BMI and Mortality: Results From a National Longitudinal Study of Canadian Adults

Transcription

1 nture publishing group BMI nd Mortlity: Results From Ntionl Longitudinl Study of Cndin Adults Hether M. Orpn 1, Jen-Mrie Berthelot 2,3, Mrk S. Kpln 4, Dvid H. Feeny 5,6, Bentson McFrlnd 7 nd Nncy A. Ross 3 Although cler risk of mortlity is ssocited with obesity, the risk of mortlity ssocited with overweight is equivocl. The objective of this study is to estimte the reltionship between BMI nd ll-cuse mortlity in ntionlly representtive smple of Cndin dults. A smple of 11,326 respondents ged 25 in the 1994/1995 Ntionl Popultion Helth Survey (Cnd) ws studied using Cox proportionl hzrds models. A significnt incresed risk of mortlity over the 12 yers of follow-up ws observed for underweight (BMI <18.5; reltive risk (RR) = 1.73, P < 0.001) nd obesity clss II+ (BMI >35; RR = 1.36, P <0.05). Overweight (BMI 25 to <30) ws ssocited with significntly decresed risk of deth (RR = 0.83, P < 0.05). The RR ws close to one for obesity clss I (BMI 30 35; RR = 0.95, P >0.05). Our results re similr to those from other recent studies, confirming tht underweight nd obesity clss II+ re cler risk fctors for mortlity, nd showing tht when compred to the cceptble BMI ctegory, overweight ppers to be protective ginst mortlity. Obesity clss I ws not ssocited with n incresed risk of mortlity. Obesity (2009) 18, doi: /oby The World Helth Orgniztion (WHO) clssifies rnges of BMI into ctegories which re ment to represent distinct levels of risk to helth (1). Hving BMI in the ctegories of underweight (BMI <18.5 kg/m 2 ), overweight (BMI 25 kg/m 2 ), or obese (BMI 30 kg/m 2 ) is considered to be risk fctor for helth problems nd premture mortlity, nd Helth Cnd hs dopted the WHO ctegories s ntionl guidelines for body weight clssifiction (2), while defining overweight s BMI 25 to <30 kg/m 2. We retin this definition throughout the rest of the pper. Using the WHO definitions, the prevlence of overweight nd obesity is rising in Western countries such s the United Sttes, Cnd, the United Kingdom, nd Austrli, nd in countries in other world regions such s Brzil, Chin, nd Isrel (3). In Cnd, the prevlence of obesity bsed on mesured height nd weight hs lmost doubled in the lst two decdes nd now ffects 23% of the dult popultion (4). Overweight nd obesity re recognized s importnt nd rpidly growing public helth concerns. Obesity hs been linked with type 2 dibetes, crdiovsculr disese, hypertension, stroke, gllbldder disese, some forms of cncer, osteorthritis, nd psychosocil problems (5). However, lthough obesity hs been clerly identified s risk for premture mortlity (6 11), the reltionship between overweight nd premture mortlity is less cler. Severl recent studies hve presented conflicting evidence s to whether overweight is risk fctor for mortlity. Recently, Adms et l. reported tht overweight t ge 50 ws ssocited with n incresed risk of deth mong both women nd men (7). Becuse of the study design of retrospective recll of body weight t ge 50 mong group of individuls belonging to the Americn Assocition of Retired Persons, this study my be subject to effects of smpling bis, recll, nd survivor effects. Similrly, nlyses by Freedmn et l. supported overweight s risk fctor for mortlity, but only in women ged 55 (ref. 12). In this study, the lowest risk for deth mong older men ws observed mong those in the overweight ctegory. In n nlysis of the Ntionl Helth nd Nutrition Exmintion Survey, Flegl et l. (6) found tht there ws no excess risk of deth ssocited with the overweight or obesity clss I ctegories, using mesured BMI. Flegl et l. (13) confirmed these findings in more recent nlysis of cusespecific deths. Their findings suggest tht the overweight ctegory might even hve protective qulities. An nlysis of within-ctegory vrition of mortlity by Gronniger (14) indicted heterogenous risk within BMI ctegories, nd lowest risk of mortlity