a total of 33 patients undergoing pancreatic surgery for chronic pancreatitis , benign pancreatic adenomas , or papillary tumors necessitating partial pancreatectomy were studied preoperatively with a 240-min oral glucose challenge . different measures of glucose control and insulin secretion were determined and compared with the fractional -cell area of the partially resected pancreas to establish potential predictors of -cell area in humans . the study protocol was approved by the ethics committee of the ruhr - university bochum ( registration no . we included 33 patients ( 17 male , 16 female ) undergoing pancreatic resections in the department of surgery , st . josef - hospital , ruhr - university bochum , between the years 2004 and 2007 . of these patients , 17 had been diagnosed with chronic pancreatitis , 13 underwent surgery for the removal of benign pancreatic adenomas , and 3 underwent partial pancreatectomy because of tumors of the ampulla of vater . the clinical diagnoses of chronic pancreatitis , pancreatic carcinoma , pancreatic adenoma , or ampullary cancer were confirmed by an independent pathologist in all cases . in patients with chronic pancreatitis , surgery was performed if conservative treatment approaches had failed to provide sufficient analgesia . in 9 patients distal pancreatectomies ( pancreas tail resection ) were performed , whereas 24 patients were treated with a proximal pancreatectomy ( pancreas head resection ) . the latter group comprised 17 patients undergoing pancreaticoduodenectomy with pylorus preservation , 4 patients undergoing duodenum - preserving pancreatic head resections according to beger , and 3 patients undergoing classic partial pancreaticoduodenectomy ( whipple 's operation ) . diabetes was previously known in six patients ( treated with insulin in four patients , glimepiride in one , and diet in one ) , whereas the other patients had no history of known diabetes . renal function was normal ( serum creatinine < 1.2 mg / dl ) in 27 patients , whereas 6 patients had mild impairments in renal function ( serum creatinine 1.22.0 mg / dl ) . there also were no differences in the concentrations of creatinine and urea between the groups with normal glucose tolerance ( ngt ) , impaired glucose tolerance ( igt)/impaired fasting glucose ( ifg ) , and diabetes ( table 1 ) . clinical characteristics of patients with ngt , igt , and/or ifg and patients with diabetes data are means  sd , n , or n ( % ) . statistical analysis used anova or  test . * significantly different vs. control subjects ( duncan 's post hoc test ) . the experiments were performed in the morning after an overnight fast with subjects in a supine position throughout the experiments . all antidiabetic treatment was withheld at least 24 h before the experiments . in insulin - treated patients , the last injection of short - acting insulin was performed on the evening before the tests , whereas all long - acting insulins were withheld for at least 24 h to avoid carryover effects . no restrictions were made regarding the intake of water until the morning of the experiments . both ear lobes were made hyperemic using finalgon ( 4 mg / g nonivamid , 25 mg / g nicoboxil ) . the experiments were started by the ingestion of the oral glucose load ( 75 g glucose in 300 ml ) over 5 min , and capillary and venous blood samples were drawn at t = 5 , 0 , 15 , 30 , 60 , 90 , 120 , 150 , 180 , 210 , and 240 min . capillary blood samples ( 100 l ) were added to naf ( microvette cb 300 ; sarstedt , nmbrecht , germany ) for the immediate measurement of glucose . venous blood was drawn into chilled tubes containing edta and aprotinin ( 20,000 kiu / ml trasylol , 200 l per 10 ml blood ; bayer , leverkusen , germany ) and kept on ice . glucose was measured as previously described ( 16 ) using a glucose oxidase method with a glucose analyzer 2 ( beckman instruments , munich ) . to adjust for the glucose concentration differences between capillary plasma and whole blood , insulin was measured as previously described ( 16 ) , using an insulin microparticle enzyme immunoassay ( imx insulin ; abbott laboratories , wiesbaden , germany ) . c - peptide was measured as previously described ( 16 ) , using an enzyme - linked immunosorbent assay from dakop ( cambrigshire , u.k . ) . pancreatic resections were fixed in formaldehyde and embedded in paraffin for subsequent analysis as previously described ( 12 ) . sequential 5-m sections were stained for insulin using a guinea pig anti - insulin antibody ( no . a 0564 , lot no . 00001500 ; dako ) at 1:400 dilution and an alkaline phosphatase / red detection system ( real envision detection system , no . 00025382 and 00025812 , respectively ; dako ) . for determination of the fractional -cell areas , entire pancreatic sections stained for insulin were imaged at 100 magnification ( 10 objective ) using a zeiss axioplan microscope equipped with a motorized stage . a tile image of the tissue section was generated using the mosaix tool of zeiss axiovision version 4.5 software . the fractional areas of the pancreas stained positive for insulin were digitally quantified using a color - based threshold using axiovision software as previously described ( 12 ) . insulin - to - glucose ratios and c - peptide  to  glucose ratios were calculated at all time points before and after oral glucose ingestion . subject characteristics are reported as the means  sd , and results are presented as the means  se . to address whether pancreatic -cell area predicts glycemic control , the fractional -cell area was first correlated to the fasting and 120-min postchallenge