hypovitaminosis d is a highly prevalent condition , especially in older adults , with an estimated prevalence of 5090% depending on the definition used and the population studied [ 1 , 2 ] .
growing evidence has established that vitamin d exhibits not only skeletal effects but also a number of nonskeletal effects [ 1 , 2 ] .
this finding explains why vitamin d plays a much greater role in human health than previously thought and why lower 25-hydroxyvitamin d ( 25ohd ) concentrations contribute to cellular dysfunction in many biological systems , which in turn is manifested by organ disorders and chronic diseases [ 14 ] .
in particular , it has been reported , among older inpatients hospitalized in geriatric acute care units , that lower serum 25ohd concentrations at the time of admission are directly associated with a greater severity of chronic diseases   and higher risks of acute decompensation   and inhospital mortality . in line with this , hypovitaminosis d has been proposed as a biomarker of longer length of stay ( los ) in acute care units [ 79 ] . for instance , lower serum 25ohd concentrations were independently associated with longer intensive care unit stay in cardiac surgical patients .
hypovitaminosis d also doubled the risk of being hospitalized more than 14 days in a geriatric acute care unit   and , in another study , the combination of hypovitaminosis d , male gender , and delirium predicted a 4.8-fold higher risk of longer los among geriatric inpatients .
importantly , the average increase in los in geriatric acute care units attributable to hypovitaminosis d remains unknown .
moreover , while existing studies report an association between hypovitaminosis d and prolonged hospitalization , it remains unclear whether an inverse linear association exists over a range of 25ohd concentrations and whether any increase in 25ohd is associated with a decrease in los , regardless of the initial level .
we had the opportunity to examine these issues in a historical cohort of older inpatients hospitalized in a geriatric acute care unit .
the objectives of this analysis were ( i ) to estimate the difference in hospitalization days between inpatients with and without hypovitaminosis d and ( ii ) to determine whether there was a linear association between serum 25ohd concentration and los .
the studied sample was a convenience of all inpatients consecutively admitted into the geriatric acute care unit of angers university hospital , france , between june and december 2008 .
study inclusion criteria included unplanned admissions to hospital for people  75  years and willingness to participate .
two hundred fifty - three inpatients met the study inclusion criteria and agreed to participate .
power analysis determined that 253 participants were sufficient to find significant correlations between serum 25ohd concentration and los with 77% power at p < 0.05 .
the study was conducted in accordance with the ethical standards set forth in the helsinki declaration ( 1983 ) .
the los was calculated in number of days using the administrative registry of angers university hospital by subtracting day of admission from day of discharge .
the age , gender , usual place of life , degree of functional independence , main reason for admission , number of acute diseases and comorbidities , use of vitamin d supplements , and creatinine clearance were assessed as part of a standardized comprehensive geriatric assessment on admission to hospital and were used as potential confounders .
functional independence was measured from 0 to 6 ( best ) with the autonomy , gerontology , iso - resources groups ( aggir ) model .
the usual place of life was also sought ( i.e. , home living versus institution dwelling defined as living in nursing home or in senior housing facilities ) .
the main reasons for admission were separated into 6 categories : gait disorders / falls / orthopedic reasons , cardiovascular disorders , respiratory disorders , digestive disorders , neuropsychiatric disorders , and social issues .
acute diseases were defined as diseases with sudden onset and rapid progression , whatever their nature or site .
comorbidities were diseases lasting at least 3 months or running a course with minimal change , whatever their nature or site .
vitamin d supplementation was systematically noted from the primary care physician 's prescription and sought by questioning the patient , whatever the dosage schedule or route of administration and regardless of the date of  commencement .
fasting early morning venous blood was collected from resting participants on the first day of hospital stay ( i.e. , the same day as the standardized anamnesis and physical examination ) for the measurement of serum 25ohd and creatinine .
serum 25ohd concentration was measured by radioimmunoassay ( diasorin inc . , stillwater , mn ) with intra- and interassay precisions of 5.2% and 11.3% , respectively .
hypovitaminosis d was defined as serum 25ohd  50  nmol / l ( to convert to ng / ml , divide by 2.496 ) according to the definition of the world health organization   and the institute of medicine .
creatinine clearance was estimated using cockcroft - gault formula ( [ ( 140  ageyears ) 
weightkg / creatininemol / l ]  1.04 for females and  1.25 for males ) .
participants ' characteristics were summarized using means and standard deviations or frequencies and percentages , as appropriate .
as the number of observations was higher than 40 , comparisons were not affected by the shape of the error distribution and no transform was applied .
first , comparisons between participants separated into two groups based on hypovitaminosis d were performed using chi - square test or student 's t - test , as appropriate .
second , univariate and multiple ( i.e. , fully adjusted direct regression and backward regression ) linear regressions were used to examine the association between los in geriatric acute care unit ( dependent variable ) and serum 25ohd concentration ( independent variable ) , while adjusting for potential confounders .
third , a pearson correlation was used to examine the relationship between serum 25ohd concentration and los in the unit .
all statistics were performed using spss ( v19.0 , ibm corporation , chicago , il ) .
among 253 inpatients included in this analysis ( mean age standard deviation86.2  6.0 years ; 66.8% female ; 100% caucasian ) , the mean serum 25ohd concentration was 33.9  21.1  nmol / l and the mean los was 14.7  8.1 days .
we found that the los was longer among older inpatients with hypovitaminosis d compared to those with adequate vitamin d status ( 15.2  8.2 days versus 12.1  7.0 days , p = 0.017 ) , underlining a mean los difference of 3.1 days between groups ( table 1 ) .
there were no other demographical and clinical differences , except for the use of vitamin d supplements ( 4.8% in participants with hypovitaminosis d versus 38.9% in their counterparts , p < 0.001 ) .