for men t BMI of 26 kg/m 2, nd for women with BMI from 23 to 24 kg/m 2. Similrly Clle et l. (9) found tht for subjects who hd never smoked nd who hd no 1 Helth Anlysis Division, Sttistics Cnd, Ottw, Ontrio, Cnd; 2 Cndin Institute for Helth Informtion, Ottw, Ontrio, Cnd; 3 Deprtment of Geogrphy, McGill University, Montrél, Québec, Cnd; 4 School of Community Helth, Portlnd Stte University, Portlnd, Oregon, USA; 5 Institute of Helth Economics nd Deprtment of Economics, University of Albert, Edmonton, Albert, Cnd; 6 Kiser Permnente Northwest Center for Helth Reserch, Portlnd, Oregon, USA; 7 Deprtment of Psychitry, Oregon Helth nd Science University, Portlnd, Oregon, USA. Correspondence: Hether Orpn (horpn@uottw.c) Received 17 December 2008; ccepted 11 My 2009; published online 18 June doi: /oby VOLUME 18 NUMBER 1 jnury

2 history of chronic disese, the reltive mortlity risks were not elevted through rnge of in men nd in women. Finlly, Jnssen (10) found tht within elderly popultions, the overweight ctegory my put individuls t n incresed risk for illness, but is ssocited with lower mortlity rtes overll. Fewer Cndin studies hve been published; however, they lso reflect conflicting results surrounding the overweight/mortlity reltionship. In study of Cndins exmining mortlity risk nd BMI, Ktzmrzyk et l. (15) reported n incresed but sttisticlly nonsignificnt risk of mortlity ssocited with the overweight ctegory. In study of Cndin women, being in the overweight BMI ctegory significntly incresed the reltive risk (RR) of deth by bout 30% s compred to those women with BMI of kg/ m 2 (ref. 16). In this pper, we use longitudinl ntionlly representtive popultion-helth survey of Cndin dults to nswer the following questions: Do Cndin dt demonstrte n incresed risk of ll-cuse mortlity mong the underweight, overweight, nd obese? Is there heterogeneity in the risk of deth within the WHO-defined ctegories? Prticipnts And Methods We nlyzed dt from the Ntionl Popultion Helth Survey, longitudinl pnel study conducted by Sttistics Cnd every 2 yers from 1994/1995 onwrd (17). Dt re vilble for follow-up to 2006/2007. The longitudinl pnel selected for the first cycle in 1994/1995 consisted of 17,276 members of privte households using multistge strtified smple of dwellings with clusters of dwellings. One prticipnt from ech selected household ws chosen to prticipte in the survey. The household response rte ws 86%. Prticipnts ged 25 in 1994/1995 were included in the present nlyses (n = 12,455). In totl, 109 women were excluded due to pregnncy in 1994/1995. After excluding prticipnts with missing dt on BMI, nd single remining prticipnt with missing dt on smoking sttus, the finl smple size ws 11,834 persons. Missing dt on physicl ctivity frequency nd lcohol consumption were coded s response ctegory in order to minimize the effect of missing dt on smple size. Respondents who were reported s decesed were mtched to the Cndin Deths Dtbse up to 31 December Subsequent deths could not be confirmed; however, historiclly there hs been very good concordnce between reported deths nd confirmed deths. Within the studied smple, 1,929 deths were observed mong the 115,225 person-yers of follow-up. Dt nd nlyticl techniques Self-reported height nd weight were used to clculte BMI in 1994/1995. Age, sex, self-reported smoking sttus, physicl ctivity frequency, nd lcohol consumption were included s covrites. Cox proportionl hzrds models were used to estimte RRs of deth by BMI ctegories in SUDAAN (18). All the nlyses were weighted using the longitudinl weights constructed to represent the totl popultion of the ten Cndin provinces in Survey bootstrp weights were used in SUDAAN to generte confidence intervls while tking into ccount the complex smpling design nd initil nonresponse. Two nlyses were conducted. The first nlysis exmined the RR of mortlity bsed on the initil WHO ctegory of BMI nd djusted for ge, sex, smoking sttus, physicl