exhibited less dependence on gender, height, and weight compared with the other physiologic variables. Thus, adjusting GFR for TBW by the regression method allows direct comparisons between individuals of different genders, weights, and heights. We propose that regression-based normalization of GFR to a standardized TBW of 40 L should replace the current practice of normalizing GFR to 1.73 m(2) of body-surface area.

Impaired fasting glucose is associated with renal hyperfiltration in the general population

OBJECTIVE

Increased glomerular filtration rate (GFR), also called hyperfiltration, is a proposed mechanism for renal injury in diabetes. The causes of hyperfiltration in individuals without diabetes are largely unknown, including the possible role of borderline hyperglycemia. We assessed whether impaired fasting glucose (IFG; 5.6-6.9 mmol/L), elevated HbA1c, or hyperinsulinemia are associated with hyperfiltration in the general middle-aged population.

RESEARCH DESIGN AND METHODS

A total of 1,560 individuals, aged 50-62 years without diabetes, were included in the Renal Iohexol Clearance Survey in Tromsø 6 (RENIS-T6). GFR was measured as single-sample plasma iohexol clearance. Hyperfiltration was defined as GFR>90th percentile, adjusted for sex, age, weight, height, and use of renin-angiotensin system inhibitors.

RESULTS

Participants with IFG had a multivariable-adjusted odds ratio of 1.56 (95% CI 1.07-2.25) for hyperfiltration compared with individuals with normal fasting glucose. Odds ratios (95% CI) of hyperfiltration calculated for a 1-unit increase in fasting plasma glucose (FPG) and HbA1c, after multivariable-adjustment, were 1.97 (1.36-2.85) and 2.23 (1.30-3.86). There was no association between fasting insulin levels and hyperfiltration. A nonlinear association between FPG and GFR was observed (df=3, P<0.0001). GFR increased with higher glucose levels, with a steeper slope beginning at FPG≥5.4 mmol/L.

CONCLUSIONS

Borderline hyperglycemia was associated with hyperfiltration, whereas hyperinsulinemia was not. Longitudinal studies are needed to investigate whether the hyperfiltration associated with IFG is a risk factor for renal injury in the general population.

Estimated GFR associates with cardiovascular risk factors independently of measured GFR

Estimation of the GFR (eGFR) using creatinine- or cystatin C-based equations is imperfect, especially when the true GFR is normal or near-normal. Modest reductions in eGFR from the normal range variably predict cardiovascular morbidity. If eGFR associates not only with measured GFR (mGFR) but also with cardiovascular risk factors, the effects of these non-GFR-related factors might bias the association between eGFR and outcome. To investigate these potential non-GFR-related associations between eGFR and cardiovascular risk factors, we measured GFR by iohexol clearance in a sample from the general population (age 50 to 62 years) without known cardiovascular disease, diabetes, or kidney disease. Even after adjustment for mGFR, eGFR associated with traditional cardiovascular risk factors in multiple regression analyses. More risk factors influenced cystatin C-based eGFR than creatinine-based eGFR, adjusted for mGFR, and some of the risk factors exhibited nonlinear effects in generalized additive models (P<0.05). These results suggest that eGFR, calculated using standard creatinine- or cystatin C-based equations, partially depends on factors other than the true GFR. Thus, estimates of cardiovascular risk associated with small changes in eGFR must be interpreted with caution.

Type 2 diabetic patients have increased gluconeogenic efficiency to substrate availability, but intact autoregulation of endogenous glucose production

OBJECTIVE

An autoregulatory mechanism involving a reciprocal relationship between gluconeogenesis and glycogenolysis regulates endogenous glucose production (EGP) in healthy individuals. In type 2 diabetes, fasting hyperglycemia may be due to increased EGP.

MATERIAL AND METHODS

To examine gluconeogenesis and autoregulation of EGP in type 2 diabetes, 9 type 2 diabetics and 8 healthy controls were studied during a 3-h infusion of 30 micromol/kg/min Na-lactate. The diabetics were also studied during a control infusion of Na-bicarbonate. To standardize levels of glucoregulatory hormones, plasma insulin, growth hormone, and glucagon were clamped at identical levels during the three experiments. Glucagon levels were elevated from basal levels to approximately 330 ng/l when the lactate or bicarbonate infusions were started, in order to mimic the hyperglucagonemia often seen in diabetes. Lactate gluconeogenesis and total EGP were measured by infusions of [6-(3)H] glucose and [U-14C] lactate.

