the trace element selenium ( se ) is essential for a number of cellular functions .
these include glutathione peroxidase ( gpx ) family members , including cellular , extracellular , phospholipid hydroperoxide and gastrointestinal gpx .
the essentiality of se relates to its incorporation in the amino acid selenocysteine , which is present in the active sites of gpx and other selenoproteins .
however , redox - active se compounds may function both as pro - oxidants and antioxidants depending on species , concentration and the redox state of the cell .
about half of the se in whole blood is found in the plasma , of which selenoprotein p ( sepp1 ) usually constitutes more than 60% and gpx constitutes 25% .
sepp1 contains up to 10 selenocysteine residues and is the means of transporting se around the body to tissues that need it .
it has an antioxidative role in the blood and , among other biological effects , it is believed to be active in endothelial protection . in order to obtain the maximum level of sepp1
, an intake of about total 100  g / day was needed in an adult uk population , meaning that for that population a supplemental intake 50  g / day , in addition to the habitual intake of ~55  g / day , was needed .
this is mainly explained by varying se content in the soil , although different food habits may have a role .
the soils in north america have a significantly higher content of se than in europe ; thus , the reported serum se levels of us citizens are generally above 120  g / l , whereas levels below 90  g / l have been reported from several european countries . the nordic countries ( finland , sweden , norway and denmark ) represent low - se areas in europe . in finland ,
the reported average intake of se in the 1970s was 25  g / day and the reported average plasma se level was 49.6 
g / l . however , based on the low intake , the authorities in finland decided to add se to the fertilizers in 1986 .
the present average plasma se level has therefore been raised to about 111  g / l . in norway ,
the import of wheat grains from northern america has resulted in levels around 90  g / l in whole blood corresponding to a mean daily intake of about 50  g / day , which is higher than in sweden and denmark .
g / l in a population of more than 1900 swedish persons , indicating a daily intake well below the recommended lower limit .
harris et al . reported a very low se intake of only 24.7  g / day in a swedish population in which breast cancer was later diagnosed .
roman vinas et al . reported that between 31% and 47% of an adult and elderly population during 19971998 had an intake of se below the estimated average requirement .
these data indicate low se levels in the swedish population in general ; however , few studies have investigated se levels nor a possible association between low se levels and negative health effects .
the aim of the present study was to investigate whether there is still low se status among elderly swedes and , furthermore , to analyze whether low se status is associated with altered mortality in the swedish population investigated .
the participants in this study were recruited from a rural municipality of 10  300 inhabitants in the south - east of sweden .
all citizens in the municipality aged between 70 and 80 years were invited to participate in the study as part of a continuous epidemiological project in the municipality that had been running since 1998 . from a population of 1130 individuals living in the municipality of the chosen age range ,
876 individuals agreed to participate and of those 675 agreed to participate in a follow - up project that started in january 2003 and concluded in february 2010 .
of the 675 that consented to participate , 668 agreed that blood samples could be drawn and they were thus included in the present study for analysis of se at the onset of the study .
this population constituted the basal se measurement population and the population from which mortality was analyzed .
owing to the fact that 219 participants were randomized to participate in another study , they did not undergo the further risk evaluations of the present study .
thus , the final study population for the current risk evaluation consists of 449 individuals . out of the 449 individuals , serum samples from 98 subjects
a new clinical history was recorded , a clinical examination was performed , doppler echocardiography and a new electrocardiogram were carried out and the new york heart association functional class was assessed .
the latter grades how a patient with heart disease experiences symptoms of tiredness , breathlessness or chest pain ; it is graded from i to iv , where iv is symptoms already at rest .
the mortality information was obtained from the national board of health and welfare in sweden , which registers all deaths of swedish citizens .
the study was approved by the regional ethical committee ( diary number 03176 ) and conforms to the ethical guidelines of the 1975 declaration of helsinki .
the blood samples were collected while the participants were resting and in a supine position .
the vials were centrifuged at 3000  g , + 4  c , plasma was transferred to new vials and was frozen at 70  c .
the serum se analyses were performed using inductively coupled plasma mass spectrometry methodology at an agilent 700 platform at kompetenzzentrum fr komplementrmedizinische diagnostik , zweigniederlassung der synlab mvz leinfelden gmbh ( leinfelden - echterdingen , germany ) .