glucose concentrations and the a1c levels because these parameters have previously been validated as markers of glucose control . to address whether pancreatic -cell area can be predicted by established indexes of -cell function , the following parameters were correlated with -cell area : 1 ) the homa -cell function index , 2 ) the fasting insulin - to - glucose ratio , 3 ) the fasting c - peptide  to  glucose ratio , 4 ) the insulin - to - glucose ratio 30 min after oral glucose ingestion , 5 ) the c - peptide  to  glucose ratio 30 min after oral glucose ingestion , 6 ) the plasma insulin levels 30 min after oral glucose ingestion , and 7 ) the plasma c - peptide levels 30 min after oral glucose ingestion . these parameters were chosen based on previous studies ( 15,1821 ) . finally , to identify the strongest predictor of -cell area , the plasma concentrations of glucose , insulin , and c - peptide as well as the c - peptide  to  glucose ratios and insulin - to - glucose ratios at all time points during the prolonged ogtt were correlated with pancreatic -cell area . the parameter yielding the greatest r value for the correlation with -cell area was considered to be the best predictor of -cell area . by these means , time course measurements were carried out by unpaired anova , using statistica version 5.0 ( statsoft europe , hamburg , germany ) . by these means , three different p values were calculated 1 ) for the determination of overall differences between the different groups ( i.e. , ngt , ifg / igt , and diabetes ) , independent of the respective time patterns ; 2 ) for the determination of differences over the time course , independent of the respective groups ; and 3 ) for the determination of differences between the groups over the time course . if a significant ( p < 0.05 ) interaction between group and time was documented , values at single time points were compared by one - way anova . correlation analyses were carried out using graphpad prism 4 using linear or nonlinear regression functions . we included 33 patients ( 17 male , 16 female ) undergoing pancreatic resections in the department of surgery , st . josef - hospital , ruhr - university bochum , between the years 2004 and 2007 . of these patients , 17 had been diagnosed with chronic pancreatitis , 13 underwent surgery for the removal of benign pancreatic adenomas , and 3 underwent partial pancreatectomy because of tumors of the ampulla of vater . the clinical diagnoses of chronic pancreatitis , pancreatic carcinoma , pancreatic adenoma , or ampullary cancer were confirmed by an independent pathologist in all cases . in patients with chronic pancreatitis , surgery was performed if conservative treatment approaches had failed to provide sufficient analgesia . in 9 patients distal pancreatectomies ( pancreas tail resection ) were performed , whereas 24 patients were treated with a proximal pancreatectomy ( pancreas head resection ) . the latter group comprised 17 patients undergoing pancreaticoduodenectomy with pylorus preservation , 4 patients undergoing duodenum - preserving pancreatic head resections according to beger , and 3 patients undergoing classic partial pancreaticoduodenectomy ( whipple 's operation ) . diabetes was previously known in six patients ( treated with insulin in four patients , glimepiride in one , and diet in one ) , whereas the other patients had no history of known diabetes . renal function was normal ( serum creatinine < 1.2 mg / dl ) in 27 patients , whereas 6 patients had mild impairments in renal function ( serum creatinine 1.22.0 mg / dl ) . there also were no differences in the concentrations of creatinine and urea between the groups with normal glucose tolerance ( ngt ) , impaired glucose tolerance ( igt)/impaired fasting glucose ( ifg ) , and diabetes ( table 1 ) . clinical characteristics of patients with ngt , igt , and/or ifg and patients with diabetes data are means  sd , n , or n ( % ) . statistical analysis used anova or  test . * significantly different vs. control subjects ( duncan 's post hoc test ) . the experiments were performed in the morning after an overnight fast with subjects in a supine position throughout the experiments . all antidiabetic treatment was withheld at least 24 h before the experiments . in insulin - treated patients , the last injection of short - acting insulin was performed on the evening before the tests , whereas all long - acting insulins were withheld for at least 24 h to avoid carryover effects . no restrictions were made regarding the intake of water until the morning of the experiments . both ear lobes were made hyperemic using finalgon ( 4 mg / g nonivamid , 25 mg / g nicoboxil ) . the experiments were started by the ingestion of the oral glucose load ( 75 g glucose in 300 ml ) over 5 min , and capillary and venous blood samples were drawn at t = 5 , 0 , 15 , 30 , 60 , 90 , 120 , 150 , 180 , 210 , and 240 min . capillary blood samples ( 100 l ) were added to naf ( microvette cb 300 ; sarstedt , nmbrecht , germany ) for the immediate measurement of glucose . venous blood was drawn into chilled tubes containing edta and aprotinin ( 20,000 kiu / ml trasylol , 200 l per 10 ml blood ; bayer , leverkusen , germany ) and kept on ice . glucose was measured as previously described ( 16 ) using a glucose oxidase method with a glucose analyzer 2 ( beckman instruments , munich ) . to adjust for the glucose concentration differences between capillary plasma and whole blood , insulin was measured as previously described ( 16 ) , using an insulin microparticle enzyme immunoassay ( imx insulin ; abbott laboratories , wiesbaden , germany ) . c - peptide was measured as previously described ( 16 ) , using an enzyme - linked immunosorbent assay from dakop ( cambrigshire , u.k . ) . pancreatic resections were fixed in formaldehyde and embedded in paraffin for subsequent analysis as previously described ( 12 ) . sequential 5-m sections were stained for insulin using a guinea pig anti - insulin antibody ( no . a 0564 , lot no . 