ctivity frequency, nd lcohol consumption. Anlyses were lso conducted strtified by sex, ge group (25 59 vs. 60+), nd smoking sttus (ever vs. never smoker). Additionlly, nlyses were conducted excluding cses of deth within Tble 1 RRs of deth by WHO BMI-ctegories, controlling for sociodemogrphics nd selected helth-relted behviors Sex Age n % RR (95% confidence intervl) Femle 6, Mle 5, ( ) , , ( ) , ( ) , ( ) , ( ) 75+ 1, ( ) BMI level < ( ) 18.5 to <25 5, to <30 4, ( ) 30 to <35 1, ( ) ( ) Smoking Dily 3, ( ) Occsionl ( ) Former 3, ( ) Never 4, Physicl ctivity Active 5, Modertely ctive 2, ( ) Inctive 3, ( ) Missing ( ) Dily lcohol consumption Never 1, ( ) Former 1, ( ) <1 Portion dily 6, ( ) 1 2 portions dily 2, portions dily ( ) Missing 52 < ( ) RR, reltive risk; WHO, World Helth Orgniztion. the first 4 yers in order to ccount for possible reverse custion, where pre- existing illness could led to lower BMI nd erlier mortlity. The second set of nlyses divided BMI into nine ctegories to explore further its ssocition with mortlity, using the most prevlent ctegory s the reference group. Becuse of well-documented reporting bises for self-reported height nd weight, ll nlyses were lso conducted using correction fctor developed by Sttistics Cnd bsed on nlysis of self- reported nd mesured height nd weight dt (19). For men, BMI ws reclculted s (1.08 BMI). For women, it ws reclculted s (1.05 BMI). obesity VOLUME 18 NUMBER 1 jnury

3 Tble 2 RRs of deth by WHO BMI-ctegories, corrected BMI nd strtified nlyses, controlling for sociodemogrphic fctors nd helth behviors Ctegory Corrected BMI (n = 11,829) Age (n = 8,371) Age 60+ (n = 3,458) RR (95 % confidence intervl) Mles (n = 5,373) Femles (n = 6,456) Ever smoked (n = 7,616) Nonsmokers (n = 4,213) < ( ) 0.87 ( ) 1.88 ( ) 2.54 ( ) 1.50 ( ) 1.69 ( ) 2.05 ( ) 18.5 to < to < ( ) 0.91 ( ) 0.81 ( ) 0.86 ( ) 0.77 ( ) 0.82 ( ) 0.76 ( ) 30 to < ( ) 0.89 ( ) 0.96 ( ) 1.10 ( ) 0.81 ( ) 0.94 ( ) 0.83 ( ) ( ) 1.53 ( ) 1.25 ( ) 1.72 ( ) 1.09 ( ) 1.21 ( ) 1.68 ( ) RR, reltive risk; WHO, World Helth Orgniztion. Results Due to the nture of the smple design, the prticipnts of our study reflect the composition of the Cndin household popultion living in the 10 provinces. There were pproximtely equl numbers of men nd women. More thn hlf the smple ws under the ge of 45. Almost hlf of the smple fell within the cceptble BMI rnge, while hlf hd excess weight. Close to 40% of the smple hd never smoked, nd hlf of the smple engged in physicl ctivity often enough to be considered ctive. Just over hlf of the smple reported drinking less thn one portion of lcohol dily, while lmost qurter reported moderte consumption t one to two portions dy. The first model (Tble 1) showed significntly incresed risk of deth for underweight RR = 1.73, 95% confidence intervl (CI) ) nd for obesity clss II+ (BMI 35, RR = 1.36, 95% CI ). The RR ws very close to one for obesity clss I (BMI from 30 to <35, RR = 0.95, 95% CI ), nd ws not significnt. As compred to those in the norml weight ctegory, overweight individuls hd lower risk of mortlity (BMI from 25 to <30, RR = 0.83, 95% CI ). The proportionl hzrds ssumption ws met. Anlyses strtified by sex, ge, nd smoking sttus yielded similr pttern of results (Tble 2), s did nlyses excluding deths in the first 4 yers of follow-up nd deths due to externl cuses. The min devition from the observed ptterns of results ws the effect of underweight by ge. For the younger prticipnts (ged 25 59), underweight ws not ssocited with n incresed risk of mortlity. However, for older prticipnts (ged 60+), being underweight ws ssocited with significntly incresed risk of mortlity (RR = 1.88 ( )). Underweight ws significnt risk fctor mong men (RR = 2.54 ( )) nd women (RR = 1.50 ( )). Obesity clss II+ ws significnt risk fctor mong men, but not mong women, while overweight ws significntly protective for women, but not men. Among both smokers nd nonsmokers, underweight