The obstructive sleep apnea syndrome is typically associated with conditions known to increase insulin resistance as hypertension, obesity, and diabetes. We investigated whether obstructive sleep apnea itself is an independent risk factor for increased insulin resistance and whether continuous positive airway pressure (CPAP) treatment improves insulin sensitivity. Forty patients (apnea–hypopnea index > 20) were treated with CPAP. Before, 2 days after, and after 3 months of effective CPAP treatment, hyperinsulinemic euglycemic clamp studies were performed. Insulin sensitivity significantly increased after 2 days (5.75 ± 4.20 baseline versus 6.79 ± 4.91 μmol/kg · min; p = 0.003) and remained stable after 3 months of treatment. The improvement in insulin sensitivity after 2 days was much greater in patients with a body mass index less than 30 kg/m2 than in more obese patients. The improved insulin sensitivity after 2 nights of treatment may reflect a decreasing sympathetic activity, indicating that sleep apnea is an independent risk factor for increased insulin resistance. The effect of CPAP on insulin sensitivity is smaller in obese patients than in nonobese patients, suggesting that in obese individuals insulin sensitivity is mainly determined by obesity and, to a smaller extent, by sleep apnea.
Obesity has a high prevalence in many societies and is closely related to the obstructive sleep apnea syndrome (OSAS) (1) as well as to hypertension (2), diabetes mellitus (3–5), and other features of the metabolic syndrome (6). Previous studies found a high incidence of OSAS within the general population (2–4% of adult men and 1–2% of adult women) (7, 8). Patients with OSAS have a higher incidence of morbidity and mortality due to cardiovascular and cerebrovascular disease, even in cases of mild obstructive sleep apnea (9). The metabolic syndrome is characterized by increased insulin resistance. Some authors speculate that in patients with OSAS the increased insulin resistance is partially mediated by an increased sympathetic activity caused by frequent nocturnal microarousals and nocturnal hypoxemia (10–14).
The standard treatment for OSAS is nasal continuous positive airway pressure (CPAP). This treatment rapidly improves the patients condition by reducing daytime sleepiness and improving vigilance. Recently, a significant reduction of mean arterial ambulatory blood pressure by effective CPAP treatment has been demonstrated (15), and CPAP is known to reduce sympathetic drive in OSAS (10). However, positive metabolic effects of CPAP treatment (e.g., improving insulin sensitivity) are still under discussion (16–18). Increased insulin resistance seems to play a key role among the mechanisms responsible for the metabolic effects of OSAS (19). The presence of increased insulin resistance has repeatedly been reported in patients with OSAS (16–18, 20–24). However, insulin resistance is not uncommon in the general population and is also associated with obesity, immobility, use of different drugs, and a variety of other conditions also frequently present in OSAS (16–25). Investigations addressing the question of whether OSAS itself is an independent risk factor of insulin resistance in these patients have led to conflicting results (16–18, 26, 27).
One study showed a tendency to improved insulin sensitivity after 4 months of CPAP treatment measured by hyperinsulinemic euglycemic clamp (16), but two other investigations (measurements after 2 and 3 months) could not confirm this result (17, 18). One reason for these conflicting findings could be the lack of statistical power due to low patient numbers. Thus, to date it is not clear whether the successful treatment of OSAS or the concomitant weight reduction and changes in lifestyle were the key factors in decreasing insulin resistance in nondiabetic patients with OSAS (20). Some authors speculate that insulin resistance in OSAS is mainly induced by increased sympathetic drive. Since the elevated sympathetic drive is mainly mediated via adrenal hormones with their short half–lives (28, 29), a significant reduction of insulin resistance should be achieved by CPAP therapy within a short period of time without any changes in body weight or lifestyle. To determine whether such rapid effects occur, using hyperinsulinemic euglycemic clamp technique, we studied patients without diabetes with moderate to severe OSAS before and 2 days after initiation of CPAP treatment (30). In addition, we investigated the insulin sensitivity 3 months after onset of CPAP treatment to determine if the effect of CPAP treatment on insulin sensitivity is maintained for a longer period of time.
Leptin is an adipocyte-derived hormone with important functions in appetite behavior and energy homeostasis. It decreases during CPAP treatment in patients with OSAS even without concomitant weight loss (24, 31). In animal models, respiratory effects of this hormone have been demonstrated clearly (32). The implications of the decrease of serum leptin in humans treated with CPAP are not that clear. Adipose tissue, the main source of leptin, is an important factor determining insulin resistance. We measured serum leptin levels at the same time points at which insulin sensitivity was measured to investigate whether changes in insulin resistance are paralleled by changes in leptin levels. Some of the preliminary results of this study have been previously reported as an abstract (33).
