when people originally from a lower altitude descend to sea level or lower altitude after high - altitude / hypoxia acclimatization , then they lose hypoxia tolerance and physiological adjustments .
in addition , they experience changes in hemoglobin and hormone levels over time , and this is known as high - altitude deacclimatization ( hada ) . this physiological process has been documented in explorers , athletes , military personnel , and workers in high - altitude mines .
following recent dramatic economic growth in plateau regions of china , such as tibet , qinghai , and xinjiang , tens of millions of lower altitude individuals temporarily migrated to high - altitude regions for work , and then they returned to lower altitudes after they finished their work each year .
when the workers returned to lower altitudes , most of them were in the process of hada and suffered from physical discomfort and symptoms .
a previous study has shown that individuals suffering from hada experienced symptoms such as sleepiness , insomnia , unresponsiveness , memory loss , fidgetiness , headache , throat pain or discomfort , coughing , sputum , chest tightness , becoming flustered , increased appetite , decreased appetite , diarrhea , abdominal distention , abdominal pain , lumbago , and arthralgia .
these symptoms are characteristic and are usually referred to as hada syndrome ( hadas ) , which affects the quality of life of these individuals [ 7 , 8 ] .
our previous and other studies showed that hadas subjects suffered a series of clinical symptoms , which could last for many years [ 6 , 911 ] .
therefore , hadas has been a public health issue in china and in other countries . in our previous study
, we found that subjects who suffered from hadas experienced hypoxia / reoxygenation ( h / r ) . the subjects lived in hypoxic conditions and then quickly returned to normoxic environments , and the levels of pao2 and so2 in hadas subjects rapidly increased from 81.58  hpa and 87.31% to 125.84  hpa and 96.78% , respectively .
evidence from studies showed that h / r induced oxidative stress and production of reactive oxygen species ( ros ) [ 12 , 13 ] and resulted in damage to tissue or cells .
superoxide dismutase ( sod ) plays an important role in removal of excess free radicals in humans experiencing oxidative stress [ 15 , 16 ] .
malondialdehyde ( mda ) is a product of lipid peroxidation , which occurs when unsaturated lipids are exposed to oxygen [ 15 , 16 ] .
elevation of mda levels leads to increased oxidative stress and oxidative - mediated damage [ 15 , 16 ] .
zhou et al .   showed that serum sod levels were elevated and mda levels were decreased when subjects returned to lower altitudes upon short - term exposure to high altitudes .
research has shown that h / r could increase generation of proinflammatory mediators , such as tumor necrosis factor- ( tnf- )   and interleukin- ( il- ) 17a , and suppress levels of anti - inflammatory factor il-10 , and then it induced apoptosis and damage to cells and tissues [ 21 , 22 ] . in our previous study , we showed that a systemic inflammatory response and myocardial injury were observed in hadas subjects 3 d after returning to a lower altitude .
however , serum levels of il-17a , il-10 , and tnf- in hadas subjects as well as correlations between these factors and occurrence rate and progression of hadas are not clear . in this study
, we evaluated the hadas score and measured serum levels of sod , mda , il-17a , tnf- , and il-10 in hadas and control groups .
we then analyzed the correlation of serum levels of sod , mda , il-17a , tnf- , and il-10 with hadas occurrence and severity .
sixty - seven healthy male subjects ( 25.1  7.6 years old ) from chongqing ( 180  m ) had worked in lhasa ( 3650  m ) for about 8 months and then returned to chongqing by airplane .
after they returned , all subjects had been diagnosed with mild - to - moderate hadas according to relevant diagnostic and scoring criteria , and they were evaluated for hadas at 3  d ( baseline ) , 50  d , and 100  d. they were considered the hadas group . in addition , 41 healthy male subjects ( 24.8  8.1 years old ) who had always lived in chongqing served as the control group . the 2 groups were not significantly different in age ( p > 0.05 ) ( table 1 ) .
this study was approved by the medical ethical committee of the second affiliated hospital , third military medical university .
