leptin is an adipose tissue - derived hormone that has been shown to be related to several metabolic , inflammatory , and hemostatic factors involved in the development of hypertension and cardiovascular disease .
experimental animal studies suggest that higher leptin levels may cause hyperglycemia , elevations in blood pressure ( mediated through increased sympathetic activity ) , and renal dysfunction . in rat models
, leptin has been shown to induce natriuresis   which may in turn result in an increase in arterial pressure so as to maintain sodium and water balance .
leptin has also been shown to serve as a cofactor of tgf - beta activation , promote renal endothelial cell proliferation , and potentially may play a role in renal glomerulosclerosis [ 57 ] .
recent studies have reported that infusion of recombinant leptin into normal rats for 3 weeks fosters the development of focal glomerulos 4.clerosis .
however , few human studies have examined the putative association between plasma leptin levels and chronic kidney disease ( ckd ) in humans . in this context
, we examined the independent relation between plasma leptin levels and ckd in a multiethnic sample of us adults , after adjusting for main confounding factors .
the current study is based on data from the third national health and nutrition examination survey ( nhanes iii ) .
detailed description of nhanes iii study design and methods are available elsewhere [ 813 ] . in brief ,
nhanes iii included a stratified multistage probability sample representative of the civilian noninstitutionalized us population .
selection was based on counties , blocks , households , and individuals within households and included the oversampling of non - hispanic blacks and mexican americans in order to provide stable estimates of these groups .
subjects were required to sign a consent form before their participation , and approval was obtained from the human subjects committee in the us department of health and human service .
the sample included in the current analysis consisted of participants aged greater than 20 years who were randomly assigned to complete an examination in the morning after an overnight fast .
we further excluded participants with self - reported cardiovascular disease ( n = 434 ) , missing serum creatinine ( n = 60 ) or who were missing data ( n = 101 ) on covariates included in the multivariable model , including systolic or diastolic blood pressure , body mass index ( bmi ) , or cholesterol levels .
serum creatinine was measured using the jaffe kinetic alkaline picrate method performed on a roche hitachi 737 analyzer .
serum creatinine values in nhanes iii were calibrated to the standard creatinine values from the cleveland clinic foundation ( ccf ) laboratory who used a roche coupled enzymatic assay method that was traceable to an isotope dilution mass spectrometric method using the following deming regression equation : standard creatinine in mg / dl = 0.960  nhanes measured serum creatinine ( in  mg / dl)0.184 .
glomerular filtration rate ( egfr ) was estimated from serum creatinine using the 4-variable modification of diet in renal disease ( mdrd ) study equation as follows : egfr = 175  ( serum creatinine in mg / dl )  ( age in years )  ( 0.742 if female )  ( 1.21 if black ) .
ckd was defined as an egfr of < 60  ml / min/1.73  m , consistent with national kidney foundation kidney disease outcomes quality initiative ( kdoqi )   stage 3 chronic kidney disease .   
age , gender , race / ethnicity , smoking status , alcohol intake ( g / day ) , level of education , history of diabetes and oral hypoglycemic intake or insulin administration , and antihypertensive medication use were assessed using standardized questionnaires .
individuals who had not smoked 100 cigarettes in their lifetimes were considered never smokers ; those who had smoked 100 cigarettes in their lifetimes were considered former smokers if they answered negatively to the question  do you smoke now ?  and current smokers if they answered affirmatively . using height and weight measured during the study examination , body mass index ( bmi )
rigorous procedures with quality control checks were used in blood collection and details about these procedures are provided in the nhanes laboratory / medical technologists procedures manual [ 8 , 11 ] .
louis , mo , usa . the assay was a radioimmunoassay ( ria ) with a polyclonal antibody raised in rabbits against highly purified recombinant human leptin .
the minimum detectable concentration of the assay was 0.5  fg / l leptin , and the limit of linearity was 100 
recovery of leptin added to serum is 99104% over the linear range of the assay .
within- and between - assay cvs ranged from 3.4% to 8.3% and from 3.6% to 6.2% , respectively [ 11 , 12 ] .  
serum glucose was measured using the modified hexokinase method at the university of missouri diabetes diagnostic laboratory .
diabetes was defined based on the guidelines of the american diabetes association as a serum glucose 126  mg / dl after fasting for a minimum of 8 hours , a serum glucose 200  mg / dl for those who fasted < 8 hours before their nhanes visit , a glycosylated hemoglobin value 6.5% , or a self - reported current use of oral hypoglycemic medication or insulin .
