Tools for Practice


#138 The skinny on BMI and mortality


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


BOTTOM LINE
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
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 >27.5Mortality 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 


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Author(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.