Forty patients (34 males, 6 females) with OSAS, mean age 53.81 ± 11.84 years, participated in the study. Mean body mass index (BMI) was 32.76 ± 6.92 kg/m2. Patients were free from severe accompanying diseases. Hyperlipidemia was present in 20 patients and hypertension was present in 24 patients. Five patients had an impaired fasting glucose. These diagnoses were made according to standard criteria (34–36). No patient had diabetes mellitus. Antihypertensives influencing insulin sensitivity were withdrawn 1 week before the clamp studies (n = 4) (25, 37). All patients had OSAS, with a mean apnea–hypopnea index (AHI) of 43.10 ± 11.38, and complained about impaired performance and daytime sleepiness (Epworth sleepiness scale: 12.9 ± 3.6; range 9–23) (38).
The 40 patients underwent a first night with diagnostic polysomnography as previously described (39, 40), a routine blood examination, and a measurement of serum leptin. Bioelectrical impedance measurements were performed and insulin responsiveness was measured by a hyperinsulinemic euglycemic clamp at 7:00 a.m. The following night, CPAP treatment was initiated according to a standard CPAP-titration protocol (41) and on the third night, CPAP was performed with the previously established minimal effective CPAP pressure followed by a second clamp the next morning. In the remaining 31 patients, the clamp studies were repeated after approximately 3 months (93.97 ± 25.22 days) and a further night with polysomnography with effective CPAP treatment. Nine of the forty patients had removed or discontinued CPAP therapy due to discomfort. The same laboratory parameter measurements were repeated as after the first night as was bioelectrical impedance analysis. The medical treatment for all patients remained unchanged throughout the study.
The study protocol was approved by the Ethics Committee of the Friedrich-Alexander-University, Erlangen-Nuremberg. All patients gave written informed consent to participate in this study.
Insulin responsiveness was measured by a hyperinsulinemic euglycemic clamp after a 10-hour overnight fasting period as described by de Fronzo and coworkers (30). Insulin (Actrapid U 40 HM; Novo Nordisk, Bagsvaerd, Denmark) was administered at a rate of 1 mU/kg · minute. Serum glucose levels were measured every 10 minutes. The insulin sensitivity index (ISI) was calculated from the insulin measurements and the corresponding glucose infusion rates during that period (given as μmol/kg · min).
Polysomnography was performed according to the recommendations of the American Thoracic Society (39) and the German Sleep Society (40, 41). Sleep staging was performed using the criteria of Rechtschaffen and Kales (42), and microarousals were defined in accordance with the definitions of the American Sleep Disorders Association (43). Variables extracted were the following: AHI, arousal index, oxygen desaturation index (number of oxygen desaturations ⩾ 4%), mean minimal arterial oxygen saturation, as described by Juhasz and coworkers (44).
Descriptive analysis was performed using mean ± SD (parametric data). Relationships between two continuous variables were analyzed using scatter plots and Spearmans ρ correlation coefficients. Differences between ISI before and after treatment were evaluated using the Wilcoxon test for paired samples. A logistic regression analysis was performed to evaluate the influence of a baseline BMI of less than 30 kg/m2 on the outcome “change in ISI greater than 0.58 after 2 days” (dichotomization according to sample median) while controlling for age, arterial hypertension, and “baseline AHI less thean 39” (dichotomization according to sample median). Two-sided p values of 0.05 or less were considered significant. All computations were performed with SPSS (Version 11.0; SPSS Inc., Chicago, IL).