seated systolic and diastolic blood pressures were measured using a mercury sphygmomanometer according to the american heart association and seventh joint national committee ( jnc7 ) recommendations .
participants were considered to have hypertension if they reported current blood pressure - reducing medication use and/or had systolic blood pressure 140  mm hg and/or diastolic blood pressure 90  mm hg .
plasma leptin was analyzed both as a continuous variable as well as a categorical variable .
for the analysis as a continuous variable , leptin values were log transformed ( base e ) as a result of their skewed distribution . using the distribution present in the nhanes iii population , we categorized plasma leptin level into quartiles ( 4.3  fg / l , 4.48.7  fg / l , 8.816.9  fg / l , > 16.9 
fg / l ) . the multivariable - adjusted odds ratio [ ( or ) ( 95% confidence interval ( ci ) ] of ckd associated with leptin quartile was calculated with the lowest quartile as the referent , using logistic regression models .
odds ratios were calculated initially after age and sex adjustment and subsequently after additional adjusting for race / ethnicity ( non - hispanic whites , non - hispanic blacks , mexican americans , and others ) , education categories ( below high school , high school , above high school ) , smoking ( never smoker , former smoker , current smoker ) , alcohol intake ( continuous ) , bmi ( continuous ) , diabetes mellitus ( absent , present ) , hypertension ( absent , present ) , and total serum cholesterol ( continuous ) .
trends in the or of ckd across increasing plasma leptin category were determined by modeling median within - quartile leptin level as a continuous variable . to examine the dose - response relationship of the observed association between plasma leptin level and ckd without linearity assumptions ,
we used flexible nonparametric logistic regression employing the generalized additive modeling approach ( r system for statistical computing , available from comprehensive r archive network [ http://www.cran.r-project.org/ ] ) to calculate odds ratio of ckds mellitus , adjusting for all covariates in the multivariable model ; the predicted odds ratio of ckd was then plotted against increasing leptin levels ( on the log scale ) .
previous studies have shown that serum leptin levels are associated with increased systemic inflammation as measured by c - reactive protein levels , hyperglycemia , high insulin levels , and increased systolic blood pressure , factors that have also been shown to be associated with ckd[17 , 21 , 22 ] .
therefore , in a supplementary analysis , to examine if the observed association between plasma leptin and ckd was explained by c - reactive protein levels , fasting insulin , glucose levels , or systolic blood pressure , we adjusted for these variables in the multivariable - adjusted model .
sample weights   that account for the unequal probabilities of selection , oversampling , and nonresponse were applied for all analyses using sudaan ( version 8.0 ; research triangle institute , research triangle park , nc ) and sas ( version 9.2 ; sas institute , cary , nc ) software ; standard errors ( se ) were estimated using the taylor series linearization method .

 table 1 presents the characteristics of the study population included in the current analysis .
overall , this study included a broad age range , multiethnic sample of americans with an approximately equal number of men and women .
the mean egfr of the study participants was 94  ml / min/1.73  m , and 3.5% had ckd . 

a positive association between higher leptin quartiles and ckd was present in the age , sex - adjusted model as well as the multivariable model . when analyzed as a continuous variable after log transformation , a positive association was present between leptin and ckd . 

table 3 presents the association between plasma leptin levels and ckd within subgroups defined by gender , bmi categories , and diabetes and hypertension .
overall , the association between leptin and ckd was consistently present within these subgroups . although some of the ors failed to reach conventional levels of statistical significance due to limited sample size and therefore statistical power , tests for interaction were not statistically significant ( each p > 0.10 for all stratified analyses ) .  
when we employed nonparametric models to graphically examine the dose - response relationship between plasma leptin levels and ckd without linearity assumptions involved in traditional regression models , we observed an overall positive association between plasma leptin and ckd , consistent with the results in tables 1 , 2 , and 3 .
however , there was a steeper association with ckd for plasma leptin levels > 16  fg / l ( figure 1 ) .   in a supplementary analysis ,
to examine if the observed association between leptin and ckd was explained by c - reactive protein , a marker of inflammation , or fasting insulin or glucose levels , or systolic blood pressure , we additionally adjusted for these variables to the multivariable - adjusted model .