RESULTS

In the bicarbonate experiments, hyperglugagonemia increased lactate gluconeogenesis in the diabetic patients from 4.3+/-1.8 to 6.1+/-2.4 micromol/kg/min (p=0.04). EGP did not change significantly (basal EGP: 15.3+/-3.9, EGP at the end of the study: 14.2+/-3.9 micromol/kg/min, p=0.14). During both lactate experiments, plasma lactate increased more than 4-fold. The increase in lactate gluconeogenesis was significantly higher in diabetics than in controls (values obtained at the end of experiments minus basal values: 10.8+/-3.6 versus 6.4+/-3.6 micromol/kg/min, p=0.03). Compared with normal subjects, the diabetic patients had higher EGP values both at basal conditions (p=0.001) and during lactate infusion (p=0.005). Despite augmented gluconeogenesis, EGP did not change during lactate and glucagon infusion in any of the groups (diabetics, basal EGP: 15.4+/-2.7 versus EGP at the end of experiments: 15.6+/-3.6 micromol/kg/min, p>0.30. Controls, basal EGP: 11.8+/-0.8 versus EGP at the end of experiments: 11.6+/-1.9 micromol/kg/min, p>0.30).

CONCLUSIONS

Although type 2 diabetics have increased EGP and increased lactate gluconeogenesis, the hepatic autoregulation of EGP during increased substrate-induced gluconeogenesis seems to be intact.

Changes in insulin sensitivity, renal function, and markers of endothelial dysfunction in hypertension--the impact of microalbuminuria: a 13-year follow-up study

Microalbuminuria (MA) clusters with the metabolic syndrome and insulin resistance and may reflect endothelial dysfunction. Microalbuminuria may also represent renal dysfunction. The aim of the present follow-up study was to assess changes over 13 years in insulin sensitivity, markers of endothelial dysfunction, and renal function in hypertensive subjects with and without MA in 1992-1993, matched for age, sex, and body mass index. Fourteen subjects with and 17 without MA at baseline (1992-1993) participated. At follow-up (2005-2006), MA status was unchanged in 75% of the subjects. The groups had comparable age, blood pressure, body mass index, markers of endothelial dysfunction, and metabolic traits, assessed by oral glucose tolerance test and hyperglycemic clamp. Estimated glomerular filtration rate decreased significantly in the MA group (P = .049) and tended to be lower in the MA than the non-MA group in 2005-2006 (79.9 +/- 24.5 vs 90.8 +/- 13.3 mL min(-1) (1.73 m(2))(-1), P = .2). Urinary albumin excretion in 1992-1993 predicted estimated glomerular filtration rate in 2005-2006 in adjusted analysis (beta = -0.47, P = .006). Estimated glomerular filtration rate less than 60 mL min(-1) (1.73 m(2))(-1) was more frequent in the MA than non-MA group at follow-up (P = .03). In conclusion, long-standing MA was not associated with progression of metabolic disturbances or markers of endothelial dysfunction in hypertensive individuals. A decline in renal function predicted by urinary albumin excretion was suggested. Microalbuminuria may not be a metabolic trait, but a marker mainly of renal endothelial dysfunction.

Predictors of change in estimated GFR: a population-based 7-year follow-up from the Tromso study

BACKGROUND

Chronic kidney disease is associated with increased cardiovascular mortality, and even mild impairment of renal function is a cardiovascular risk factor. Several studies have investigated the risk factors for the development of end-stage renal disease, but little is known about predictors of change in renal function in the general population.

METHODS

The present study included 2249 men and 2192 women without signs of kidney disease at baseline who were followed for 7 years from 1994 to 1995 in the Tromsø Study. Estimated glomerular filtration rate (eGFR) was calculated from the Modification of Diet in Renal Disease study equation. Gender-specific multiple linear regression analyses were used to assess predictors of change in eGFR (DeltaGFR).

RESULTS

Change in eGFR, measured in ml/min/1.73 m(2)/year, was associated with systolic blood pressure (SBP) [beta-value for a 10-mmHg increase in SBP, men = -0.14, 95% confidence interval (CI) = -0.18 to -0.09; women = -0.07, 95% CI = -0.11 to -0.03] and fibrinogen [beta-value for 1 SD increase in fibrinogen, men (1 SD: 0.85 g/L) = -0.12, 95% CI -0.20 to -0.03; women (1 SD: 0.80) = -0.11, 95% CI -0.20 to -0.02]. High alcohol consumption in men and high physical activity in women predicted an increase in eGFR. Higher albumin/creatinine ratio was associated with a decline in eGFR in men only.

CONCLUSIONS

Some risk factors for change in GFR seem to be gender specific but both high SBP and high levels of fibrinogen contribute to a more rapid decline in GFR for both men and women.

Proinsulin:insulin and insulin:glucose ratios as predictors of carotid plaque growth: a population-based 7 year follow-up of the Tromsø Study

AIMS/HYPOTHESIS

Proinsulin is increased in persons at cardiovascular risk. Increased secretion of proinsulin relative to insulin has been suggested as a sign of defective conversion of proinsulin to insulin and C-peptide and is associated with beta cell dysfunction. It has also been suggested that proinsulin has more of a pro-atherogenic effect than insulin, the levels of which are also increased in the insulin resistance state. In this prospective population-based study, we examined whether the proinsulin:insulin ratio (PIR) or insulin:glucose ratio (IGR, an insulin resistance surrogate) predicted carotid plaque size in nondiabetic participants.

