***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: PURPOSE Exercise is associated with a decrease in bone mineral density under certain conditions . One potential mechanism is increased bone resorption due to an exercise-induced increase in parathyroid hormone ( PTH ) , possibly triggered by dermal calcium loss . The purpose of this investigation was to determine whether calcium supplementation either before or during exercise attenuates exercise-induced increases in PTH and C-terminal telopeptide of Type I collagen ( CTX ; a marker of bone resorption ) . METHODS Male endurance athletes ( n = 20 ) completed three 35-km cycling time trials under differing calcium supplementation conditions : 1 ) 1000 mg of calcium 20 min before exercise and placebo during , 2 ) placebo before and 250 mg of calcium every 15 min during exercise ( 1000 mg total ) , or 3 ) placebo before and during exercise . Calcium was delivered in a 1000-mg·L(-1 ) solution . Supplementation was double-blinded , and trials were performed in r and om order . PTH , CTX , bone-specific alkaline phosphatase ( BAP ; a marker of bone formation ) , and ionized calcium ( iCa ) were measured before and immediately after exercise . RESULTS CTX increased and iCa decreased similarly in response to exercise under all test conditions . When compared with placebo , calcium supplementation before exercise attenuated the increase in PTH ( mean ± SE : 55.8 ± 15.0 vs 74.0 ± 14.2 pg·mL(-1 ) , P = 0.04 ) ; there was a similar trend ( 58.0 ± 17.4 , P = 0.07 ) for calcium supplementation during exercise . There were no effects of calcium on changes in CTX , BAP , and iCa . CONCLUSIONS Calcium supplementation before exercise attenuated the disruption of PTH . Further research is needed to determine the effects of repeated increases in PTH and CTX on bone ( i.e. , exercise training ) and whether calcium supplementation can diminish any exercise-induced demineralization Context : Low bone mineral density ( BMD ) is a serious public health problem . Osteoporotic fractures are associated with low bone mass , occurring frequently in the hip and spine . Previous studies have demonstrated a positive relationship between BMD and weightbearing exercise but not a similar positive relationship with nonweightbearing exercise . There is concern that cycling , a weight-supported sport , does not benefit bone health . Objective : To systematic ally review the evidence suggesting that cyclists have impaired bone health at the femoral neck and lumbar spine . Data Sources : Articles in PubMed , Cochrane Library , and CINAHL were identified in December 2009 What is the start date for the search ? based on the following terms and combinations : bicycling , bone density , cyclist . Study Selection : Thirteen studies satisfied inclusion criteria : 2 prospect i ve studies ( level of evidence 2b ) and 11 cross-sectional studies ( level of evidence 2c ) . Data Extraction : Data included sample size , demographics , description of cycling and control criteria , and BMD ( g/cm2 ) at the lumbar spine , femoral neck , and hip . Results : Two prospect i ve studies showed a decrease in femoral neck , total hip , or lumbar spine BMD in cyclists over the study period . Four cross-sectional studies compared cyclists with sedentary controls , and 3 found cyclists ’ lumbar spine and femoral neck BMD similar to that of controls , whereas 1 found cyclists ’ BMD to be lower than that of controls . Seven cross-sectional studies compared cyclists with active controls : 2 found no differences in femoral neck and lumbar spine BMD between cyclists and controls ; 4 found that cyclists had lower lumbar spine BMD than did active controls , including runners ; and 1 reported a trend toward lower lumbar spine BMD in cyclists versus controls . Conclusions : There is concerning but inconsistent , limited- quality disease-oriented evidence —primarily from cross-sectional data —indicating that cyclists may be at risk for low bone mass , particularly at the lumbar spine . Additional longitudinal controlled intervention trials are needed The focus of this prospect i ve , observational study was to determine the effect of sport-specific training and calcium intake on bone mineral density ( BMD ) in female master cyclists , runners and non-athletes . Thirty women ( 12 cyclists , 9 runners , 9 controls ) , mean age of 49.6 + /- 7.9 years , were assessed at baseline and 18 months for calcium intake ( 4-day records ) , current exercise activity ( recall question naire ) , and BMD of the lumbar spine and hip ( DXA ) . A three ( cyclists , runners , controls ) by two ( 0 and 18 months ) repeated measures ANOVA demonstrated a significant interaction effect of sport and time at the femoral neck ( p < .04 ) . Tukey post hoc analysis indicated that the BMD of the femur was maintained in cyclists and runners but declined in controls ( p < .05 ) . A significant time effect was noted in BMD at the lumbar spine ( p < . 001 ) and the trochanter ( p < .003 ) . BMD of the lumbar spine was maintained in runners but declined in cyclists ( p < .007 ) and in controls ( p < .03 ) , while trochanteric BMD declined in all groups ( p < .01 ) . No significant interaction effect of sport and dietary calcium intake was noted for BMD at any site OBJECTIVES To examine whether different sedentary behaviours are associated with the risk of  low bone mineral content in adolescents , and if so , whether extracurricular physical-sporting activity influences this association . MATERIAL S AND METHODS A total of 277 adolescents from Zaragoza ( 168 females and 109 males ) aged 13.0 - 18.5 yr within frame work of the multicentre AVENA study participated in this study . Bone mineral content ( BMC ) , lean mass , and fat mass were measured with DXA . Physical activity and sedentary independent variables : participation in extracurricular physical-sporting activity ( PA ) , h/d of television watching , playing video/computer games during school days and on weekend days and doing homework/ study ing . They all were assessed by question naire . The main outcome was low BMC , as defined by BMC Z-score for age and sex < percentile 10 . Logistic regression was used to test the interaction and association of PA and sedentary variables with low BMC , after controlling for confounders like height , maturational status or lean mass . RESULTS Among the sedentary variables studied , only television watching > or =3 h/d was associated with an increased risk