00001500 ; dako ) at 1:400 dilution and an alkaline phosphatase / red detection system ( real envision detection system , no . for determination of the fractional -cell areas , entire pancreatic sections stained for insulin were imaged at 100 magnification ( 10 objective ) using a zeiss axioplan microscope equipped with a motorized stage . a tile image of the tissue section was generated using the mosaix tool of zeiss axiovision version 4.5 software . the fractional areas of the pancreas stained positive for insulin were digitally quantified using a color - based threshold using axiovision software as previously described ( 12 ) . insulin - to - glucose ratios and c - peptide  to  glucose ratios were calculated at all time points before and after oral glucose ingestion . subject characteristics are reported as the means  sd , and results are presented as the means  se . to address whether pancreatic -cell area predicts glycemic control , the fractional -cell area was first correlated to the fasting and 120-min postchallenge glucose concentrations and the a1c levels because these parameters have previously been validated as markers of glucose control . to address whether pancreatic -cell area can be predicted by established indexes of -cell function , the following parameters were correlated with -cell area : 1 ) the homa -cell function index , 2 ) the fasting insulin - to - glucose ratio , 3 ) the fasting c - peptide  to  glucose ratio , 4 ) the insulin - to - glucose ratio 30 min after oral glucose ingestion , 5 ) the c - peptide  to  glucose ratio 30 min after oral glucose ingestion , 6 ) the plasma insulin levels 30 min after oral glucose ingestion , and 7 ) the plasma c - peptide levels 30 min after oral glucose ingestion . these parameters were chosen based on previous studies ( 15,1821 ) . finally , to identify the strongest predictor of -cell area , the plasma concentrations of glucose , insulin , and c - peptide as well as the c - peptide  to  glucose ratios and insulin - to - glucose ratios at all time points during the prolonged ogtt were correlated with pancreatic -cell area . the parameter yielding the greatest r value for the correlation with -cell area was considered to be the best predictor of -cell area . by these means , time course measurements were carried out by unpaired anova , using statistica version 5.0 ( statsoft europe , hamburg , germany ) . by these means , three different p values were calculated 1 ) for the determination of overall differences between the different groups ( i.e. , ngt , ifg / igt , and diabetes ) , independent of the respective time patterns ; 2 ) for the determination of differences over the time course , independent of the respective groups ; and 3 ) for the determination of differences between the groups over the time course . if a significant ( p < 0.05 ) interaction between group and time was documented , values at single time points were compared by one - way anova . correlation analyses were carried out using graphpad prism 4 using linear or nonlinear regression functions . oral glucose tolerance was normal in 8 subjects , 2 subjects exhibited impaired fasting glucose alone , impaired glucose tolerance alone was present in 10 patients , 2 patients presented with both ifg and igt , and 11 patients had a diabetic glucose tolerance . patients with igt and ifg were collectively presented as one group for subsequent analyses . as expected , postchallenge glucose excursions were significantly higher both in patients with overt diabetes as well as in patients with igt / ifg compared with patients with normal glucose tolerance ( p < 0.0001 ) ( fig . this was accompanied by a marked reduction in both insulin and c - peptide concentrations after the oral glucose load in the patients with diabetes ( fig . insulin and c - peptide levels were not significantly lower in igt / ifg patients compared with ngt subjects . concentrations of glucose ( a ) , insulin ( b ) , and c - peptide ( c ) in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) after oral ingestion of 75 g glucose . statistics were carried out using repeated - measures anova and denote differences between the experiments ( a ) , differences over time ( b ) , and differences due to the interaction of experiment and time ( ab ) . * significant ( p < 0.05 ) differences vs. control subjects at individual time points ( one - way anova and duncan 's post hoc test ) . the fractional -cell area of the pancreas was 1.22  0.14 , 1.14  0.13 , and 0.43  0.12% in subjects with ngt , igt / ifg , and diabetes , respectively ( p = 0.0003 ) , corresponding to a 65% -cell deficit in patients with diabetes ( fig . 