MATERIALS AND METHODS

The study included 1,859 men and 1,998 women aged 25-82 years from the Tromsø Study, who were examined with B-mode high resolution ultrasound at baseline in 1994-1995 and at follow-up in 2001-2002. All images were computer processed to yield mm(2) measures of plaque. Proinsulin and insulin were measured at baseline. All analyses were stratified for sex.

RESULTS

After adjusting for age, baseline plaque area, BMI, cholesterol, HDL-cholesterol, HbA(1c), IGR, albumin:creatinine ratio, fibrinogen, BP and lifestyle factors (tobacco smoking, alcohol consumption, physical activity), PIR was significantly associated with plaque size at follow-up in women but not men. For each SD in the PIR in women, the mean plaque area increased by 0.97 mm(2) (95% CI 0.44-1.50). IGR was not associated with carotid plaque size.

CONCLUSIONS/INTERPRETATION

The PIR is associated with progressive carotid artery plaque size in women.

Metabolic risk factors associated with serum creatinine in a non-diabetic population

Insulin resistance, low HDL-cholesterol and microalbuminuria are important components of the metabolic syndrome as defined by WHO. Insulin resistance and low HDL-cholesterol are also common in chronic kidney disease (CKD), but it is not clear whether they are early or late phenomenons in the development of renal failure. This study examined whether low-grade albuminuria (microalbuminuria), lipoprotein fractions, and the insulin/glucose ratio (IGR)-a surrogate marker of insulin resistance-were related to renal function (expressed as serum creatinine) in persons without diabetes and with apparently normal renal function. The study included 4,131 men and women aged 55-75 years from the cross-sectional Tromsø IV survey (1994-1995). Lifestyle factors, waist circumference and blood pressure were included in the analyses. Gender stratified multivariate analysis was used to assess the relationship between serum creatinine and microalbuminuria, lipoprotein fractions and IGR. Serum creatinine was positively associated with microalbuminuria in men (beta = 2.50, 95% confidence interval (CI) 0.66-4.34), but not in women. HDL-cholesterol and IGR were strongly associated with creatinine in both genders (HDL-cholesterol: Men: beta = -4.82, 95% CI -6.27 to -3.37; women: beta = -2.12, 95% CI -3.28 to -0.96. IGR: Second, third and fourth quartile compared with first quartile, men: beta = 0.94, 95% CI -0.63 to 2.51; 2.10, 95% CI 0.52-3.69 and 2.40, 95% CI 0.75-4.04; women: beta = 1.91, 95% CI 0.59-3.22; 2.61, 95% CI 1.28-3.95 and 3.20, 95% CI 1.80-4.60). These findings suggest that even early impairment of renal function may be associated with insulin resistance and dyslipidemia, regardless of renal albumin leakage.

[Staging and measurement of renal function in chronic renal conditions]

Kidney failure has traditionally been categorised as mild, moderate or severe. Other poorly defined terms like uraemia and end-stage kidney disease have commonly been applied. During the last few years, an international consensus has emerged staging chronic kidney disease into 5 stages: stage 1 being glomerular filtration rate (GFR) > 90 ml/min and signs of kidney damage, stage 2 GFR 60-89 ml/min and signs of kidney damage, stage 3 being GFR 30-59 ml/min, stage 4 being GFR 15-29 ml/min, and stage 5 GFR < 15 ml/min. Kidney function should be calculated from creatinine-based formulas, or measured by isotope or iohexol clearances rather than as renal clearance of creatinine in timed urine samples. We address the importance and implications of these new definitions.

Quantitative measurement of muscle fiber composition in a normal population

To obtain normative muscle morphology data on a healthy population recruited from a population survey, we examined vastus lateralis biopsies from 58 men and 33 women, aged 26-67 years. Biopsies were measured with automated, computer-aided techniques. Data were analyzed according to gender and age, and the influence of blood pressure, body mass index (BMI), and smoking habits was also examined. Men had larger muscle fibers (fiber area approximately 5400 microm(2)) than women ( approximately 4000 microm(2), P = 0.003). No gender differences were seen in fiber composition, fiber roundness, percentage of connective tissue, or capillary density. Blood pressure did not influence fiber size or composition, but was correlated with fiber roundness in men. BMI was associated with fiber area in men, but not in women. Variations in age, smoking habits, and physical activity did not influence muscle morphology data substantially. Thus, in a normal population, men have larger muscle fibers than women, but similar fiber composition. Variation in gender, BMI, blood pressure, and physical activity may influence morphological features to a minor degree.