CPAP treatment could initially be effectively established in all 40 patients. AHI and daytime sleepiness were normalized. There were no significant changes of body weight or body fat mass during the 3 months of CPAP treatment. Further characteristics of the patients during the course of treatment are given in Table 1
Before CPAP Treatment
After 2 days CPAP Treatment
After 3 Months CPAP Treatment
|No. of patients||40||40||31|
|BMI (kg/m2)||32.76 ± 6.92||ND||32.33 ± 6.69|
|Percent Body Fat||23.10 ± 9.89||ND||21.29 ± 8.72|
|AHI||43.10 ± 11.38||5.50 ± 4.39||4.43 ± 3.40|
|ARI||37.81 ± 2.12||5.09 ± 3.41||5.43 ± 3.74|
|ODI||40.12 ± 16.41||5.12 ± 4.79||4.82 ± 4.13|
|MMAO2 (%)||88.72 ± 4.16||92.98 ± 0.93||91.31 ± 0.84|
|ESS||12.90 ± 3.60||ND||6.90 ± 4.00|
|Leptin (μg/l)||20.56 ± 17.04||20.63 ± 18.67||10.19 ± 10.90|
ISI was significantly improved after 2 days of CPAP treatment in the 40 patients (p = 0.003). In the 31 patients followed-up after 3 months, there was no further significant improvement of ISI, but the difference from baseline remained statistically significant (p = 0.001; Figure 1). Similar results could be found in a subgroup with BMI less than 30 kg/m2 (normal weight and overweight grade I, per World Health Organization criteria, n = 16). In patients with a BMI of 30 kg/m2 or more (overweight grade II and III, per World Health Organization criteria, n = 24), no significant improvement of ISI could be demonstrated after 2 days of CPAP (Table 2)
ISI (Whole Group, n = 40) (After 3 Months n = 31) (μmol/kg · min)
ISI (BMI < 30 kg/m2 Group, n = 16) (After 3 Months n = 13) (μmol/kg · min)
ISI (BMI > 30 kg/m2 Group, n = 24) (After 3 Months n = 18) (μmol/kg · min)
|Baseline||5.75 ± 4.20||8.53 ± 4.48||3.89 ± 2.80|
|After 2 days CPAP therapy||6.79 ± 4.91||10.47 ± 5.09||4.33 ± 2.87|
|Improvement compared to baseline||p = 0.003||p = 0.001||p = 0.13|
|After 3 months CPAP therapy*||7.54 ± 4.84||10.71 ± 4.95||5.26 ± 3.50|
|Improvement compared to baseline||p = 0.001||p = 0.001||p = 0.03|
Because body weight seems to influence the effect of CPAP on insulin responsiveness, a logistic regression analysis of the BMI on the changes between ISI before and after 2 days after CPAP treatment was performed (Table 3)
OR (95% CI)
|BMI < 30 kg/m2||2.19||0.02||8.89 (1.51–52.22)|
|Presence of hypertension||0.73||0.39||2.07 (0.39–10.91)|
|Baseline AHI < 39||1.36||0.09||3.91 (0.81–18.90)|
CPAP adherence could be a further factor influencing the changes in ISI. Nine patients discontinued CPAP treatment subsequently, but 31 patients showed good compliance during the first 3 months of treatment (the built-in data stores of the CPAP devices were read out, the number of days of use within the past 42-day period established, and the mean duration of use per treatment night calculated). CPAP devices were used on 38.1 ± 6.4 nights (range: 23–41). The mean duration of use per night was 5.2 ± 0.91 h (range: 2.3–7.8). The possible association between mean duration of CPAP use per night and change in insulin sensitivity from baseline to 3 months was evaluated by scatterplot and by calculation of the Spearman correlation coefficient, and no significant association could be demonstrated (r = 0.21, p = 0.27).
Parameters of the severity of OSAS were studied to investigate a possible influence on and changes in ISI. We found only minor to moderate correlations between the insulin sensitivity before treatment and the parameters AHI (r = −0.30, p = 0.06), arousal index (r = −0.30, p = 0.06), oxygen desaturation index (r = −0.35, p = 0.03) and mean minimal arterial oxygen saturation (r = 0.12, p = 0.54) and between the difference in insulin sensitivity at baseline and after 2 days of treatment and the parameters AHI (r = −0.32, p = 0.047), arousal index (r = −0.11, p = 0.50), oxygen desaturation index (r = −0.31, p = 0.05) and mean minimal arterial oxygen saturation (r = 0.71, p < 0.001). Furthermore, in an additional exploratory logistic regression analysis, no significant independent influence (p = 0.08) of the variable “baseline AHI less than 39” on the variable “baseline ISI less than 3.94” was found when controlling for the interfering variables “percent body fat less than 21.5”, “BMI less than 30”, or hypertension. Due to the limited case number, inclusion of the variable age was not feasible in this additional analysis.