2 ) . when the two subjects with isolated ifg were excluded , the fractional -cell area in the remaining individuals with igt was 1.08  0.14% . -cell area in the two individuals with isolated ifg ( fasting glucose levels 107 and 102 mg / dl , 120-min glucose levels 106 and 120 min ) was 1.48 and 1.45% , respectively . homa -cell function ( including all patients examined ) was 81.1  13.0 , 37.5  30.0 , and 135.1  76.9% for those with ngt , igt / ifg , and diabetes , respectively ( p = 0.36 ) . there also were no differences in homa -cell function when two outliers ( calculated homa -cell function 876 and 336% ) were excluded ( p = 0.69 ) ( fig . the fasting insulin - to - glucose ratio and the fasting c - peptide  to  glucose ratio were not different between the groups ( p = 0.76 and p = 0.067 , respectively ) ( fig . , the insulin - to - glucose ratio and the c - peptide  to  glucose ratio 30 min after oral glucose ingestion were significantly lower in the patients with diabetes ( p = 0.033 and p < 0.0001 , respectively ) ( fig . fractional -cell area ( a ) , fasting insulin - to - glucose ratio ( b ) , insulin - to - glucose ratio 30 min after oral glucose ingestion ( c ) , homa -cell function ( excluding two outlier patients ) ( d ) , fasting c - peptide  to  glucose ratio ( e ) , and c - peptide  to  glucose ratio 30 min after oral glucose ingestion ( f ) in eight individuals with ngt , 14 individuals with ifg and/or igt , and 11 patients with diabetes . data are the means  se . statistics were carried out by anova and duncan 's post hoc test . there was a significant inverse association between fasting glucose concentrations and the fractional -cell area ( r = 0.53 , p = 0.0014 ) ( fig . an even closer association than in the fasting state was obtained when fractional -cell area was expressed in relation to respective glucose excursions 120 min after an oral glucose load . however , although the correlation between -cell area and fasting glucose levels tended to follow a linear relationship , the association with 120-min postchallenge glucose concentrations was best described by an exponential decay function using the following equation : y = 358.6  exp(3.21  x ) + 134.9 ( r = 0.89 ) ( fig . there also was a significant exponential relationship between fractional -cell area and a1c levels ( r = 0.82 ) ( fig . correlation analyses between fractional -cell area ( a , b , and c ) or plasma c - peptide levels ( d , e , and f ) and basal blood glucose levels ( a and d ) , blood glucose levels 120 min after oral glucose ingestion ( b and e ) , and a1c levels ( c and f ) in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) . r values in panels b , c , e , and f were calculated by nonlinear regression analyses using an exponential decay function . dotted lines in panels a , b , d , and e denote the respective margins of normal and impaired fasting glucose and normal and impaired glucose tolerance , respectively . c- peptide 30 , c - peptide 30 min after oral ingestion . because c - peptide levels 30 min after oral glucose ingestion yielded the closest association with postchallenge glucose excursions , this parameter was chosen for subsequent correlation analyses . there was no significant relationship between fasting glycemia and postchallenge c - peptide levels ( r = 0.14 , p = 0.44 ) ( fig . , there was an inverse exponential relationship between c - peptide levels at t = 30 min after oral glucose ingestion and both the 120-min glucose levels ( r = 0.82 ) ( fig . the associations between these parameters of glucose control and c - peptide excursions were weaker than the respective associations with fractional -cell area ( fig . there was a significant linear relationship between fractional -cell area and the fasting insulin - to - glucose ratio ( r = 0.51 , p = 0.0024 ) , the fasting c - peptide  to  glucose ratio ( r = 0.64 , p < 0.0001 ) , the insulin - to - glucose ratio 30 min after oral glucose ingestion ( r = 0.60 , p = 0.0002 ) , the c - peptide  to  glucose ratio 30 min after oral glucose ingestion ( r = 0.68 , p < 0.0001 ) , plasma insulin levels 30 min after oral glucose ingestion ( r = 0.51 , p = 0.0027 ) , and c - peptide levels 30 min after oral glucose ingestion ( r = 0.57 , p = 0.0005 ) ( fig . 4 ) . in contrast , no significant relationship was found between -cell area and the homa index of -cell function ( r = 0.03 , p = 0.88 ) . also , after excluding two outlier patients , this association failed to reach statistical significance ( r = 0.23 , p = 0.21 ) ( fig . linear regression analyses between fractional -cell area and established indexes of insulin secretion . a f : correlations with the fasting insulin - to - glucose ratio ( a ) , plasma insulin levels 30 min after oral glucose administration ( b ) , the insulin - to - glucose ratio 30 min after oral glucose administration ( c ) , the fasting c - peptide  to  glucose ratio ( d ) , plasma c - peptide levels 30 min after oral glucose administration ( e ) , and the c - peptide  to  glucose ratio 30 min after oral glucose administration ( f ) in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) . linear regression analyses between fractional -cell area and the homa index of -cell function in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) . a : the correlation including all patients examined . fractional -cell area was correlated with the glucose , insulin , and c - peptide levels as well as with the respective insulin - to - glucose ratios and c - peptide  to  glucose ratios obtained before and after oral glucose ingestion . based on these analyses , the plasma glucose levels measured 120 min after oral glucose ingestion yielded the closest association with -cell area in an exponential model ( fig . 6 ) . when a linear regression model was applied , the plasma glucose concentrations at 60 min after oral glucose ingestion showed the highest degree of correlation ( r = 0.80 , p < 0.0001 ) ( figs . 