briefly , adult individuals who were less than 60 years old returned to a lower altitude from a higher altitude where they had worked for 412 months .
they suffered from 3 or more of the following symptoms : fatigue , sleepiness , insomnia , unresponsiveness , memory loss , fidgetiness , headache , and throat pain or discomfort .
the principal exclusion criteria included symptoms directly attributable to primary diseases affecting the cardiovascular , respiratory , nervous , urinary , and hematological systems , cancer or leukemia , and a recent history of influenza , upper respiratory tract infection , infectious diarrhea , or similar symptoms .
hadas symptom scores ( hadas scores ) were evaluated according to the scoring criteria for hadas .
scores from 6 to 15 indicated a mild reaction , and scores from 16 to 25 indicated a moderate reaction .
morning fasting venous blood ( 3  ml ) was collected , centrifuged at 4000  r / min for 10  min to separate serum , and stored at 80c before assay .
human serum il-17a , tnf- , and il-10 elisa kits were purchased from r&d systems ( abingdon , uk ) .
serum il-17a , tnf- , and il-10 levels were detected according to the manufacturer 's instructions .
sod and mda assay kits were purchased from the nanjing jiancheng bioengineering institute ( nanjing , china ) .
briefly , the serum of subjects was mixed with the reagents and incubated for 40  min at 37c . after the reaction , absorbance at 560 
the samples were then mixed with trichloroacetic acid and incubated for 40  min at 95c .
all data are presented as the arithmetic mean values ( sd ) or mean ( se , 95% ci ) .
the changes in hadas scores and levels of serum factors between hadas and control group were examined across the study time course with a linear mixed effects modeling approach , and age and times were considered as covariates .
correlations of scores , sod , mda , il-17a , il-10 , tnf- , and ages of hadas subjects at baseline were analyzed using pearson 's correlation test .
the effect of cytokine with hadas occurrence and severity was analyzed via covariance adjusted logistic regression analysis , respectively , and age was considered as covariance .
based on logistic regression analysis , significant variables associated with hadas occurrence were incorporated into a multivariate logistic regression model .
receiver operating characteristic ( roc ) curve analysis was used to determine the value of serum il-17a and il-10 levels and in predicting hadas .
the hadas scores were evaluated by physicians on the 3rd , 50th , and 100th days upon returning to low - altitude areas following 6 months of exposure to high altitudes and are shown in figure 1 .
the scores of subjects were 13.59 ( 0.41 , 12.7714.40 ) at baseline , 5.72 ( 0.25 , 5.226.22 ) at the 50th day , and 3.06 ( 0.19 , 2.693.43 ) at the 100th day , respectively .
the differences in mean scores between baseline and the 50th day and 100th day were 7.87 ( 0.18 , 7.418.32 ) ( p < 0.001 ) and 10.52 ( 0.23 , 9.5411.09 ) ( p < 0.001 ) , respectively .
the difference in mean scores between the 50th day and 100th day was 2.66 ( 0.10 , 2.422.90 ) ( p < 0.001 ) .
the serum concentrations of sod , mda , il-17a , il-10 , and tnf- were measured and compared between hadas and control groups ( figures 2 and 3 ) . at baseline ,
mean serum sod and il-10 levels were significantly lower in the hadas group [ serum sod : 59.25 ( 0.52 , 58.2260.28 )  nu / ml versus 83.39 ( 0.67 , 82.0884.71 )  nu / ml , p < 0.001 ; serum il-10 : 42.29 ( 0.41 , 41.4743.10 )  pg / ml versus 56.55 ( 0.53 , 55.5157.59 )  pg / ml , p < 0.001 ] .