Autoregulation of endogenous glucose production during hyperglucagonemia

Increased endogenous glucose production (EGP) contributes to fasting hyperglycemia in type II diabetes. In nondiabetic subjects, increased gluconeogenesis from lactate does not increase EGP. Type 2 diabetes is associated with hyperglucagonemia. The present study was undertaken to examine whether physiologic elevation of plasma glucagon overrides autoregulation of EGP. Eight healthy volunteers were studied on 2 occasions, once during a 3-hour infusion of 30 micromol/kg/min Na-lactate and once during a control infusion of Na-bicarbonate. Plasma glucagon, insulin, and growth hormone were clamped at identical levels in both experiments. Rates of appearance of glucose, lactate, and gluconeogenesis from lactate were measured by tracer techniques. Glucagon infusion rate was elevated when the lactate or bicarbonate infusions were started to induce physiologic hyperglucagonemia. Plasma glucagon increased from baseline levels (234 +/- 21 ng/L and 211 +/- 23 ng/L) to 313 +/- 47 ng/L (bicarbonate experiments) and 329 +/- 43 ng/L (lactate experiments, means +/- SE, P >.3). Lactate infusion increased plasma lactate concentrations from 1.1 +/- 0.9 to 4.6 +/- 0.5 mmol/L (P =.0003). Lactate conversion to glucose increased from 1.5+/-0.3 to 2.8+/-0.8 micromol/kg/min (P =.03) and from 1.7 +/- 0.3 to 8.1 +/- 0.8 micromol/kg/min (P =.0003) in the bicarbonate and lactate experiments, respectively. The increments in lactate conversion to glucose differed significantly (P =.0008). Nevertheless, plasma glucose and EGP were not different in the bicarbonate and lactate experiments: 5.4 +/- 0.5 versus 6.6 +/- 0.7 mmol/L (P =.21), and 10.5 +/- 0.6 versus 11.6 +/- 0.6 micromol/kg/min (P =.19). We conclude that in normal volunteers, neither hyperglucagonemia nor the combination of hyperglucagonemia and increased substrate availability alters the autoregulation of EGP.

Microalbuminuria in hypertension is not a determinant of insulin resistance

BACKGROUND

Microalbuminuria (MA) clusters with metabolic derangements linked to the insulin resistance syndrome, and is associated with increased risk of cardiovascular disease in both diabetes and hypertension. This study questioned if MA, reflecting endothelial damage, is directly linked to impaired insulin action.

METHODS

MA was measured in two 24-hour urine samples in 84 persons with untreated hypertension recruited from a population survey (diastolic blood pressures 90 to 105 mm Hg). Thirty-one percent had MA values>20 microg/min (MA group, N = 26), and these were matched according to age, gender, and body-mass index with hypertensive persons without MA (non-MA group, N = 32) for comparison of the metabolic profile. Insulin sensitivity was measured with clamp techniques.

RESULTS

The MA and non-MA groups were similar in their fasting and post-load glucose and insulin levels, in the first (930 +/- 594 vs. 1097 +/- 707 pmol/L) and second (1111 +/- 662 vs. 1163 +/- 702 pmol/L) phases of insulin release during a hyperglycemic clamp, and in their insulin sensitivity indices (0.16 +/- 0.10 vs. 0.17 +/- 0.13, P> 0.3 for all). The MA group had higher systolic blood pressure (157 +/- 13 vs. 150 +/- 12 mm Hg, P = 0.05) and a higher serum level of circulating advanced glycation end products (AGEs; 11.0 +/- 3.0 vs. 7.9 +/- 3.5 U/mL, P = 0.05) than the controls. No associations were found between MA and the insulin sensitivity index, or glucose and insulin levels. Weak associations were found with systolic blood pressure (r = 0.25, P = 0.05), AGEs (r = 0.27, P = 0.05), and smoking habits (r = 0.39, P = 0.01).

CONCLUSION

In hypertension, MA is not a determinant of insulin resistance, provided confounding factors such as degree of adiposity are carefully controlled.

Effect of exercise training on skeletal muscle fibre characteristics in men with chronic heart failure. Correlation between skeletal muscle alterations, cytokines and exercise capacity

BACKGROUND

In patients with congestive heart failure (CHF) there is a shift from aerobic type I muscle fibres to less aerobic type II fibres. Exercise training has been shown to have beneficial effects on exercise performance, peripheral pathology and the neurohumoral profile in stable patients with CHF. This study evaluated the effect of a 3 month exercise training program on skeletal muscle characteristics and the correlation of these to cytokines and exercise capacity in CHF patients.

METHODS

Skeletal muscle biopsies for enzyme-histochemical analysis were performed in 15 CHF patients in New York Heart Association classes II-III, with a mean ejection fraction of 33+/-5% before and after a 12 week training period. The patients were trained for 30 min, five times a week at 80% of the peak heart rate achieved at baseline ergometer cycle test. Fifteen healthy men were used as controls. Plasma samples were examined by enzyme immunoassays for levels of pro-inflammatory cytokines.