the accuracy of the measurements was checked by analyzing two external reference materials with certified values of 63 and 103  g / l ( control programme offered by the society for advancement of quality assurance in medical laboratories , instand e.v . , dsseldorf , germany ) , showing values within 90%110% of certified concentrations . a round - robin test with instand e.v .
the precision of the method , checked by repetitive analyses of the same sera , showed an average coefficient of variation of 5.7% .
the doppler echocardiographic examinations ( accuson xp-128c , siemens healthcare , siemens ag , erlangen , germany ) were performed with the participant in the left lateral position .
the cardiac function expressed as ejection fraction readings were categorized into four classes , with interclass limits placed at 30% , 40% and 50% .
normal systolic function was defined as ejection fraction 50% , whereas severely impaired systolic function was defined as ejection fraction < 30% .
the student 's unpaired two - sided t - test was used for continuous variables . for variables on the continuous scale level analysis of variance evaluations
have been performed , and for variables on the nominal scale level multiple -tests have been performed .
meier analyses were based on 449 participants illustrating the association between levels of se and all - cause , and cardiovascular mortality during up to 2500 days .
basal level of se was analyzed in 668 participants and the values were ordered into quartiles where the first quartile ( that is , levels < 57.2  g / l ) was evaluated against the second , third and fourth quartiles .
censored participants were those still living at the end of the study or those who had died for reasons other than all - cause / cardiovascular diseases .
completed participants were those who had died owing to all - cause / cardiovascular diseases .
risk assessment in the serum se quartiles was performed using univariate and multivariate cox proportional hazard regressions .
factors that influence the measured se levels include certain diseases and smoking . in the multivariate model ,
adjustments were made for the following clinical variables : male gender , smoking , ischemic heart disease , diabetes , chronic obstructive pulmonary disease and ejection fraction < 40% according to echocardiography .
p - values < 0.05 were considered significant , based on two - sided evaluations .
the participants in this study were recruited from a rural municipality of 10  300 inhabitants in the south - east of sweden .
all citizens in the municipality aged between 70 and 80 years were invited to participate in the study as part of a continuous epidemiological project in the municipality that had been running since 1998 . from a population of 1130 individuals living in the municipality of the chosen age range ,
876 individuals agreed to participate and of those 675 agreed to participate in a follow - up project that started in january 2003 and concluded in february 2010 .
of the 675 that consented to participate , 668 agreed that blood samples could be drawn and they were thus included in the present study for analysis of se at the onset of the study .
this population constituted the basal se measurement population and the population from which mortality was analyzed .
owing to the fact that 219 participants were randomized to participate in another study , they did not undergo the further risk evaluations of the present study .
thus , the final study population for the current risk evaluation consists of 449 individuals . out of the 449 individuals , serum samples from 98 subjects
a new clinical history was recorded , a clinical examination was performed , doppler echocardiography and a new electrocardiogram were carried out and the new york heart association functional class was assessed .
the latter grades how a patient with heart disease experiences symptoms of tiredness , breathlessness or chest pain ; it is graded from i to iv , where iv is symptoms already at rest .
the mortality information was obtained from the national board of health and welfare in sweden , which registers all deaths of swedish citizens .
the study was approved by the regional ethical committee ( diary number 03176 ) and conforms to the ethical guidelines of the 1975 declaration of helsinki .
the blood samples were collected while the participants were resting and in a supine position .
the vials were centrifuged at 3000  g , + 4  c , plasma was transferred to new vials and was frozen at 70  c .
the serum se analyses were performed using inductively coupled plasma mass spectrometry methodology at an agilent 700 platform at kompetenzzentrum fr komplementrmedizinische diagnostik , zweigniederlassung der synlab mvz leinfelden gmbh ( leinfelden - echterdingen , germany ) .
the accuracy of the measurements was checked by analyzing two external reference materials with certified values of 63 and 103  g / l ( control programme offered by the society for advancement of quality assurance in medical laboratories , instand e.v . ,
dsseldorf , germany ) , showing values within 90%110% of certified concentrations . a round - robin test with instand e.v .
the precision of the method , checked by repetitive analyses of the same sera , showed an average coefficient of variation of 5.7% .
the doppler echocardiographic examinations ( accuson xp-128c , siemens healthcare , siemens ag , erlangen , germany ) were performed with the participant in the left lateral position .
the cardiac function expressed as ejection fraction readings were categorized into four classes , with interclass limits placed at 30% , 40% and 50% .