serum levels of sod and il-10 were not significantly different between hadas and control groups at the 50th day and 100th day [ serum sod at the 50th day : 81.63 ( 0.52 , 80.6082.66 )  nu / ml versus 82.68 ( 0.67 , 81.3784.00 )  nu / ml , p = 0.21 ; serum il-10 at the 50th day : 56.22 ( 0.41 , 55.4157.03 )  pg / ml versus 57.01 ( 0.53 , 55.9758.05 )  pg / ml , p = 0.24 ; serum sod at the 100th day : 83.58 ( 0.52 , 82.5584.61 )  nu / ml versus 84.24 ( 0.67 , 82.9285.56 )  nu / ml , p = 0.44 ; serum il-10 at the 100th day : 56.68 ( 0.41 , 54.8756.49 )  pg / ml versus 55.52 ( 0.53 , 54.4856.56 )  pg / ml , p = 0.81 ] . moreover ,
serum sod and il-10 levels of hadas subjects were lower at baseline than at the 50th and 100th days ( p < 0.001 ) .
mean serum levels of mda , il-17a , and tnf- were significantly higher in the hadas group at baseline [ serum mda : 7.64 ( 0.03 , 7.597.69 )  mol / ml versus 4.89 ( 0.04 , 4.824.96 )  mol / ml , p < 0.001 ; serum il-17a : 821.41 ( 9.52 , 802.68840.14 )  pg / ml versus 578.38 ( 12.17 , 554.44602.33 )  pg / ml , p < 0.001 ; serum tnf- : 259.03 ( 1.48 , 256.16261.89 ) 
pg / ml versus 231.11 ( 1.86 , 227.46233.57 )  pg / ml , p < 0.001 ] .
compared with the control group , serum il-17a was higher at the 50th day [ 747.36 ( 9.52 , 728.63766.09 )  pg / ml versus 568.58 ( 12.17 , 544.63592.52 )  pg / ml , p < 0.001 ] .
il-17a concentrations were not significantly different between hadas and control groups at the 100th day [ 566.90 , 9.52 ( 548.17585.64 ) 
pg / ml versus 573.12 , 12.17 ( 549.17597.06 )  pg / ml , p = 0.69 ] .
serum mda and tnf- levels were not significantly different between hadas and control groups at the 50 and 100th days [ serum mda at the 50th day : 4.79 ( 0.03 , 4.744.85 )  mol / ml versus 4.819 ( 0.04 , 4.754.89 )  mol / ml , p = 0.56 ; serum tnf- at the 50th day : 232.19 ( 1.46 , 229.32235.05 )  pg / ml versus 229.91 ( 1.86 , 226.24233.57 )  pg / ml , p = 0.34 ; serum mda levels at the 100th day : 4.86 ( 0.03 , 4.814.92 )  mol / ml versus 4.85 ( 0.04 , 4.784.92 )  mol / ml , p = 0.83 ; tnf- levels at the 100th day : 232.19 ( 1.46 , 229.32235.05 )  pg / ml versus 229.91 ( 1.86 , 226.24233.57 )  pg / ml , p = 0.34 ] . in addition , serum mda , il-17a , and tnf- levels of hadas subjects were higher at baseline than at the 50th and 100th days ( p < 0.001 ) , and the il-17a concentration at the 50th day was higher than at the 100th day ( p < 0.001 ) . to understand correlations of scores , sod , mda , il-17a , il-10 , and tnf- of hadas subjects at baseline , all baseline data from hadas subjects were subjected to correlation analysis ( table 2 ) .
hadas score was positively correlated with serum levels of il-17a and tnf- ( r = 0.44 , p < 0.001 ; r = 0.67 , p < 0.001 , resp . ) and negatively correlated with serum levels of il-10 ( r = 0.56 , p < 0.001 ) . however
, hadas score was not correlated with serum sod or mda or age of hadas subjects ( p > 0.05 ) . serum il-17a level
was negatively correlated with il-10 and tnf- levels ( r = 0.33 , p = 0.003 ; r = 0.33 , p = 0.003 , resp . ) .
however , serum il-17a level was not correlated with serum sod or mda or age of hadas subjects ( p > 0.05 ) .
it is interesting that serum il-10 level was negatively correlated with tnf- level and subject ages ( r = 0.45 , p < 0.001 ; r = 0.24 , p = 0.03 , resp . ) .