the positive association between leptin and ckd was attenuated , but still present . compared to quartile 1 of plasma leptin ( referent ) ,
the multivariable or ( 95% ci ) of ckd was 1.31 ( 0.70 to 2.44 ) in quartile 2 , 1.22 ( 0.57 to 2.62 ) in quartile 3 , and 2.72 ( 1.14 to 6.48 ) in quartile 4 ; p - trend = 0.0475 .
  in a multi - ethnic , population - based sample of us adults , we found that higher plasma leptin levels were positively associated with ckd .
this association appeared to be independent of confounders such as age , race - ethnicity , education , bmi , diabetes , and hypertension and appeared to be consistently present in both men and women .
furthermore , the observed association between plasma leptin levels and ckd was present even after adjusting for c - reactive protein and fasting insulin levels , suggesting an association between this adipokine and ckd that is independent of these factors .  
several lines of recent evidence suggest that an association between leptin and ckd is plausible .
this includes the role of leptin in activating the sympathetic nervous system and causing chronic elevations in blood pressure and renal dysfunction , inducing natriuresis   which may result in an increase in arterial pressure so as to maintain sodium and water balance , serving as a cofactor of tgf - beta activation , promoting renal endothelial cell proliferation , and subsequent glomerulosclerosis [ 57 ] .
it was recently shown that , in rats , infusion of recombinant leptin caused the development of focal glomerulosclerosis .
also , leptin is also reported to be related to insulin resistance   and high c - reactive protein levels , both of which have been shown to be related to ckd [ 21 , 25 ] .  
a previous study conducted among women with type 1 diabetes reported that plasma leptin levels are independently related to reduced renal function . in a study from south africa conducted among approximately 300 black subjects , okpechi et al . reported that plasma leptin levels were inversely related to egfr . in another study , common polymorphisms in the lep gene
overall , our findings of a positive association between plasma leptin and ckd are in agreement with these previous studies ; in addition , we were able to study a large multiethnic sample that includes both men and women and also adjusts for multiple confounders such as bmi , diabetes , hypertension , lipid levels , and c - reactive protein .  
the main strengths of our study include its population - based nature , inclusion of a representative multiethnic sample , adequate sample size , and the availability of data on confounders for multivariable adjustment .
furthermore , all data were collected following rigorous methodology , including a study protocol with standardized quality control checks .
the main limitation of our study is the cross - sectional nature of nhanes , which precludes conclusions regarding the temporal nature of the association between plasma leptin and ckd .
second , defining ckd as egfr < 60  ml / min/1.73  m introduces some ascertainment bias .
this bias is likely to result in under- or overestimation of odds ratios presented in this report .   in summary , in a multiethnic sample of us adults
, we found that higher plasma leptin levels are associated with ckd , independent of traditional factors such as age , sex , smoking , alcohol intake , bmi , diabetes , hypertension and serum cholesterol .
our results suggest that leptin may explain part of the reported association between obesity and kidney disease
. however , future prospective cohort studies are needed to confirm or refute our findings .
 
 background . 
 leptin is an adipose tissue - derived hormone shown to be related to several metabolic , inflammatory , and hemostatic factors related to chronic kidney disease . 
 recent animal studies have reported that infusion of recombinant leptin into normal rats for 3 weeks fosters the development of glomerulosclerosis . 
 however , few studies have examined the association between leptin and ckd in humans . therefore , we examined the association between plasma leptin levels and ckd in a representative sample of us adults . methods . 
 we examined the third national health and nutrition examination survey participants > 20 years of age ( n = 5820 , 53.6% women ) . 
 plasma leptin levels were categorized into quartiles ( 4.3  fg / l , 4.48.7  fg / l , 8.816.9  fg / l , > 16.9  
 fg / l ) . 
 ckd was defined as a glomerular filtration rate of < 60  ml / min/1.73  m2 estimated from serum creatinine . 
 results . 
 higher plasma leptin levels were associated with ckd after adjusting for age , sex , race / ethnicity , education , smoking , alcohol intake , body mass index ( bmi ) , diabetes , hypertension , and serum cholesterol . 
 compared to quartile 1 of leptin ( referent ) , the odds ratio ( 95% confidence interval ) of ckd associated with quartile 4 was 3.31 ( 1.41 to 7.78 ) ; p - trend = 0.0135 . 
 subgroup analyses examining the relation between leptin and ckd by gender , bmi categories , diabetes , and hypertension status also showed a consistent positive association . 
 conclusion . 
 higher plasma leptin levels are associated with ckd in a representative sample of us adults . 
 