normal systolic function was defined as ejection fraction 50% , whereas severely impaired systolic function was defined as ejection fraction < 30% .
the student 's unpaired two - sided t - test was used for continuous variables . for variables on the continuous scale level analysis of variance evaluations
have been performed , and for variables on the nominal scale level multiple -tests have been performed .
meier analyses were based on 449 participants illustrating the association between levels of se and all - cause , and cardiovascular mortality during up to 2500 days .
basal level of se was analyzed in 668 participants and the values were ordered into quartiles where the first quartile ( that is , levels < 57.2  g / l ) was evaluated against the second , third and fourth quartiles .
censored participants were those still living at the end of the study or those who had died for reasons other than all - cause / cardiovascular diseases .
completed participants were those who had died owing to all - cause / cardiovascular diseases .
risk assessment in the serum se quartiles was performed using univariate and multivariate cox proportional hazard regressions .
factors that influence the measured se levels include certain diseases and smoking . in the multivariate model ,
adjustments were made for the following clinical variables : male gender , smoking , ischemic heart disease , diabetes , chronic obstructive pulmonary disease and ejection fraction < 40% according to echocardiography .
p - values < 0.05 were considered significant , based on two - sided evaluations .
the characteristics of the study population ( n=668 ) divided into the four quartiles of se are presented in table 1 .
similar gender ( female to male ) ratios were found in the different quartiles , except in the fourth quartile . in the first quartile ,
the group was older and more participants in the third quartile were in the new york heart association functional class ii .
no significant difference was found between the four quartiles with respect to any other variable , including laboratory measures of inflammation . at inclusion ,
serum se levels were determined again after 48 months in a subset of 98 randomized individuals . in these individuals , the values from the second measurement ( for females : 70.8 
g / l in average and for males : 72.5  g / l in average ) showed higher concentrations at the second measurement after 48 months .
a detailed analysis of the changes in serum se concentration is shown in table 3 .
the current smokers had a lower serum se concentration ( mean 63.1  g / l ; s.d .
17.3 ) compared with the non - smoking group ( mean 67.7  g / l ; s.d . 16.6 ) .
of the 449 participants in the total study population in which the risk evaluations were performed , 122 participants ( 27.2% ) died of all - cause mortality and 85 participants ( 18.9% ) died of cardiovascular mortality during the follow - up period of 2500 days ( 6.85 years ) . a significant difference in mortality rates was found in persons with serum se values grouped in the lowest se ( first ) quartile versus those in the fourth quartiles . in the first quartile , 41 individuals out of 107 died versus 25 out of 111 in the fourth quartile , from all causes combined (  : 6.44 ; p=0.01 ) . regarding cardiovascular mortality , the same trend
was noted : 29/107 in the first quartile and 16/111 in the fourth quartile (  : 5.35 ; p=0.02 ) .
an evaluation of the frequency of cancer mortality in the different quartiles of serum se has also been performed .
a kaplan  meier analysis illustrating all - cause ( figure 2 ) and cardiovascular mortality ( figure 3 ) of the first quartile versus the second - to - fourth quartiles during the follow - up period in the population is shown . comparing the first quartile with the second - to - fourth quartiles , a 67% increased risk of all - cause mortality ( hazard ratio : 1.67 ; 95% confidence interval ( ci ) : 1.152.44 ; p=0.007 ) in the univariate cox proportional hazard regressions analyses
was found , and regarding cardiovascular mortality a 73% increased risk ( hazard ratio : 1.73 ; 95% ci : 1.102.72 ; p=0.02 ) was found . applying a multivariate model where well - known variables influencing risk of mortality as well as serum se
were included , an independent and significantly increased risk of 43% for all - cause mortality and 56% for cardiovascular mortality was found ( table 4 ) .
the characteristics of the study population ( n=668 ) divided into the four quartiles of se are presented in table 1 .
similar gender ( female to male ) ratios were found in the different quartiles , except in the fourth quartile . in the first quartile ,
the group was older and more participants in the third quartile were in the new york heart association functional class ii .
no significant difference was found between the four quartiles with respect to any other variable , including laboratory measures of inflammation .
at inclusion , the mean serum se concentration was 67.1  g / l ( s.d .
serum se levels were determined again after 48 months in a subset of 98 randomized individuals . in these individuals , the values from the second measurement ( for females : 70.8  g / l in average and for males : 72.5  g / l in average ) showed higher concentrations at the second measurement after 48 months .
a detailed analysis of the changes in serum se concentration is shown in table 3 .
the current smokers had a lower serum se concentration ( mean 63.1  g / l ; s.d .