moreover , there was no correlation between il-10 and sod and mda ( p > 0.05 ) .
correlations of il-17a , tnf- , and il-10 with hadas occurrence and severity were analyzed via logistic regression .
these data showed that serum il-17a level was associated with hadas severity ( p < 0.05 ) ( table 3 ) , and serum il-17a , tnf- , and il-10 levels were associated with hadas occurrence ( p < 0.01 ) .
in addition , sod and mda levels and subject ages were not significantly associated with hadas occurrence ( p = 0.984 , p = 0.994 , and p = 0.984 , resp . ) .
multivariate logistic regression analysis showed that il-17a level ( or = 1.025 , p = 0.044 ) and il-10 level ( or = 0.681 , p = 0.017 ) were predictive of hadas ( table 4 ) . to study further the role of serum levels of il-17a and il-10 in hadas ,
these data demonstrated that serum il-17a predicted hadas with sensitivity of 93.9% and specificity of 77.7% [ auc = 0.941  0.025 ( se ) , 95% ci : 0.8920.991 , p < 0.001 ] .
these data demonstrated that a serum il-17a level > 726.41  pg / ml predicted hadas with sensitivity of 83.8% and specificity of 93.9% [ auc
= 0.941  0.025 ( se ) , 95% ci : 0.8920.991 , p < 0.001 ] .
a serum il-10 level < 48.76  pg / ml predicted hadas with specificity of 89.2% and sensitivity of 93.9% [ auc = 0.973  0.022 ( se ) , 95% ci : 0.0001.000 , p <
it is interesting that combining il-17a and il-10 levels provided an additional benefit for predicting hadas with specificity of 97.3% and sensitivity of 97.0% [ auc = 0.984  0.016 ( se ) , 95% ci : 0.0001.000 , p < 0.001 ] .
the findings indicated that the combination of serum il-17a and il-10 levels was a better diagnostic predictor of hadas than serum il-17a or il-10 levels alone .
our data showed that the hadas group had a significantly higher serum level of il-17a compared with the control group at baseline and the 50th day and serum levels of il-17a and hadas score decreased over time in the hadas group .
furthermore , serum levels of mda and tnf- were significantly higher and sod and il-10 levels were lower in the hadas group than in the control group at baseline .
serum levels of il-17a , il-10 , and tnf- were significantly correlated with hadas score .
serum il-17a was correlated with hadas severity , and serum levels of il-17a and il-10 were significantly correlated with hadas occurrence rate .
thus , serum levels of il-17a and il-10 could be novel diagnostic predictors of hadas .
sod is an antioxidant that reduces oxidative stress and protects tissues from damage induced by ros .
suppression of serum sod and elevation of mda levels was observed in hadas subjects at baseline .
serum sod and mda levels were not significantly different between the hadas and control groups at the 50th and 100th days .
this suggests that the levels of oxidative stress in hadas subjects returned to normal levels .
in addition , serum levels of sod and mda in hadas subjects were not correlated with hadas scores , severity , or occurrence rate at baseline .
this could be because baseline serum levels of sod and mda reflected the very early stages of oxidative stress and antioxidant status in hadas subjects , and they were indirectly involved in the appearance of symptoms .
h / r - mediated oxidative stress could induce release of mediators of inflammation and activate many signaling pathways , which result in cellular apoptosis and tissue damage [ 21 , 2529 ] . in our study , levels of proinflammatory mediators , tnf- and il-17a , were elevated , and levels of anti - inflammatory factor il-10 were reduced at baseline in the hadas group .
this data suggests that an h / r - induced inflammatory response was involved in the occurrence of hadas . moreover , serum levels of il-17a decreased over time in the hadas group and were not different between hadas and control groups at 100  d. similar to serum levels of sod and mda , serum levels of il-10 and tnf- were not significantly different from the control group at the 50th and 100th days .
these data suggest that the h / r - induced inflammatory response decreased with time .