for low BMC in males ( OR , 95 % CI : 7.01 , 1.73 to 28.40 ) , after controlling for sexual maturation . When PA was in the models , television watching was not any longer associated with low BMC , while PA was so ( OR , 95 % CI : 0.23 , 0.09 to 0.55 ) . Involvement in such activity reduced the risk of low bone mass by 76 % ( P<0.01 ) independently of body mass , height and fat mass , but not of the lean mass . CONCLUSION Watching television for 3 or more h/d seems to be associated with an increased risk for  low BMC in male adolescents . However , this association is mediated by participation in PA , suggesting that negative consequences of excessive television watching on adolescent bone health could be counteracted by sport participation . Longitudinal data and r and omized controlled trials will confirm or contrast our findings UNLABELLED Male cyclists have been found to have low BMD  in cross-sectional studies . Changes in BMD values over 1 yr of training and competition were studied in 14 male cyclists .  BMD decreased significantly at the total hip , neck , trochanter , and shaft regions but not the lumbar spine . This first prospect i ve study of cyclists showed a decrease in BMD over the course of 1 yr . INTRODUCTION Cross-sectional studies have shown that some endurance athletes , and cyclists in particular , have low BMD . Whether vigorous cycle training is causally related with low BMD remains unknown . MATERIAL S AND METHODS Changes in BMD values over 1 yr of training and competition were studied in 14 male road cyclists , 27 - 44 yr of age . Subjects were r and omized to receive 1500 ( 500 mg with meals ) or 250 mg of supplemental calcium citrate daily . BMD measurements were obtained at pre- , mid- , post- , and off-season time points over 1 yr . Dermal calcium loss during exercise was estimated using a patch collection technique to examine calcium loss as a potential mediator of changes in BMD . RESULTS Using paired t-tests , BMD was found to decrease significantly from pre- to off-season at the total hip , neck , shaft , and trochanter regions ( relative changes of -1.5 + /- 2.1 % , -0.7 + /- 2.1 % , -0.9 + /- 2.1 % , and -1.0 + /- 1.2 % , respectively , all p < 0.05 ) . The 1.0 + /- 1.2 % decrease in BMD at the lumbar spine failed to reach statistical significance ( p = 0.079 ) . There were no differences in changes in BMD between the calcium supplementation groups . The 2-h dermal calcium loss was estimated at 136.5 + /- 60.5 mg . Higher dermal calcium losses were associated with lower baseline BMD values at the total hip , neck , and shaft ( all p < 0.05 ) , but were not significantly associated with changes in BMD . CONCLUSIONS This study suggests that high intensity cycle training may adversely affect BMD . Excessive dermal calcium loss during exercise may be a contributing factor , but mechanisms remain to be eluci date PURPOSE This study investigated the influence of different exercise types and differences in anatomical distribution of mechanical loading patterns on bone mineral density ( BMD ) in elite female cyclists , runners , swimmers , triathletes , and controls ( N = 15 per group ) . Associations between leg strength and BMD were also examined . METHODS Areal BMD ( g x cm(-2 ) ) was assessed by duel-energy x-ray absorptiometry ( DXA ) ( total body ( TB ) , lumbar spine ( LS ) , femoral neck ( FN ) , legs , and arms ) . Right knee flexion and extension strength was measured using a Cybex Norm isokinetic dynamometer at 60 degrees x s(-1 ) . RESULTS Runners had significantly higher unadjusted TB , LS , FN , and leg BMD than controls ( P < 0.05 ) ; higher TB , FN , and leg BMD than swimmers ( P < 0.05 ) ; and greater leg BMD than cyclists ( P < 0.05 ) . Absolute knee extension strength was significantly ( P < 0.01 ) correlated ( 0.33 < or = r < or = 0.44 ) with TB , FN , LS , and leg BMD for all groups combined . Weaker but still significant correlations ( 0.28 < or = r < or = 0.33 ) existed for normalized ( per leg lean tissue mass ) knee extension strength and all BMD sites , except FN BMD . There were no significant correlations between absolute or normalized knee flexion strength and any of the BMD variables . Absolute knee extension strength was entered as the second independent predictor for LS and leg BMD in stepwise multiple linear regression analysis ( MLRA ) , accounting for increments of 4 % and 12 % , respectively , in total explained variation . CONCLUSION We conclude that running , a weight bearing exercise , is associated with larger site-specific BMD than swimming or cycling , that the generalized anatomical distribution of loads in triathlon appears not to significantly enhance total body BMD status , and that knee extension strength is only a weak correlate and independent predictor of BMD in adolescent females This article describes the system for rating the quality of medical evidence  developed and used during creation of the Agency for Health Care Policy and Research -sponsored heart failure guideline . Previous approaches to rating evidence were not design ed for use in the setting of clinical practice guidelines . The present system is based on the tenet that flaws in research design are serious to the extent they threaten the validity of the results of studies . A taxonomy of major and minor flaws based on that tenet was developed for r and omized controlled trials and for cohort and medical registry studies . The use of the system is described in the context of two difficult clinical issues considered by the Panel : the role of coronary artery revascularization and the use of metoprolol 
    Output:
    
Despite some controversial results , it has been observed that adult road cyclists participating in regular training have low bone mineral density in key regions ( for example , lumbar spine ) .
Conversely , other types of cycling ( such as mountain biking ) , or combination with other sports could reduce this unsafe effect .
These results can not yet be explained by differences in dietary patterns or endocrine factors .
Conclusions From our comprehensive survey of the current available literature it can be concluded that road cycling does not appear to confer any significant osteogenic benefit .
The cause of this may be related to spending long hours in a weight-supported position on the bike in combination with the necessary enforced recovery time that involves a large amount of time sitting or lying supine , especially at the competitive level