RESULTS

(a) At baseline we found muscle atrophy in five of the patients. The percent area of type I fibres (40.7+/-12.0 vs. 56.4+/-11.0%, P<0.05) and the thickness of type IIA (56.10+/-7.8 vs. 71.6+/-11.9 microm, P<0.001) and B-fibres (49.0+/-8.9 vs. 63.9+/-10.6 microm, P<0.001) were reduced, whereas the percent area of type IIA fibres (52.1+/-13.3 vs. 36.4+/-9.9%, P<0.05) was increased in heart failure patients compared to healthy controls. There was a modest correlation between fibre thickness and the level of interleukin 6 (r=-0.657, P=0.008). (b) After exercise training there was a reduction in muscle area examined by light-microscopy, measured as a percentage of field (-2.7, P=0.003) with an concomitant increase in interstitium. This reduction correlated to the increase in the 6-min walk test (r=-0.558, P=0.031). The thickness of type IIB fibres increased (+5.6 microm, P=0.068) and the area of type I fibres decreased (-6.1%, P=0.062).

CONCLUSIONS

Patients with CHF have a relatively increased area of type IIA fibres and a relatively decreased area of type I fibres compared to healthy individuals. The thickness of type IIA and type IIB fibres is decreased compared to normal individuals. A modest negative correlation between the level of interleukin 6 and fibre thickness at baseline, suggests that inflammatory cytokines may be involved in the pathogenesis of the CHF related myopathy. A significant correlation between the reduction of muscle area, with increased interstitum, and the increase in the 6-min walk test may indicate that the improvement is due to increased capillary density permitting better flow reserve to exercising muscles.

Population-based study of the relationship among muscle morphology, insulin action, and hypertension

To examine whether changes in muscle morphology are linked to the metabolic abnormalities associated with the insulin resistance syndrome, muscle morphology and the metabolic profile were examined in 52 individuals with untreated hypertension (mean arterial pressure [MAP] = 117+/-7 mm Hg) and 38 carefully matched controls (MAP = 89+/-5 mm Hg). Oral glucose tolerance tests and hyperglycemic clamps were performed for measurements of insulin action on glucose disposal and suppression of nonesterified fatty acids (NEFA). Fully automated, computer-aided techniques were used for morphometric measurements of muscle biopsies from m. vastus lateralis. The hypertensive and normotensive groups did not differ in insulin sensitivity to glucose disposal (0.18+/-0.16 v. 0.19+/-0.13 micromol/kg/min/pmol/L; P = .20) and NEFA suppression (87.5+/-7.3 v. 87.2+/-9.4%, P>0.30) during a hyperglycemic clamp. The groups were similar in the proportion of types 1, 2a, and 2b muscle fibers, fiber size, and capillary density. Fiber roundness (ratio of fiber perimeter squared to fiber area) differed in the hypertensive (1.51+/-0.07) and normotensive (1.58+/-0.12, P = .004) groups, showing that the muscle fibers in the hypertensive group were more rounded in shape, a nonspecific change often seen after minimal ischemic lesions. The quotient expressing fiber roundness was associated with systolic (r = -0.29, P = .01) and diastolic (r = -0.32, P = .005) blood pressure. We conclude that persons with mild and moderate hypertension do not have abnormalities in muscle morphology that could explain the impairment of insulin action often observed in this condition. However, hypertensive individuals have increased muscle fiber roundness. It is wondered whether hypertension may be a condition with defects in the regulation of the transmembranous ion transport, leading to raised intracellular sodium concentration, swelling of the cytoplasma, and roundening of the fibers.

Insulin resistance in hypertension is associated with body fat rather than blood pressure

The insulin resistance syndrome has been characterized by hypertension, upper body obesity, insulin resistance, hyperinsulinemia, glucose intolerance, and hypertriglyceridemia. Previous studies are inconsistent regarding the relationship between blood pressure and insulin resistance. We therefore compared the metabolic profile in 60 hypertensive subjects (mean+/-SD arterial pressure, 116+/-7 mm Hg) and 60 normotensive subjects (mean arterial pressure, 88+/-5 mm Hg) matched for age, gender, and body mass index. Hypertensives had significantly higher waist-to-hip ratio than normotensives (P=0.002). The groups did not differ in fasting plasma glucose (0.2 mmol/L, P=0.09), insulin (6 pmol/L, P=0.14), insulin sensitivity index (-0.01 micromol x kg(-1) x min(-1) x pmol/L(-1), P=0.7), and suppression of nonesterified fatty acids during a hyperglycemic clamp (1%, P=0.40). There were significant differences in fasting levels of C-peptide (50 pmol/L, P=0.004) and proinsulin (2 pmol/L, P=0.01), 2-hour postload levels of glucose (0.8 mmol/L, P=0.01) and insulin (84 pmol/L, P=0.01) after oral glucose challenge, and hepatic glucose production during the clamp (2.87 micromol x kg(-1) x min(-1), P=0.02). These differences were not significant when controlling for waist-to-hip ratio. Body mass index and waist-to-hip ratio were similarly associated with the insulin sensitivity index in the hypertensive (r=-0.59, P=0.0001 and r=-0.32, P=0.05) and normotensive (r=-0.58, P=0.0001 and r=-0.39, P=0.05) groups. Hypertension per se is not associated with insulin resistance. However, even small increments in both body mass index and waist-to-hip ratio, as often seen in hypertension, may lead to impairment in insulin sensitivity, probably mediated through altered lipid metabolism.