6 and 7 ) . correlation coefficients ( squared ) for the relationship between fractional -cell area and the respective insulin , c - peptide , and glucose levels obtained before and after oral glucose ingestion in eight individuals with ngt , 14 individuals with ifg and/or igt , and 11 patients with diabetes determined by linear regression analysis ( dashed lines ) or by an exponential decay function ( solid lines ; shown for glucose only ) . , glucose ( linear ) ;  , insulin ( linear ) ;  , c - peptide ( linear ) ;  , glucose ( exponential ) . linear regression analyses between fractional -cell area and glucose concentrations 60 min after oral glucose administration ( a ) and the c - peptide  to  glucose ratio 15 min after oral glucose administration ( b ) in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) . these parameters were identified as strongest predictors of -cell area based on the respective r values . the closest associations between -cell area and individual insulin and c - peptide levels were found at t = 30 min after the oral glucose load ( r = 0.51 , p = 0.0027 , and r = 0.57 , p = 0.0005 ; respectively ) ( fig . an even greater degree of correlation was obtained when insulin and c - peptide levels were expressed in relation to the respective glucose concentrations . by these means , the closest correlations with fractional -cell area were found for the c - peptide  to  glucose ratio 15 min after oral glucose ingestion ( r = 0.72 , p < 0.0001 ) ( fig . 7 ) and for the insulin - to - glucose ratio determined 30 min after the glucose load ( r = 0.60 , p = 0.0002 ) ( fig . oral glucose tolerance was normal in 8 subjects , 2 subjects exhibited impaired fasting glucose alone , impaired glucose tolerance alone was present in 10 patients , 2 patients presented with both ifg and igt , and 11 patients had a diabetic glucose tolerance . patients with igt and ifg were collectively presented as one group for subsequent analyses . as expected , postchallenge glucose excursions were significantly higher both in patients with overt diabetes as well as in patients with igt / ifg compared with patients with normal glucose tolerance ( p < 0.0001 ) ( fig . this was accompanied by a marked reduction in both insulin and c - peptide concentrations after the oral glucose load in the patients with diabetes ( fig . insulin and c - peptide levels were not significantly lower in igt / ifg patients compared with ngt subjects . concentrations of glucose ( a ) , insulin ( b ) , and c - peptide ( c ) in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) after oral ingestion of 75 g glucose . statistics were carried out using repeated - measures anova and denote differences between the experiments ( a ) , differences over time ( b ) , and differences due to the interaction of experiment and time ( ab ) . * significant ( p < 0.05 ) differences vs. control subjects at individual time points ( one - way anova and duncan 's post hoc test ) . the fractional -cell area of the pancreas was 1.22  0.14 , 1.14  0.13 , and 0.43  0.12% in subjects with ngt , igt / ifg , and diabetes , respectively ( p = 0.0003 ) , corresponding to a 65% -cell deficit in patients with diabetes ( fig . 2 ) . when the two subjects with isolated ifg were excluded , the fractional -cell area in the remaining individuals with igt was 1.08  0.14% . -cell area in the two individuals with isolated ifg ( fasting glucose levels 107 and 102 mg / dl , 120-min glucose levels 106 and 120 min ) was 1.48 and 1.45% , respectively . homa -cell function ( including all patients examined ) was 81.1  13.0 , 37.5  30.0 , and 135.1  76.9% for those with ngt , there also were no differences in homa -cell function when two outliers ( calculated homa -cell function 876 and 336% ) were excluded ( p = 0.69 ) ( fig . the fasting insulin - to - glucose ratio and the fasting c - peptide  to  glucose ratio were not different between the groups ( p = 0.76 and p = 0.067 , respectively ) ( fig . the insulin - to - glucose ratio and the c - peptide  to  glucose ratio 30 min after oral glucose ingestion were significantly lower in the patients with diabetes ( p = 0.033 and p < 0.0001 , respectively ) ( fig . 2 ) . fractional -cell area ( a ) , fasting insulin - to - glucose ratio ( b ) , insulin - to - glucose ratio 30 min after oral glucose ingestion ( c ) , homa -cell function ( excluding two outlier patients ) ( d ) , fasting c - peptide  to  glucose ratio ( e ) , and c - peptide  to  glucose ratio 30 min after oral glucose ingestion ( f ) in eight individuals with ngt , 14 individuals with ifg and/or igt , and 11 patients with diabetes . data are the means  se . statistics were carried out by anova and duncan 's post hoc test . there was a significant inverse association between fasting glucose concentrations and the fractional -cell area ( r = 0.53 , p = 0.0014 ) ( fig . an even closer association than in the fasting state was obtained when fractional -cell area was expressed in relation to respective glucose excursions 120 min after an oral glucose load . however , although the correlation between -cell area and fasting glucose levels tended to follow a linear relationship , the association with 120-min postchallenge glucose concentrations was best described by an exponential decay function using the following equation : y = 358.6  exp(3.21  x ) + 134.9 ( r = 0.89 ) ( fig . there also was a significant exponential relationship between fractional -cell area and a1c levels ( r = 0.82 ) ( fig . correlation analyses between fractional -cell area ( a , b , and c ) or plasma c - peptide levels ( d , e , and f ) and basal blood glucose levels ( a and d ) , blood glucose levels 120 min after oral glucose ingestion ( b and e ) , and a1c levels ( c and f ) in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) . r values in panels b , c , e , and f were calculated by nonlinear regression analyses using