ws ssocited with significntly incresed risk of mortlity, while overweight ws significntly protective. However, nonsignificnt results should be interpreted cutiously becuse the reduced smple sizes my hve decresed the power to detect effects. Anlyses conducted using BMI djusted by the correction fctor yielded the sme pttern of results, indicting tht the results reported here pper to be robust. We lso rn nlyses controlling for income s n indictor of socioeconomic sttus nd the results were not substntively different. Tble 3 presents the results with BMI divided into nine groups. The most prevlent BMI group (BMI from 22.5 to <25) ccounted for 24% of the Cndin dult popultion nd ws used s the reference group. Overll, the results support the well-documented U-shped ssocition between BMI nd mortlity nd show heterogeneity within the WHO-defined cceptble rnge ctegory. The lowest risks were ssocited with individuls in the 27.5 to <30 nd BMI 32.5 to <35 ctegories. However, these groups were not significntly different from the reference group, which is not surprising given the smll numbers of individuls in ech of these ctegories. The RR for the BMI ctegory rnging from 32.5 to <35 ws lower thn nticipted but nonsignificnt t 0.92 (95% CI ). This result my be due to the smll smple size for this group, which represented 3% of Cndins living in households ged 25. Discussion We found tht lthough the Cndin dt demonstrted significntly incresed risk of mortlity over 12 yers of follow-up mong individuls in the underweight nd obesity clss II+ ctegories, being overweight ws ssocited with significnt protective effect s compred to those in the cceptble weight ctegory. Obesity clss I ws not ssocited with significntly incresed risk of mortlity. Further nlyses demonstrted U-shped reltionship between BMI nd mortlity when smller increments of BMI were studied. As compred to the reference group of BMI from 22.5 to <25, severl BMI ctegories in the rnge of 25 hd lower (lthough not sttisticlly significntly) RRs of 12-yer mortlity, indicting hetero geneity of risk within the cceptble nd overweight ctegories. Our findings re consistent with those of Flegl et l. who demonstrte in multiple studies tht overweight does not pper to be risk fctor for mortlity (6,13). Both the results of our study nd those by Flegl et l. hve good generlizbility due to ntionlly representtive smples. Even though there re differences in the methodology of the surveys, there is tendency to obtin lower excess risk thn hd been found 216 VOLUME 18 NUMBER 1 jnury

4 Tble 3 RRs of deth by nine BMI ctegories djusted for sociodemogrphics nd selected helth-relted behviors n % Adjusted RR (95 % confidence intervl) < ( ) 18.5 to < ( ) 20 to <22.5 2, ( ) 22.5 to <25 2, to <27.5 2, ( ) 27.5 to <30 1, ( ) 30 to < ( ) 32.5 to < ( ) ( ) RR, reltive risk. using clinicl or cohort studies (6), nd results from cohort studies with select popultions such s nurses or members of voluntry ssocition my not be generlizble to the generl popultion. Becuse of the limited number of control vribles included in the present nlyses, cution should be tken with respect to inferring cuslity. Further reserch documenting the mechnisms through which weight ffects mortlity would be useful. In fct, our inclusion of physicl ctivity s co vrite my hve ttenuted effects, s physicl inctivity not only contributes to excess weight, but is lso ssocited with ge nd poor helth, both risk fctors for mortlity. However, the coefficients relted to BMI were only mrginlly different between models excluding nd including physicl ctivity. In order to ddress the potentil for reverse custion, we conducted nlyses excluding deths occurring in the first 4 yers of the study. The results were not significntly ffected. In ddition to demonstrting tht overweight ws not ssocited with n incresed risk of deth, our study lso suggests tht there might hve been reduction in the impct of excess weight on