Insulin kinetics, insulin action, and muscle morphology in lean or slightly overweight persons with impaired glucose tolerance

Glucose intolerance is influenced by body fat mass, as well as muscle fiber composition. To examine the relation between the metabolic profile and muscle morphology in this condition, we performed muscle biopsies and hyperglycemic clamps to determine insulin secretion and clearance, and the insulin effects on glucose disposal and nonesterified fatty acids (NEFA) in 45 glucose intolerant persons (body mass index [BMI], 27.8 +/- 3.0 kg/m2) and 45 normoglycemic controls (BMI, 25.8 +/- 2.7 kg/m2) (P = .001). After adjustment for BMI, glucose-intolerant subjects had lower first-phase insulin release (726 v 954 pmol/L, P = .04). Glucose-intolerant subjects and controls differed in fasting insulin, insulin clearance, and insulin sensitivity to glucose disposal before, but not after, standardizing for BMI. During the clamp, glucose-intolerant subjects had less NEFA suppression and elevated levels of NEFA compared with controls (85% +/- 9% v 90% +/- 6%, P = .02; and 70 +/- 42 micromol/L v 45 +/- 28 micromol/L, P = .01). Glucose-intolerant subjects also had a higher percentage of insulin-insensitive, type 2b muscle fibers, which are not adapted for fat oxidation (7% +/- 9% v 9% +/- 9%, P = .003). BMI was not associated with NEFA suppression or the percentage of type 2b muscle fibers in either group. In conclusion, glucose-intolerant persons have impaired first-phase insulin release, an elevated percentage of type 2b muscle fibers, and increased NEFA availability. Reduced insulin clearance, hyperinsulinemia, and insulin resistance were associated with small increments in BMI.

Fibrinolytic function after dietary supplementation with omega3 polyunsaturated fatty acids

Hypertension is associated with derangements in glucose and lipid metabolism. Increased levels of plasminogen activator inhibitor type 1 (PAI-1) are thought to potentiate the development of coronary events in this condition. Fish oil (omega3 polyunsaturated fatty acids [PUFAs]) have lipid-lowering effects, but the cardioprotective potential has been questioned because fish oil has been found to increase PAI-1 activity. This study was performed to determine the effects of omega3 PUFAs on the fibrinolytic function in hypertension. Seventy-eight persons with untreated hypertension were included in a 16-week, double-blind, randomized, controlled intervention study with 4 g/d of eicosapentaenoic and docosahexaenoic acids or corn oil placebo. Plasma PAI-1 activity, tissue plasminogen activator (tPA) activity, levels of fibrinogen and factor VII(c), and platelet count were measured before and after intervention (mean+/-SE). PAI-1 activity changed similarly in the fish oil and corn oil groups (1.8+/-1.0 U/mL versus 3.5+/-1.2 U/mL, P=.25), as did tPA (-0.02+/-0.02 IU/mL versus -0.13+/-0.03 IU/mL, P=.28), levels of factor VII(c) (6+/-5% versus 5+/-4%, P>.3), and platelet count (2+/-7x10(9)/L versus 3+/-5x10(9)/L, P>.3). None of these variables changed from pretreatment levels during fish oil intake. Fibrinogen levels increased significantly both during fish oil (0.6+/-0.1 g/L, P=.0001) and corn oil (0.4+/-0.1 g/L, P=.002) intake. There was no between-group difference (P>.3). In conclusion, a daily intake of 4 g omega3 PUFAs does not affect PAI-1 and tPA activity in persons with hypertension. A modest increase in fibrinogen levels was observed after both fish oil and corn oil intake.