an exponential decay function . dotted lines in panels a , b , d , and e denote the respective margins of normal and impaired fasting glucose and normal and impaired glucose tolerance , respectively . because c - peptide levels 30 min after oral glucose ingestion yielded the closest association with postchallenge glucose excursions , this parameter was chosen for subsequent correlation analyses . there was no significant relationship between fasting glycemia and postchallenge c - peptide levels ( r = 0.14 , p = 0.44 ) ( fig . , there was an inverse exponential relationship between c - peptide levels at t = 30 min after oral glucose ingestion and both the 120-min glucose levels ( r = 0.82 ) ( fig . the associations between these parameters of glucose control and c - peptide excursions were weaker than the respective associations with fractional -cell area ( fig . 4 ) . there was a significant linear relationship between fractional -cell area and the fasting insulin - to - glucose ratio ( r = 0.51 , p = 0.0024 ) , the fasting c - peptide  to  glucose ratio ( r = 0.64 , p < 0.0001 ) , the insulin - to - glucose ratio 30 min after oral glucose ingestion ( r = 0.60 , p = 0.0002 ) , the c - peptide  to  glucose ratio 30 min after oral glucose ingestion ( r = 0.68 , p < 0.0001 ) , plasma insulin levels 30 min after oral glucose ingestion ( r = 0.51 , p = 0.0027 ) , and c - peptide levels 30 min after oral glucose ingestion ( r = 0.57 , p = 0.0005 ) ( fig . 4 ) . in contrast , no significant relationship was found between -cell area and the homa index of -cell function ( r = 0.03 , p = 0.88 ) . also , after excluding two outlier patients , this association failed to reach statistical significance ( r = 0.23 , p = 0.21 ) ( fig . linear regression analyses between fractional -cell area and established indexes of insulin secretion . a f : correlations with the fasting insulin - to - glucose ratio ( a ) , plasma insulin levels 30 min after oral glucose administration ( b ) , the insulin - to - glucose ratio 30 min after oral glucose administration ( c ) , the fasting c - peptide  to  glucose ratio ( d ) , plasma c - peptide levels 30 min after oral glucose administration ( e ) , and the c - peptide  to  glucose ratio 30 min after oral glucose administration ( f ) in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) . linear regression analyses between fractional -cell area and the homa index of -cell function in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) . a : the correlation including all patients examined . fractional -cell area was correlated with the glucose , insulin , and c - peptide levels as well as with the respective insulin - to - glucose ratios and c - peptide  to  glucose ratios obtained before and after oral glucose ingestion . based on these analyses , the plasma glucose levels measured 120 min after oral glucose ingestion yielded the closest association with -cell area in an exponential model ( fig . . when a linear regression model was applied , the plasma glucose concentrations at 60 min after oral glucose ingestion showed the highest degree of correlation ( r = 0.80 , p < 0.0001 ) ( figs . 6 and 7 ) . correlation coefficients ( squared ) for the relationship between fractional -cell area and the respective insulin , c - peptide , and glucose levels obtained before and after oral glucose ingestion in eight individuals with ngt , 14 individuals with ifg and/or igt , and 11 patients with diabetes determined by linear regression analysis ( dashed lines ) or by an exponential decay function ( solid lines ; shown for glucose only ) . , glucose ( linear ) ;  , insulin ( linear ) ;  , c - peptide ( linear ) ;  , glucose ( exponential ) . linear regression analyses between fractional -cell area and glucose concentrations 60 min after oral glucose administration ( a ) and the c - peptide  to  glucose ratio 15 min after oral glucose administration ( b ) in eight individuals with ngt (  ) , 14 individuals with ifg and/or igt (  ) , and 11 patients with diabetes (  ) . these parameters were identified as strongest predictors of -cell area based on the respective r values . the closest associations between -cell area and individual insulin and c - peptide levels were found at t = 30 min after the oral glucose load ( r = 0.51 , p = 0.0027 , and r = 0.57 , p = 0.0005 ; respectively ) ( fig . an even greater degree of correlation was obtained when insulin and c - peptide levels were expressed in relation to the respective glucose concentrations . by these means , the closest correlations with fractional -cell area were found for the c - peptide  to  glucose ratio 15 min after oral glucose ingestion ( r = 0.72 , p < 0.0001 ) ( fig . 7 ) and for the insulin - to - glucose ratio determined 30 min after the glucose load ( r = 0.60 , p = 0.0002 ) ( fig . the current study was designed to assess the validity of functional indexes of insulin secretion to predict -cell area in humans in vivo . using a combination of oral glucose tolerance tests carried out before pancreatic surgery and morphometric analyses of the respective pancreatic tissue samples , we report that 1 ) glucose control deteriorates with declining -cell area in humans , 2 ) -cell area is significantly related to different functional measures of insulin secretion , and 3 ) a c - peptide  to  glucose ratio determined 15 min after oral glucose ingestion appears to predict -cell area better than fasting measures of insulin secretion , such as the homa index . the lack of reliable imaging techniques suitable to determine -cell mass in vivo has prompted great interest in the functional assessment of -cell