mortlity in Cnd over recent yers. RRs clculted by Ktzmrzyk et l. through 13-yer mortlity follow-up of the 1981 Cnd Fitness Survey were: 1.6 for underweight, 1.2 for overweight, 1.3 for obesity clss I (BMI of kg/m 2 ), nd 3.0 for obesity clss II+ (BMI >35 kg/m 2 ) (ref. 15). The risk ssocited with underweight ws lower thn we found in our study, but the risks for overweight nd obesity clsses I nd II were considerbly higher thn the risks observed in our study. This my be result of differences in the ge rnge studied. Our study includes individuls ged 25, while the study by Ktzmrzyk et l. focused on Cndins ge Nevertheless, it would pper tht the pttern observed by Flegl et l. in the United Sttes of reduction of the excess risk of mortlity ssocited with obesity my lso be occurring in Cnd. The documented reduction in the United Sttes of crdiovsculr risk fctors for ll ctegories of BMI my explin prt of this, lthough these findings would need to be replicted in Cnd (20). The nlysis of the shpe of the ssocition between BMI nd mortlity rises n importnt question bout which reference group to use for the clcultion of excess deths ssocited with excess weight. Using the cceptble BMI ctegory s defined by the WHO will include individuls with higher risk of deth thn those in the overweight ctegory. This my hve significnt impct on the RR used in the clcultion of excess deths ttributble to excess body weight. For exmple, the RR for obesity clss II+ is estimted to be 1.36 when cceptble BMI is used s the reference ctegory vs when the overweight BMI ctegory (25 to <30) is used. This study contributes new evidence from Cnd to the debte surrounding the ssocition between body weight nd mortlity risk. Strengths of this study include the representtive smple of Cndins from wide ge rnge. A limittion of our study is tht height nd weight were self-reported for the 1994/1995 Ntionl Popultion Helth Survey. It is well documented tht respondents hve tendency to underestimte their weight nd/or overestimte their height (4). However, selfreported height nd weight re considered vlid for identifying reltionships in epidemiologicl studies (20), with self-reported vlues being strongly correlted with mesured vlues (21,22). Our nlyses using correction fctor developed by Sttistics Cnd showed the sme pttern of results, thus we believe tht our results re robust (19). As well, this study hs reltively short follow-up, t 12 yers. It is possible tht the pttern of results could chnge given longer period of follow-up. There is lso evidence tht the distribution of ft mtters; it is possible tht the underlying reltionships between weight nd the risk of mortlity cnnot be identified dequtely when relying on BMI lone (23,24). This study exmined BMI t n initil point in time nd relted it to mortlity over the subsequent 12 yers, wheres others studies hve shown tht mid life BMI ctegory is importnt for subsequent mortlity in older cohorts (7). Cre should be tken before extrpolting results on mortlity to morbidity. Overweight nd obesity hve been clerly ssocited with morbid conditions like hert disese, hyper tension, nd type 2 dibetes (5,10). The threshold for morbidity my differ from the threshold for mortlity, indicting the need for the use of summry mesures of popultion helth tht incorporte both mortlity nd morbidity consequences of excess weight. This is n importnt public helth messge, becuse while overweight my not be risk fctor for mortlity, becoming overweight is necessry step between being of cceptble weight nd becoming obese. Other nlyses using the Ntionl Popultion Helth Survey dt demonstrted tht lmost qurter of Cndins who hd been overweight in 1994/1995 hd become obese by 2002/2003 (ref. 25) nd Cndin dults within ll BMI ctegories continue to gin weight (26). Becuse using cceptble weight s the reference ctegory for estimting excess deths due to excess weight is being questioned, more reserch is required to understnd the ssocition mong BMI, morbidity, mortlity, nd the evolution of BMI over time. This gol cn be ccomplished by further in-depth nlyses of existing longitudinl popultion bsed dt sets. obesity VOLUME 18 NUMBER 1 jnury