univariate and multiple linear regression models showed an inverse association between serum 25ohd concentration and los ( unadjusted  = 0.05 [ 95% confidence interval ( ci ) : 0.10 ; 0.01 ] , p = 0.028 ;  = 0.07 [ 95% ci : 0.14 ; 0.02 ] , p = 0.043 for adjusted model ;  =  0.08 [ 95% ci : 0.14 ; 0.02 ] , p = 0.015 for backward model ) .
the creatinine clearance was also retained as an explanatory variable of los in the backward model (  =  0.02 [ 95% ci : 0.05 ; 0.01 ] , p = 0.081 ) . finally ,
as illustrated in figure 1 , we found an inverse correlation between serum 25ohd concentration and los ( r = 0.14 , p = 0.028 ) .
our results show that , in a geriatric acute care unit , older inpatients with serum 25ohd  50  nmol / l at time of admission had a los extended by approximately 3 days compared to those with 25ohd > 50 
nmol / l . additionally , we report the first evidence of a linear association between 25ohd concentration and los , suggesting that any increase in 25ohd was associated with a decrease in los whatever the initial vitamin d status .
vitamin d is a secosteroid hormone that exhibits multiple biological actions mediated by the vitamin d receptor present in many cells and target tissues [ 13 ] .
since vitamin d is necessary for the regulation of cellular growth , differentiation , and function , lower vitamin d status leads to multiple organ dysfunction , disability , and unstable health status , which are all causes of deconditioning , polypharmacy , and longer los [ 1416 ] .
consistently , previous literature from our group has provided compelling evidence that hypovitaminosis d could be regarded as a biomarker of age - related disease burden [ 4 , 5 ] and consequent longer los [ 79 ] .
for instance , it has been reported that the risk of being in the highest tertile of los was multiplied by approximately 2 in case of hypovitaminosis d at baseline among geriatric inpatients .
however , the number of additional hospitalization days in case of hypovitaminosis d has not been examined yet .
thus , the results of the current study provide extra information , specifically , that older inpatients with hypovitaminosis d were hospitalized 3 days more in our geriatric acute care unit than those with adequate vitamin d status ; that is , they had a 21 percent longer los ( table 1 ) .
the clinical relevance is that longer los exposes older inpatients to greater risks of disability , nosocomial infections , deconditioning , and higher healthcare costs [ 16 , 17 ] .
consequently , any decrease in los is beneficial to older inpatients . in the current study
, we found a 0.14-day decrease in los per 1  nmol / l increase in serum 25ohd concentration ( figure 1 ) , that is , a 1-day decrease in los per 7.1  nmol / l increase in 25ohd .
it has been previously reported that vitamin d supplementation produces an elevation of serum 25ohd concentration of approximately 2.5 
it can thus be inferred that each additional 300  iu vitamin d per day is associated with a 1-day shortening of los among geriatric inpatients .
such estimates may help to justify , plan , evaluate , and compare the effectiveness of interventions aiming at shortening the los in geriatric acute care units with vitamin d supplements
firstly , the study cohort was restricted to caucasian older inpatients hospitalized in a single geriatric acute care unit who might be unrepresentative of all older inpatients .
thirdly , although we were able to control for many important characteristics that could bias the results , residual prehospital health conditions influencing both vitamin d status and los , such as the body mass index , were not considered .
finally , it has to be kept in mind that , in some cases , los does not only depend on bioclinical aspects but is influenced by other factors , including number of beds available , human resource employees , and health insurance guidelines , which could not be taken into account here .
increased serum 25ohd concentrations were linearly associated with shorter los in this cohort of acute care geriatric inpatients .
those with an adequate vitamin d status were hospitalized 3 days less than those with hypovitaminosis d. our findings reinforce the conceptualization of vitamin d as a biomarker of the health status of inpatients in geriatric acute care unit and provide an additional rationale for prescribing vitamin d supplements in this population .
multicenter randomized controlled trials are required to investigate the benefits of vitamin d supplementation on los with higher levels of evidence .
 
 background . 
 hypovitaminosis d is linked to unstable health in older adults . 
 our objectives were to determine ( i ) the difference in length of stay ( los ) in geriatric acute care unit between inpatients with and without hypovitaminosis d and ( ii ) whether there was a linear association between serum 25-hydroxyvitamin d ( 25ohd ) concentration and los . 
 methods . 
 253 inpatients admitted in 2008 to the geriatric acute care unit of angers university hospital , france , ( mean agestandard deviation , 86.2  6.0 years ; 66.8% female ) were included in this historical cohort study . 
 los was calculated by subtracting day of admission from day of discharge . 
 hypovitaminosis d was defined as 25ohd50 nmol / l at the time of admission . 
 age , gender , place of life , functional independence , reason for admission , number of acute diseases and comorbidities , use of vitamin d supplements , and creatinine clearance were used as confounders . results . 
 participants with hypovitaminosis d had longer los than their counterparts ( 15.2  8.2 days versus 12.1  7.0 days , p = 0.017 ) , underlining a mean difference of 3 days . 
 25ohd concentration inversely correlated ( r = 0.14 , p = 0.028 ) and was inversely associated with los ( adjusted  = 0.07 [ 95%ci : 0.14 ; 0.02 ] , p = 0.043 ) . conclusions . 
 we found an inverse linear association between serum 25ohd concentrations and los in a geriatric acute care unit . 
 participants with 25ohd>50  nmol / 
 l were hospitalized on average 3 days less than those with 25ohd50  nmol / l . 
1. Introduction
2. Materials and Methods
3. Results
4. Discussion
5. Conclusions