mass ( 22 ) . thus , a number of studies have been performed in different animal models of diabetes ( rats , dogs , baboons , and minipigs ) ( 2327 ) . the results of these studies have recently been elegantly reviewed by robertson ( 22 ) . as a rule , significant linear associations between the insulin secretory responses to arginine or glucagon administration and pancreatic -cell mass have been reported in these animal studies ( 22 ) . in humans , available information about the relationship between insulin secretion and -cell mass is rather sparse , and two different studies have previously addressed this issue . ( 28 ) determined insulin secretory responses to intravenous glucose and arginine administration in eight subjects undergoing islet autotransplantation . in this study , all measures of insulin secretion were closely correlated with the transplanted islet cell mass . a similar association between the number of transplanted islet equivalents and insulin levels after arginine administration was found in patients with type 1 diabetes studied after islet allotransplantation ( 29 ) . the current study extends these results by providing for the first time direct evidence for a linear correlation between intrapancreatic -cell mass and insulin secretion in humans with and without diabetes in vivo . of note , none of the imaging methods available so far has proven to predict -cell mass with a comparable accuracy as the functional measures applied herein ( 13 ) . although this study and the prior studies mentioned above lend strong support to the use of functional tests for the assessment of -cell area in humans , it is important to bear in mind the potential limitations of such an approach . in fact , even though the overall association between -cell area and insulin secretion in this study was rather close , any functional measure of insulin secretion can be confounded by a number of factors other than merely the amount of pancreatic -cells . along these lines , insulin secretion can be increased by as much as two- to threefold under conditions of obesity or insulin resistance ( 30 ) , although the actual increase in -cell mass in obese patients has been estimated to be only 30% ( 2 ) . furthermore , the insulin responses to intravenous glucose administration in patients with type 2 diabetes can be enhanced acutely by > 300% after the infusion of a glp-1 analogue over 5 h ( 31 ) , a time period during which any gain in -cell mass is far from being realistic . consistent with this , the current study has shown that despite the significant relationship between insulin secretion and -cell area , there was still considerable variability in insulin secretion between different individuals with a similar extent of -cell area . in this regard , it is also important to stress that any association between functional indexes of insulin secretion and -cell area is only valid in the absence of concomitant antidiabetic treatment . moreover , because a large number of functional parameters were compared with -cell area , there is a certain likelihood of significant results attributable to multiple testing . however , the overall number of significant associations identified in this study was far in excess of the expected random probability , and the relationships observed were in good agreement with the primary hypothesis . the close inverse relationship between -cell area and 120-min glucose levels during the ogtt is another intriguing finding from this study . as much as 80% of the variations in postchallenge glucose excursions could be attributed to differences in -cell area . . taken together , these results suggest that postprandial glucose control strictly depends on a sufficient extent of insulin - secreting -cells , whereas fasting glucose levels may be affected by a number of additional factors as well . collectively , these findings further emphasize the importance of -cell mass for the development of diabetes . in this regard , it is noteworthy that the associations between the different parameters of glucose control and c - peptide secretion were even weaker than the respective associations with -cell area , which may suggest that defects in -cell mass rather than impairments in -cell function primarily determine the development of diabetes . although -cell area was significantly reduced in the patients with overt hyperglycemia , there was only a very small reduction in -cell area in the pre - diabetic patients with ifg and igt . examined -cell area in ifg subjects , whereas the majority of patients in this study were characterized by igt . given the marked differences in the pathogenesis of ifg and igt ( 32 ) , these findings suggest that the rise in postchallenge glucose levels is a rather early phenomenon during the development of diabetes , whereas fasting hyperglycemia may be indicative of a more pronounced -cell loss . second , in the study by butler et al . , the diagnostic threshold for ifg was 110 mg / dl ( 33 ) , whereas the revised ada criteria with a glucose threshold of 100 mg / dl were used in this study ( 34 ) . by these means , patients with relatively mild alterations in fasting glycemia ( i.e. , fasting glucose levels between 100 and 110 mg / dl ) were included in the ifg / igt group in this study as well , whereas the degree of fasting hyperglycemia was clearly more pronounced in the previous study . finally , it is important to emphasize that all patients examined in this study underwent pancreatic surgery for underlying pancreatic disorders ( especially chronic pancreatitis ) , whereas butler et al . ( 2 ) studied a group of patients more typical