5 Acknowledgments We thnk Kthryn O Grdy nd Seed Khn for conducting initil nlyses nd to Jessie Mndle for providing other reserch support. The study ws supported by Ntionl Institutes of Helth grnts AG from the Ntionl Institute on Aging nd DK from the Ntionl Institute of Dibetes nd Digestive nd Kidney Diseses, s well s Cndin Studies Fculty Reserch Grnt from the Cndin Embssy in Wshington, DC (to M.K.). Disclosure The uthors declred no conflict of interest The Obesity Society References 1. Obesity: Preventing nd Mnging the Globl Epidemic. Report of WHO Consulttion on Obesity. World Helth Orgniztion: Genev, Helth Cnd. Cndin guidelines for body weight clssifiction in dults. Report No. H49-179/2003E. Helth Cnd: Ottw, Flegl KM. The obesity epidemic in children nd dults: current evidence nd reserch issues. Med Sci Sports Exerc 1999;31:S509 S Tjepkem M. Adult obesity in Cnd: mesured height nd weight. Report No MWE. Sttistics Cnd: Ottw, Kpln MS, Huguet N, Newsom JT, McFrlnd BH, Lindsy J. Prevlence nd correltes of overweight nd obesity mong older dults: findings from the Cndin Ntionl Popultion Helth Survey. J Gerontol A Biol Sci Med Sci 2003;58: Flegl KM, Grubrd BI, Willimson DF, Gil MH. Excess deths ssocited with underweight, overweight, nd obesity. JAMA 2005;293: Adms KF, Schtzkin A, Hrris TB et l. Overweight, obesity, nd mortlity in lrge prospective cohort of persons 50 to 71 yers old. N Engl J Med 2006;355: Jee SH, Sull JW, Prk J et l. Body-mss index nd mortlity in Koren men nd women. N Engl J Med 2006;355: Clle EE, Thun MJ, Petrelli JM, Rodriguez C, Heth CW. Body-mss index nd mortlity in prospective cohort of U.S. dults. N Engl J Med 1999;341: Jnssen, MA. Elevted body mss index nd mortlity risk in the elderly. Obes Rev 2007;8: Sturm R. Increses in morbid obesity in the USA: Public Helth 2007;121: Freedmn DM, Ron E, Bllrd-Brbsh R, Doody MM, Linet MS. Body mss index nd ll-cuse mortlity in ntionwide US cohort. Int J Obes (Lond) 2006;30: Flegl KM, Grubrd BI, Willimson DF, Gil MH. Cuse-specific excess deths ssocited with underweight, overweight, nd obesity. JAMA 2007;298: Gronniger JT. A semiprmetric nlysis of the reltionship of body mss index to mortlity. Am J Public Helth 2006;96: Ktzmrzyk PT, Crig CL, Bouchrd C. Originl rticle underweight, overweight nd obesity: reltionships with mortlity in the 13-yer follow-up of the Cnd Fitness Survey. J Clin Epidemiol 2001;54: Jin MG, Miller AB, Rohn TE et l. Body mss index nd mortlity in women: follow-up of the Cndin Ntionl Brest Screening Study cohort. Int J Obes (Lond) 2005;29: Tmby J-L, Ctlin G. Smple design of the Ntionl Popultion Helth Survey its longitudinl nture. Helth Rep 1995;7: SUDAAN (computer progrm). Reserch Tringle Institute: NC, Connor Gorber S, Shields M, Trembly MS, McDowell I. The fesibility of estblishing correction fctors to djust self-reported estimtes of obesity. Helth Rep 2008;19: Gregg EW, Cheng YJ, Cdwell BL et l. Seculr trends in crdiovsculr disese risk fctors ccording to body mss index in US dults. JAMA 2005;293: Spencer EA, Appleby PN, Dvey GK, Key TJ. Vlidity of self-reported height nd weight in 4808 EPIC-Oxford prticipnts. Public Helth Nutr 2002;5: Stevens J, Keil JE, Wid LR, Gzes PC. Accurcy of current, 4-yer, nd 28-yer self-reported body weight in n elderly popultion. Am J Epidemiol 1990;132: Guo SS, Zeller C, Chumle WC, Siervogel RM. Aging, body composition, nd lifestyle: the Fels Longitudinl Study. Am J Clin Nutr 1999;70: Cundiff DK. BMI: poor surrogte for diet nd exercise in ssessing risk of deth. Int J Obes (Lond) 2006;30: Le Petit C, Berthelot JM. Obesity growing issue. Helth Rep 2006;17: Orpn HM, Trembly MS, Finès P. Trends in weight chnge mong Cndin dults. Helth Rep 2007;18: VOLUME 18 NUMBER 1 jnury