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#138 The skinny on BMI and mortality


CLINICAL QUESTION
QUESTION CLINIQUE
 What is the association between body mass index (BMI) and mortality?

BOTTOM LINE
RÉSULTAT FINAL
 Normal (20-25) to overweight (25-30) BMI carry the lowest risk of mortality, with ~25 appearing lowest (in elderly ~27.5). Mortality increases when BMI is below “low-normal” (BMI <20) and obese (BMI ≥30), more at the extremes.   


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EVIDENCE
DONNÉES PROBANTES
 ≥8 systematic reviews of observational studies in general population evaluating all-cause mortality at various BMI ranges.1-10 Focusing on the largest and most recent: 
  • Most studies1 (97 studies, 2.88 million participants) compared to normal (BMI 18.5-24.9), relative risks (RR): 
    • Overweight (BMI 25-29.9): RR=0.94. 
    • Obese Grade I (BMI 30-35): RR=0.95.  
    • Obese Grade ≥II (BMI >35): RR=1.29.  
  • Most participants2 (eight studies, 5.8 million participants) comparing to high normal BMI (22.5-25), hazard ratios (HR) for men:  
    • Low (BMI <18.5): HR=1.88. 
    • Low normal (BMI 18.5-20): HR=1.39. 
    • Mid normal (BMI 20-22.5): HR=1.15. 
    • High normal (BMI 22.5-25): HR=1.00. 
    • Low overweight (BMI 25-27.5): HR=0.97. 
    • High overweight (BMI 27.5-30): HR=1.04. 
    • Obesity Grade I (BMI 30-35): HR=1.18. 
  • Third largest3 (19 studies, 1.46 million participants) compared to BMI 22.5-24.9 for women: 
    • BMI <18.5-20: Increase mortality (HR=1.25). 
    • BMI 20-27.4: Very similar risk throughout range (HR=1.03-1.05). 
    • BMI >27.5: Mortality increases with BMI, examples: 
      • BMI 27.5-30: HR=1.14.  
      • BMI 40-50: HR=2.13.  
  • Others found similar.4-8 
  • Meta-analysis in specific populations: 
    • Diabetes:9 Similar to above. 
    • Elderly (age ≥65):1,10,11 Overweight lower risk (best ~27.5 BMI).10 
    • Pre-existing CVD,12-15 COPD,16 hemodialysis:17 Overweight and Grade I obesity similar risk12 or reduced risk13-17 relative to normal weight BMI.
Context: 
  • Confidence intervals not presented above: Trends of risk are more informative. Highest risk occurs at extremes of BMI with lowest risk occurring around 25 (27.5 in elderly). Minimal differences in HR/RR around 1 (e.g. 0.9-1.1) are likely of little clinical importance. 
  • Observational studies cannot prove causation.   
  • BMI indicates weight for height: Weight (in kilograms) divided by height (in metres) squared. BMI does not indicate fitness level.18  
  • Guidelines recommend the use of BMI as an assessment for obesity and intervention in individuals who are overweight and obese.19,20 

Jennifer Douek August 30, 2024

This really is not terribly helpful because there is no control for reverse causality in lower BMI ranges. Many life-limiting illnesses cause weight loss. This could lead to the type of interpretation error that had physicians recommending daily moderate alcohol as a health promoting intervention.

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Author(s)
Auteur(s)
  • Braiden Hellec BScPharm
  • G. Michael Allan MD CCFP

1. Flegal KM, Kit BK, Orpana H, et al. JAMA. 2013; 309(1):71-82.

2. Flegal KM, Kit BK, Graubard BI. Am J Epidemiol. 2014; 180(3):288-96.

3. Berrington de Gonzalez A, Hartge P, Cerhan JR, et al. N Eng J Med. 2010; 363(23):2211-9.

4. Prospective Studies Collaboration. Lancet. 2009; 373(9669):1083-96.

5. McGee DL, Diverse Populations Collaboration. Ann Epidemiol. 2005; 15:87-97.

6. Lenz M, Richter T, Mühlhauser I. Dtsch Arztebl Int. 2009; 106(40):641-8.

7. Janssen I, Mark AE. Obes Rev. 2007; 8(1):41-59.

8. Troiano RP, Frongillo EA Jr, Sobal J, et al. Int J Obes Relat Metab Disord. 1996; 20(1):63-75.

9. Tobias DK, Pan A, Jackson CL, et al. N Engl J Med. 2014; 370(3)233-44.

10. Winter JE, MacInnis RJ, Wattanapenpaiboon N, et al. Am J Clin Nutr 2014; 99:875-90.

11. Donini LM, Savina C, Gennaro E, et al. J Nutr Health Aging. 2012; 16(1):89-98.

12. Romero-Corral A, Montori VM, Somers VK, et al. Lancet. 2006; 364:666-78.

13. Oreopoulos A, Padwal R, Kalantar-Zadeh K, et al. Am Heart J. 2008; 156:13-22.

14. Padwal R, McAlister FA, McMurray JJV, et al. Int J Obes. 2014; 38(8):1110-4.

15. Sharma A, Valakati A, Einstien AJ, et al. Mayo Clin Proc. 2014; 89(8):1080-100.

16. Cao C, Wang R, Wang J, et al. PLoS ONE. 2012; e43892.

17. Jialin W, Yi Z, Weijie Y. Nephron Clin Pract. 2012; 121(3-4):c102-11.

18. Goyal A, Nimmakayala KR, Zonszein J. Cardiol Review. 2014; 22:163-70.

19. Brauer P, Connor Grober S, Shaw E, et al. CMAJ. 2015; 187(3):184-95.

20. Jensen MD, Ryan DH, Apovian CM, et al. Circulation. 2014; 129(25 Suppl 2):S102-38.

Authors do not have any conflicts to disclose.