Serum leptin was studied before and during the course of CPAP treatment. Serum leptin and the ISI were well correlated before CPAP treatment, at 2 days, and at 3 months of CPAP treatment (r = −0.40, r = −0.51, r = −0.61; p = 0.01 in all cases). We found no significant changes between baseline fasting leptin levels after 2 days of CPAP treatment (Table 1). Serum fasting leptin levels were significantly lower at 3 months of CPAP treatment when compared with baseline levels (p < 0.05). Serum fasting leptin levels at baseline showed a strong correlation with the BMI (r = 0.71), as well as the percentage of body fat (r = 0.57). The correlation with AHI was low (r = 0.34).
There is a growing body of literature suggesting that sleep-disordered breathing may be a causal factor for glucose intolerance and insulin resistance (26, 27). Thus, the goal of our study was to investigate whether OSAS itself is an independent risk factor for increased insulin resistance and whether effective CPAP treatment improves insulin sensitivity. Previous studies have already investigated the effect of CPAP treatment on insulin sensitivity in patients with OSAS by euglycemic clamp studies, the standard method used since 1979 to measure insulin sensitivity (30). Those studies had conflicting results. Whereas the study of Brooks and coworkers (16), which employed hyperinsulinemic euglycemic clamp in severely obese and diabetic patients, showed a tendency towards improved insulin sensitivity after 4 months of CPAP treatment, two other investigations (measurements after 2 and 3 months of treatment) could not confirm this result in patients with OSAS who did not have diabetes (17, 18). A major problem of those investigations was the small patient number (always less than 10 patients), which may not offer enough statistical power to detect significant associations between insulin sensitivity and indices of sleep-disordered breathing. Furthermore, if the reassessment of insulin sensitivity is done 2–4 months after onset of CPAP treatment, other factors influencing insulin sensitivity (e.g., weight, body fat distribution, treatment of concomitant diseases, changes in dietary behavior, smoking, alcohol consumption, and physical activity) also may be of considerable importance. Changes in body weight or fat distribution must especially be taken into consideration as important factors influencing insulin sensitivity when interpreting these previous studies (12). In these studies, the BMI did not change significantly, which is in accordance with our observations, but the percentage of body fat was not measured in those earlier investigations. However, in recent studies, a growing body of data suggests an independent association between OSAS and insulin resistance independent of body weight. Using the homeostasis assessment method to determine insulin sensitivity, and applying stepwise multiple linear regression analysis, Ip and colleagues (26) were able to demonstrate that obesity was the major determinant of insulin resistance in a group of 270 patients with OSAS, but that parameters of sleep-disordered breathing (AHI and minimal oxygen saturation) were also independent determinants of insulin resistance. Punjabi and colleagues (27) also demonstrated an association between an increasing AHI and insulin resistance independent of obesity in mildly obese men.
In our study, we wanted to investigate direct effects of OSAS on insulin resistance with hyperinsulinemic euglycemic clamp studies performed soon after the onset of CPAP treatment to remove confounding variables, such as fluctuations in body fat or body composition. Assuming that increased nocturnal sympathetic drive is one important mechanism leading to increased insulin resistance in OSAS, and considering the short half-lives of adrenal hormones mainly mediating the sympathetic drive (28, 29), this effect should be quickly reversible after effective treatment of OSAS. It has to be pointed out that the hypothesis of an elevated sympathetic drive is only one possible mechanism; alterations of the hypothalamic-pituitary-adrenal function due to sleep disruption and/or hypoxemia also have to be taken into consideration. Such alterations should also be reversible soon. To summarize our results, we could clearly demonstrate an improvement in insulin sensitivity as early as 2 days after onset of effective CPAP therapy in most of our patients.
In our study, we did not perform computed tomography to differentiate between visceral fat accumulation and subcutaneous fat. Changes in body fat distribution may have an impact on insulin sensitivity, but significant changes of the body fat distribution within 2 days can be excluded. However, after 3 months of CPAP treatment, changes in body fat distribution might have taken place. The difference in results compared with those of Saarelainen and colleagues (17) and Smurra and colleagues (18) can be explained by a lack of statistical power in these studies. With this in mind, changes in lifestyle that are difficult to record completely (e.g., physical activity, diet, nicotine and alcohol consumption) will also influence the statistical calculations more in small samples.