Regulation of gluconeogenesis by glutamine in normal postabsorptive humans

There is evidence that glutamine may act as a regulator of protein, free fatty acid, and glycogen metabolism. To test the hypothesis that glutamine may act as a physiological regulator of gluconeogenesis, we infused 16 normal postabsorptive volunteers with glutamine at a rate (11.4 micromol kg(-1) x min(-1)) estimated to approximate its appearance in plasma after a protein meal and assessed changes in production of glucose from glutamine, systemic glucose appearance and disposal, and uptake and release of glucose, glutamine, and alanine by forearm skeletal muscle. Although infusion of glutamine increased plasma glutamine concentration and turnover only threefold (from 0.63 +/- 0.03 to 1.95 +/- 0.10 mmol/l and from 5.43 +/- 0.24 to 14.85 +/- 0.66 micromol x kg(-1) x min(-1), respectively; P < 0.001), formation of glucose from glutamine increased sevenfold from 0.55 +/- 0.03 to 3.74 +/- 0.28 micromol x kg(-1) x min(-1) (P < 0.001). Formation of glucose from alanine was also stimulated (0.52 +/- 0.05 vs. 0.75 +/- 0.04 micromol x kg(-1) x min(-1); P < 0.001) in the absence of a change in plasma alanine concentration. Furthermore, glutamine infusion decreased its own de novo synthesis (4.55 +/- 0.22 vs. 2.81 +/- 0.62 micromol x kg(-1) x min(-1);P < 0.02) while increasing that of alanine (2.82 +/- 0.32 vs. 3.56 +/- 0.32 micromol x kg(-1) x min(-1); P < 0.002). Systemic glucose appearance, systemic glucose disposal, and forearm balance of glucose and alanine were not altered. Because the stimulatory effects of glutamine on gluconeogenesis occurred in the absence of changes in plasma insulin and glucagon levels, these results provide evidence that, in humans, glutamine may act both as a substrate and as a regulator of gluconeogenesis as well as a modulator of its own metabolism.

Gender differences in the relationships between plasma plasminogen activator inhibitor-1 activity and factors linked to the insulin resistance syndrome in essential hypertension

Impaired fibrinolysis due to elevated levels of plasma plasminogen activator inhibitor type 1 (PAI-1) is a risk factor for thromboembolic disease. Hypertension, obesity, derangements in lipid and glucose homeostasis, and elevated levels of PAI-1 are features of the insulin resistance syndrome. The interrelationships between PAI-1 and the metabolic disturbances seen in this condition are unsettled. We investigated the associations between PAI-1 activity and components of the insulin resistance syndrome in 53 men and 31 women with untreated hypertension. In men, PAI-1 activity correlated significantly with plasma glucose (r = .41, P = .002), insulin sensitivity (r = -.35, P = .01), and insulin-induced suppression of nonesterified fatty acid (NEFA) (r = -.43, P = .007). Plasma glucose and NEFA suppression were independently associated with PAI-1 activity in a multivariate analysis. In women, PAI-1 activity correlated with body mass index (r = .62, P = .0005), waist-to-hip ratio (r = .75, P = .0001), plasma glucose (r = .50, P = .007), insulin (r = .49, P = .009), proinsulin (r = .57, P = .002), C-peptide (r = .60, P = .0009), insulin sensitivity (r = -.74, P = .0001), NEFA suppression (r = -.64, P = .003), and triglycerides (r = .58, P = .001). In multivariate analyses, insulin sensitivity and NEFA suppression were independently associated with PAI-1 if waist-to-hip ratio was not included in the model. After introduction of waist-to-hip ratio into the model, waist-to-hip ratio was the only independent predictor of PAI-1 activity. We conclude that in women, waist-to-hip ratio, body mass index, and insulin-induced NEFA suppression are determinants for PAI-1 activity. In men, insulin-induced NEFA suppression and plasma glucose are independently associated with PAI-1 activity.

Effects of n-3 polyunsaturated fatty acids on glucose homeostasis and blood pressure in essential hypertension. A randomized, controlled trial

OBJECTIVE

To determine whether dietary supplementation with fish oil adversely affects glycemic control in patients with hypertension.

DESIGN

Randomized, double-blind, placebo-controlled study.

PATIENTS

78 persons with untreated hypertension recruited from a population survey.

INTERVENTION

Participants were randomly assigned to receive eicosapentaenoic and docosahexaenoic acids, 4 g/d, or corn oil placebo, 4 g/d, for 16 weeks.

MEASUREMENTS

An oral glucose tolerance test; assessments of insulin release, glucose disposal, and insulin sensitivity done using the hyperglycemic clamp technique to keep plasma glucose levels at 10 mmol/L for 180 minutes; assessment of insulin sensitivity done using a euglycemic hyperinsulinemic clamp technique (infusing insulin and glucose to keep plasma glucose levels at 5 mmol/L); assessments of lipid levels and blood pressure. Measurements were done before and after intervention.