17.3 ) compared with the non - smoking group ( mean 67.7  g / l ; s.d . 16.6 ) .
the participants were evaluated regarding possible association between se level and mortality . of the 449 participants in the total study population in which the risk evaluations were performed , 122 participants ( 27.2% ) died of all - cause mortality and 85 participants ( 18.9% ) died of cardiovascular mortality during the follow - up period of 2500 days ( 6.85 years ) . a significant difference in mortality rates
was found in persons with serum se values grouped in the lowest se ( first ) quartile versus those in the fourth quartiles . in the first quartile , 41 individuals out of 107 died versus 25 out of 111 in the fourth quartile , from all causes combined (  : 6.44 ; p=0.01 ) . regarding cardiovascular mortality , the same trend was noted : 29/107 in the first quartile and 16/111 in the fourth quartile (  : 5.35 ; p=0.02 ) .
an evaluation of the frequency of cancer mortality in the different quartiles of serum se has also been performed .
no difference in cancer mortality was found ( data not shown ) . a kaplan  meier analysis illustrating all - cause ( figure 2 ) and cardiovascular mortality ( figure 3 ) of the first quartile versus the second - to - fourth quartiles during the follow - up period in the population is shown . comparing the first quartile with the second - to - fourth quartiles ,
a 67% increased risk of all - cause mortality ( hazard ratio : 1.67 ; 95% confidence interval ( ci ) : 1.152.44 ; p=0.007 ) in the univariate cox proportional hazard regressions analyses was found , and regarding cardiovascular mortality a 73% increased risk ( hazard ratio : 1.73 ; 95% ci : 1.102.72 ; p=0.02 ) was found . applying a multivariate model where well - known variables influencing risk of mortality as well as serum se
were included , an independent and significantly increased risk of 43% for all - cause mortality and 56% for cardiovascular mortality was found ( table 4 ) .
in the present study , serum se concentrations from a population of elderly healthy swedes were low ( ~ 67.1  g / l ) compared with the reported se levels in the united states and also relatively low in comparison with values in other regions in europe .
the difference in concentrations between smokers and non - smokers in the present study was not significant , which may be an effect of the small size of the smoking group ( n=40 ; 9.0% ) , as some studies have found decreased levels in current smokers . however , there are also studies where such an association has not been found .
the present nordic recommendations are 50  g / day for females and 60  g / day for males .
the serum se concentrations measured in our study are lower than the values achieved by these recommendations .
thus , a recent norwegian study indicated that an average dietary intake of 50  g se / day resulted in serum levels around 90  g / l .
the most interesting finding in our study was a significantly increased mortality among those in the lowest se quartile ( < 57.2 
this segment of the population had 43% increased risk for all - cause mortality and 56% increased risk for cardiovascular mortality .
as a low se level could be a result of disease , we therefore evaluated the low se level and adjusted for well - known clinical conditions that could influence mortality risk .
low se level was still a significant and independent risk factor for both all - cause and cardiovascular mortality , and the difference in mortality could not obviously be explained by different inflammatory response , as two biomarkers for inflammation did not differ across the four se quartiles .
suadacini et al . from denmark showed in a prospective cohort of more than 3000 males that serum se concentrations below 60  g / l were associated with increased risk of myocardial infarction .
finnish studies from the 1970s have also observed this association between low serum se and increased risk of cardiovascular mortality .
a study of an elderly french population , referred to as the eva study , also reported an association between low plasma concentration of se and increased mortality . in a small study from spain , milln adame et al .
reported that half of the evaluated population was se deficient , as their plasma concentration was insufficient for maximal gpx activity , which requires 90  g / l .
letsiou et al . reported in a study on more than 500 individuals from greece that > 87% of the participants had a lower se concentration than needed for optimal function of gpx .
the same message was reported from the czech republic , where a majority of the 3000 blood donors investigated had lower se concentration than needed for optimal activity of gpx . finally , in 100 individuals living in central poland , a mean serum se concentration of 51.1  g /
l was reported , which the authors showed was lower than required for cytoprotective gene expression .
these observations are also consistent with a meta - analysis of se and coronary heart disease , which included 25 observational ( 14 cohort and 11 case  control studies ) and six randomized controlled trials . in the meta - analysis ,
the pooled relative risks between the highest and lowest blood or toenail se categories were 0.85 ( 95% ci 074099 ) .
a higher mean baseline se intake was found in the six randomized controlled trials mentioned above , compared with our study . however , a more recent meta - analysis of 12 different trials by rees et al .