In our attempt to identify other factors influencing the change in insulin sensitivity, we investigated the BMI of our patients as well as the glucose metabolism (normal fasting glucose versus impaired fasting glucose, according to the American Diabetes Association criteria). Fasting glucose at baseline was investigated and five patients had an impaired fasting glucose. Unfortunately, due to this low sample size, there was insufficient statistical power to detect an effect of the elevated fasting glucose on the changes in insulin sensitivity (Figure 2). We also analyzed the impact of the severity of OSAS on insulin sensitivity, as there is experimental evidence suggesting that the exposure to hypoxia can induce insulin resistance (45, 46). Furthermore, an independent inverse association between AHI and insulin resistance, independent of obesity, has recently been reported (26, 27). In our study, we were unable to demonstrate a substantial association between parameters indicating the severity of OSAS with the degree of initial insulin resistance or the degree of improvement of insulin sensitivity 2 days after onset of CPAP treatment. It is unlikely that the different methods used for the measurement of insulin sensitivity (homeostasis assessment method versus euglycemic hyperinsulinemic clamp method) could explain this discrepancy. It is likely that our study had insufficient power to demonstrate such effects. However, it should be pointed out that AHI cannot be considered a parameter that strongly reflects the severity of all aspects of OSAS, such as loss of vigilance, daytime activity, and cardiovascular effects, and may also be a poor predictor of metabolic effects in OSAS.
In our study, we identified obesity as the main predictor for an improvement in insulin sensitivity during CPAP. The leaner patient group had a more rapid improvement in insulin sensitivity, but the effect could also be demonstrated at a later stage of treatment in the more obese group. At all time points, insulin sensitivity was significantly greater in the lean group.
In an attempt to characterize further important factors associated with metabolic effects in OSAS, we measured serum levels of leptin, the product of the ob-gene. Leptin is produced within the adipose tissue, which is known to be a main determinant of insulin resistance. We observed close correlations between serum leptin and the insulin sensitivity index before as well as 2 days and 3 months after the initiation of CPAP treatment. These data suggest a linkage between insulin resistance and leptin, perhaps via the common main determinant, the adipose tissue, and do less to support concepts of leptin as a respiratory stimulus, as demonstrated in animal models (32). The decrease in serum leptin after 3 months of treatment may be related to factors other than sleep-disordered breathing as another distribution of adipose tissue or changes in dietary habits.
In our study, we were able to demonstrate that effective CPAP treatment rapidly improves insulin sensitivity in patients with OSAS. Nevertheless, our study also shows that obesity is a more important determinant of insulin resistance than OSAS. The less obese the patients are, the greater is the improvement in insulin sensitivity brought about by CPAP treatment. Our data are in agreement with the recent study by Ip and colleagues (26). A controversial question still discussed is whether insulin resistance or hyperinsulinemia are independent cardiovascular risk factors. The question is controversial (47–50) because insulin resistance is typically accompanied by clinical conditions, such as obesity, diabetes mellitus, or hypertension, per se putting the patient at an elevated cardiovascular risk. In the insulin resistance syndrome, the main pathophysiology is probably the dissociation between intermediate metabolic effects of insulin and its growth-promoting effects. However, recent studies, such as the Quebec Cardiovascular Study (47), have provided data that suggest that insulin resistance could be an independent risk factor for cardiovascular disease, with plasma insulin levels predicting a 1.6-fold higher odds ratio for coronary heart disease with every increase of one standard deviation in insulin concentration. Data from the United Kingdom Prospective Diabetes Study (50) indicated no relationship between (exogenous) insulin treatment and cardiovascular disease. In a more recent article by Resnick and colleagues (51), insulin resistance was found to predict diabetes mellitus type 2, but not cardiovascular disease, in American Indians. Rutter and colleagues (52) found insulin resistance associated with increased left ventricular mass, a premier risk factor for cardiovascular disease events in women but not in men. Data from the prospective Insulin Resistance Atherosclerosis Study (53) supported the concept of a protective association between greater insulin sensitivity and a reduced risk for the development of hypertension and cardiovascular disease. The authors claim that these findings support the conclusion that interventions improving insulin sensitivity may be beneficial in reducing the atherogenic risk. There is evidence that CPAP treatment can lower blood pressure (16). Furthermore, in medically treated patients with heart failure, CPAP treatment reduces systolic blood pressure and improves left ventricular systolic function (54).
Together with an improvement of blood pressure and heart function due to CPAP therapy, the improvement in insulin sensitivity, clearly demonstrated in our study, might be an important factor contributing to a reduction of cardiovascular risk in patients with OSAS treated with CPAP.
The authors are grateful to Professor Dr. Thomas Pieber and his team (L.K.H., University of Graz, Austria) for introduction and training in hyperinsulinemic euglycemic clamping and to Dr. Patrick Michaeli for his help with the graphic presentation.
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