RESULTS

Changes in integrated glucose and insulin response after the oral glucose challenge did not differ between the fish oil and corn oil groups after intervention (-0.6 +/- 0.7 compared with -1.0 +/- 0.6 mmol/L [P > 0.3] for integrated glucose and 143 +/- 76 compared with 169 +/- 84 pmol/L [P > 0.3] for insulin response). Changes in first-phase insulin release (34 +/- 72 pmol/L in the fish oil group compared with 191 +/- 112 pmol/L in the corn oil group [P > 0.3]), second-phase insulin release (179 +/- 66 pmol/L compared with 257 +/- 122 pmol/L [P > 0.3]), and insulin sensitivity index (-0.03 +/- 0.01 compared with -0.01 +/- 0.01 [mumol/kg.min divided by pmol/L]; P > 0.3) were also similar in both groups after treatment. Fish oil lowered systolic blood pressure by 3.8 mm Hg more than control (P = 0.04) and lowered diastolic blood pressure by 2.0 mm Hg more than control (P = 0.10). After fish oil treatment, triglyceride levels decreased by 0.28 +/- 0.08 mmol/L more than control (P = 0.01), and very-low-density lipoprotein cholesterol levels decreased by 0.13 +/- 0.04 mmol/L more than control (P = 0.01).

CONCLUSION

Fish oil, in doses that reduce blood pressure and lipid levels in hypertensive persons, does not adversely affect glucose metabolism.

Glutamine: a major gluconeogenic precursor and vehicle for interorgan carbon transport in man

To compare glutamine and alanine as gluconeogenic precursors, we simultaneously measured their systemic turnovers, clearances, and incorporation into plasma glucose, their skeletal muscle uptake and release, and the proportion of their appearance in plasma directly due to their release from protein in postabsorptive normal volunteers. We infused the volunteers with [U-14C] glutamine, [3-13C] alanine, [2H5] phenylalanine, and [6-3H] glucose to isotopic steady state and used the forearm balance technique. We found that glutamine appearance in plasma exceeded that of alanine (5.76 +/- 0.26 vs. 4.40 +/- 0.33 mumol.kg-1.min-1, P < 0.001), while alanine clearance exceeded glutamine clearance (14.7 +/- 1.3 vs. 9.3 +/- 0.8 ml.kg-1.min-1, P < 0.001). Glutamine appearance in plasma directly due to its release from protein was more than double that of alanine (2.45 +/- 0.25 vs. 1.16 +/- 0.12 mumol.kg-1.min-1, P < 0.001). Although overall carbon transfer to glucose from glutamine and alanine was comparable (3.53 +/- 0.24 vs 3.47 +/- 0.32 atoms.kg-1.min-1), nearly twice as much glucose carbon came from protein derived glutamine than alanine (1.48 +/- 0.15 vs 0.88 +/- 0.09 atoms.kg-1.min-1, P < 0.01). Finally, forearm muscle released more glutamine than alanine (0.88 +/- 0.05 vs 0.48 +/- 0.05 mumol.100 ml-1.min-1, P < 0.01). We conclude that in postabsorptive humans glutamine is quantitatively more important than alanine for transporting protein-derived carbon through plasma and adding these carbons to the glucose pool.

Simultaneous assessment of insulin secretion and insulin sensitivity using a hyperglycemia clamp

A hyperglycemic clamp is an established method to assess insulin secretion and is generally used only for this purpose. To determine whether it could also be used to assess insulin sensitivity, we compared insulin sensitivity indices (ISI) obtained during euglycemic and hyperglycemic clamp experiments in 22 nonobese volunteers (body mass index, 23.9 +/- 0.6 kg/m2) and in 20 obese individuals (body mass index, 30.8 +/- 1.3 kg/m2) matched for age and gender. The ISI values (micromoles per kg.min/pmol) of the obese group assessed during hyperglycemic (0.088 +/- 0.011) and euglycemic (0.050 +/- 0.005) clamp experiments were both significantly lower than the ISI of the nonobese group assessed in hyperglycemic and euglycemic clamp experiments (0.179 +/- 0.024 and 0.096 +/- 0.009, respectively; both P less than 0.01). Although the ISI values obtained with hyperglycemic clamps were consistently greater than those obtained with euglycemic clamp (0.137 +/- 0.016 vs. 0.075 +/- 0.007; P less than 0.001), they were highly correlated (r = 0.84; P less than 0.0001). Moreover, when these indices were converted to clearance rates, thereby correcting for the mass action effects of glucose on glucose disposal, the values obtained with the hyperglycemic clamp (0.0137 +/- 0.0016 mL/kg.min/pmol) were statistically identical to those obtained with the euglycemic clamp (0.0142 +/- 0.0013 mL/kg.min/pmol), as indicated by a regression equation having an intercept of 0 and a slope (1.03) not different from 1. We, therefore, conclude that the hyperglycemic clamp and the euglycemic clamp yield comparable estimates of insulin sensitivity and that, under appropriate conditions, the hyperglycemic clamp technique may be used to assess both insulin sensitivity and insulin secretion in the same individual in a single experiment.