of ( pre- ) type 2 diabetes . in line with this , the mean bmi of the patients in the ifg population studied previously was 37 kg / m , whereas the current group of patients with ifg / igt had a mean bmi of 23.4 it is therefore likely that the mechanisms underlying the -cell destruction in the patients studied herein were different from those typically found in patients with type 2 diabetes . therefore , even though our current study provides novel information regarding the relationship between -cell area and insulin secretion as well as glycemic control , the findings should not be generalized to the majority of patients with type 2 diabetes . a number of different functional indexes have been proposed to estimate -cell function in vivo . among these , insulin responses to intravenous glucagon or arginine administration ( with or without glucose potentiation ) appear to provide the most reliable estimates of -cell function ( 22 ) . however , broad application of such tests is clearly limited by their laborious procedures , which prevent the routine examination of larger groups of individuals . in contrast , the oral glucose tolerance test can easily be performed under everyday conditions and is therefore commonly applied to detect disturbances in glucose control . the current data showing a close relationship between both insulin and c - peptide excursions during the ogtt and the fractional -cell area of the pancreas lend further support to the broad use of this test and suggest that postchallenge glucose excursions may not only predict the risk of developing diabetes , but also allow for some conclusions regarding the residual -cell area . because pancreatic weight can not readily be measured at surgery in humans , the fractional -cell area rather than the total -cell mass has been determined in this study . therefore , the actual -cell mass of the patients may still vary to some extent , depending on the overall size of the pancreas . however , previous studies including > 1,800 patients have demonstrated that pancreatic volume is not affected by the presence of type 2 diabetes and remains rather constant during adulthood in humans , suggesting that the overall impact of this factor on the current results should be rather minor ( 35 ) . the close relationship between postchallenge glycemia and the fractional -cell area observed in this study is consistent with the postulate that fractional -cell area rather than pancreatic weight is the primary determinant of -cell mass and suggests that in humans , -cell mass may be readily estimated from the determination of the -cell area in pancreatic tissue samples . one important goal of this study was to identify simple and reliable predictors of -cell area in humans . judging from the respective r values alone , the best correlation was obtained for the 120-min glucose levels during the ogtt . however , because this relationship followed an exponential rather than a linear function , the predictive value of such postchallenge glucose excursions may be low , especially in individuals with a relatively normal -cell area ( i.e. , > 50% of normal ) . a better estimation of -cell area may be derived from a c - peptide  to  glucose ratio at t = 15 min after oral glucose ingestion , which in the current study explained as much as 51% of the variations in -cell area . however , even a simple c - peptide  to  glucose ratio under fasting conditions already provides a reasonable estimate of -cell area . in contrast , the homa -cell function index , which is commonly applied as a measure of -cell function in clinical studies ( 36,37 ) , failed to predict pancreatic -cell area in the setting of this study . the results of this study may have important consequences for the design and interpretation of future longitudinal studies about the potential effects of different antidiabetic agents on pancreatic -cell area and the progression of diabetes . thus , although there is solid evidence for a progressive decline in -cell mass and function with advancing type 2 diabetes from a number of cross - sectional studies ( 38,39 ) , the only longitudinal studies that have examined the course of islet dysfunction in patients with type 2 diabetes ( ukpds [ u.k . prospective diabetes study ] and adopt [ a diabetes outcome progression trial ] ) have applied the homa index as a measure of -cell function , which , according to the current results , does not necessarily predict changes in -cell mass ( 36,40 ) . in a similar fashion , a number of recent clinical trials have suggested improvements in -cell function and presumably -cell mass during treatment with dipeptidyl peptidase-4 inhibitors and glp-1 analogues based on such calculations ( 37,41 ) . the current study suggests that although the homa index may have some relevance to estimate the functional integrity of insulin secretion , it represents an odd surrogate of -cell area in vivo . future studies in this area should therefore consider the use of a c - peptide  to  glucose ratio after oral glucose ingestion as a functional estimate of -cell area . in conclusion , the current studies have revealed a close inverse relationship between postchallenge glycemia and -cell area in humans in vivo . a c - peptide  to  glucose ratio after oral glucose ingestion or even in the fasting state can provide a reasonable estimate of pancreatic -cell area , whereas the homa -cell function index appears to be less suitable in this regard . such indexes may be useful to assess the natural course of -cell loss in longitudinal studies and to estimate the residual -cell area in individual patients with diabetes .