cytokine il-17a is a member of the il-17 family , and it is the hallmark cytokine of th17 cells .
serum levels of il-17a are elevated in several chronic inflammatory diseases [ 3032 ] , and il-17a plays an important role in regulating inflammatory mediators and the inflammatory response .
the mechanism of il-17a in inflammation involves mediating recruitment of neutrophils to sites of inflammation and activating a number of proinflammatory chemokines and matrix metalloproteases .
recently , the role of il-17a in h / r or ischemia / reperfusion ( i / r ) has been explored .
barry and colleagues showed that elevation of il-17a played a fundamental role in inflammation and apoptotic response in myocardial i / r injury .
similarly , another study demonstrated that levels of serum il-17a were increased by neutrophils and cd4 t cells produced following experimental i / r injury in mice .
friedrich and colleagues   showed that il-17a strongly induced tnf- expression in inflammatory bowel disease .
xue and colleagues   showed that increased il-17a not only enhanced production of proinflammatory cytokines but also impaired production of anti - inflammatory factors such as il-10 , resulting in renal tissue injury after i / r .
.   demonstrated that il-17a played a critical role in intestinal , renal , and liver injury after i / r , and il-17a knockout or inactivation significantly alleviated intestinal i / r injury and subsequent liver and kidney dysfunction .
based on the above research , we speculated that il-17a may play a more important role than other inflammatory mediators , such as il-10 and tnf- , in the occurrence of hadas .
the diagnostic and scoring criteria of hadas   included the essential diagnostic criteria , auxiliary diagnostic criteria , and symptom scores of hadas .
twenty - one symptoms had to be evaluated in scoring of hadas symptoms by physicians and researchers .
furthermore , symptom severity of hadas subjects was based on subjective feelings , which resulted in some error for hadas symptom scores .
the present findings showed that serum levels of il-17a , tnf- , and il-10 were significantly correlated with the hadas score .
it is interesting that il-17a level was significantly associated not only with severity of hadas , but also with hadas occurrence based on multivariate logistical regression analysis .
serum level of il-10 also correlated with occurrence of hadas , and there was no correlation between il-10 level and hadas severity .
this indicated that the serum level of il-17a was a better independent predictor of hadas than il-10 .
in addition , roc indicated that the combination of serum levels for il-17a and il-10 was a better predictor of hadas occurrence than serum levels of il-17a or il-10 alone .
these data suggest that a combination of serum levels for il-17a and il-10 may be a novel diagnostic predictor of hadas .
first , all of the subjects were male , and the majority of subjects were an average of 25 years old .
second , the spans between evaluation time points were too long to evaluate some parameters such as sod , mda , il-10 , and tnf- , which had already returned to normal levels in the hadas group .
h / r - mediated oxidative stress and an inflammatory response are involved in occurrence of hadas , and il-17a level could be a novel predictive index of hadas .
 high - altitude deacclimatization syndrome ( hadas ) is emerging as a severe public health issue that threatens the quality of life of individuals who return to lower altitude from high altitude . in this study , we measured serum levels of sod , mda , il-17a , il-10 , tnf- , and hadas score in hadas subjects at baseline and 50th and 100th days and to evaluate the relationship between interleukins , including il-17a , and hadas . 
 our data showed that and the serum il-17a levels and hadas score decreased over time in the hadas group , and serum il-17a levels were significantly higher in the hadas group at baseline and 50th day compared with controls ( p < 0.05 ) . 
 furthermore , baseline serum levels of mda and tnf- were significantly higher , while sod and il-10 levels were lower in hadas subjects compared with controls ( p < 0.05 ) . 
 it is interesting that serum levels of il-17a were clearly interrelated with hadas incidence and severity ( p < 0.05 ) . 
 roc curve analysis showed that combined serum il-17a and il-10 levels were a better predictor of hadas incidence than serum levels of il-17a or il-10 alone . 
 these data suggest that serum levels of il-17a are a novel predictive index of hadas . 
1. Introduction
2. Methods
3. Results
4. Discussion