( including the american selenium and vitamin e cancer prevention trial ( select ) and nutritional prevention of cancer ( npc ) trial ) could not support the finding that supplementation with se decreased cardiovascular mortality .
it should , however , be noted as mentioned by hatfield and gladyshev that both the select and npc studies included participants with mean plasma se levels higher than in all european countries ( 135 and 113  g / l respectively ) , and that those two studies comprised the vast majority of participants of the meta - analysis performed by rees et al .
analysis of the participant group with two se measurements in our study ( at inclusion and at 48 months ) revealed a statistically significant , but not high , correlation coefficient ( r=0.34 ) between the first and the second measurement .
this low correlation could be a result of the discussion in the community where the study was performed , resulting in a novel interest in using se supplementation .
thus , we observed that 63.3% ( 62 out of 98 ) had a higher serum se concentration at the second measurement .
the consequence is that results of those assigned to the first quartile may even be underestimated . from the basal characteristics of the study population ( table 1 )
, no difference could be found in the concentration of two biomarkers high - sensitivity c - reactive protein and soluble p - selectin between the four quartiles of se , showing that it is unlikely that the mortality difference result from different levels of inflammation .
one possible mechanism for the association between low se and mortality could be found in the fact that selenoproteins ( the gpx family of proteins , txnrd and sepp1 ) have a central role in the defense against oxidative stress , and that increased oxidative stress is reported in patients with ischemic heart disease . in heart failure ,
the balance between cytosolic and mitochondrial reactive oxygen species have been reported to become distorted , leading to oxidative imbalance , mitochondrial dysfunction and eventually to cell death . as an insufficient level of se will result in only partially functioning anti - oxidative defense
, this could explain the increased mortality in the present study among those with low serum se .
in the present study , serum se levels in 668 elderly healthy swedish persons were analyzed . the mean concentration , 67.1  g / l , is insufficient to achieve optimum function of important selenoproteins .
significantly increased all - cause mortality and cardiovascular mortality were observed in persons with low serum se concentrations in an almost 7-year follow - up .
as indicated in our previous report from the same swedish region , we suggest that se supplementation should be recommended to persons with serum se concentration below 57  g / l .
 background / objectives : the daily dietary intake of selenium ( se ) , an essential trace element , is still low in sweden in spite of decades of nutritional information campaigns and the effect of this on the public health is presently not well known . 
 the objective of this study was to determine the serum se levels in an elderly swedish population and to analyze whether a low se status had any influence on mortality.subjects/methods:six-hundred sixty - eight ( n=668 ) elderly participants were invited from a municipality and evaluated in an observational study . 
 individuals were followed for 6.8 years and se levels were re - evaluated in 98 individuals after 48 months . 
 clinical examination of all individuals included functional classification , echocardiography , electrocardiogram and serum se measurement . 
 all mortality was registered and endpoints of mortality were assessed by kaplan  meier plots , and cox proportional hazard ratios adjusted for potential confounding factors were calculated.results:the mean serum se level of the study population ( n=668 ) was 67.1  g / l , corresponding to relatively low se intake . 
 after adjustment for male gender , smoking , ischemic heart disease , diabetes , chronic obstructive pulmonary disease and impaired heart function , persons with serum se in the lowest quartile had 43% ( 95% confidence interval ( ci ) : 1.022.00 ) and 56% ( 95% ci : 1.032.36 ) increased risk for all - cause and cardiovascular mortality , respectively . 
 the result was not driven by inflammatory effects on se concentration in serum.conclusion:the mean serum se concentration in an elderly swedish population was 67.1  
 g / l , which is below the physiological saturation level for several selenoprotein enzymes . 
 this result may suggest the value of modest se supplementation in order to improve the health of the swedish population . 
Introduction
Materials and methods
Participants
Blood samples
Se analysis
Echocardiography
Statistics
Results
Population baseline characteristics
Serum Se concentrations
Mortality rates in persons with low versus high Se
Discussion
Conclusion