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Several recent studies raise questions about the predictive power of body mass index (BMI) and highlight its limitations.
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In a British study evaluating the relationship of BMI, waist-to-hip ratio (WHR), and waist circumference to mortality, researchers followed 14,833 men and women >75 years of age for 5.9 years and found that WHR was a better indicator than BMI of mortality risk in the elderly.1
Men with a WHR >0.99 and women with a ratio of >0.90 had an increased risk of death from cardiovascular events. When classified by BMI, those with the lowest BMI (<23 in men, <22.3 in women) were at the highest risk of death. The overweight (BMI 25–29.9) and obese (BMI 30–34.9) groups did not show a higher mortality rate when compared with the normal BMI group.
Astrid Fletcher, PhD, of the London School of Hygiene and Tropical Medicine and one of the study's authors, recommends using WHR to stratify risk for elderly patients because it is a better measure of abdominal obesity.
"Waist-hip ratio is much more informative in identifying those at high risk of cardiovascular mortality, which is the 20% of people with a WHR around 1," Fletcher says. "WHR is a little more time-consuming to measure than BMI and does require people to undress to their undergarments. But we think it will be worthwhile for primary care physicians and nurses to familiarize themselves with the measurements and make it a routine measure. People with a WHR of 1 should be counseled regarding diet and exercise. Of course, our results apply only to the over-75-year population."
Another recent study found that overweight and mildly obese patients with preexisting coronary artery disease had lower mortality rates.2 Again, the highest risk of death was seen in persons with the lowest BMI. The authors emphasize that the inverse association between death and obesity does not suggest obesity is benign, but that BMI has limitations when used to assess cardiovascular risk.
INTERPRETATION MATTERS
These studies did not show an increased mortality in overweight or obese patients, but clinicians should still counsel those patients about lifestyle changes and weight loss. The studies suggest that clinicians need to carefully interpret a patient's BMI, which does not distinguish between fat and lean mass. A patient with a BMI of 27 who has a large proportion of muscle mass does not have the same risk of metabolic abnormalities as another patient with the same BMI who has a higher proportion of abdominal fat. Research shows that excess visceral fat increases insulin resistance, diabetes, and cardiovascular disease.3,4 It is important to look at where a person stores fat, and WHR is a better marker of abdominal fat stores.
Despite the inaccuracy of BMI in determining body composition, it is still a useful anthropometric index. But getting a true measure of body fat is difficult because adipose tissue is deposited diffusely throughout the body.
For most clinicians, the gold standards of determining body fat—underwater weighing and dual-energy X-ray absorptiometry—are not practical. They are left with BMI, WHR, and waist circumference as options. WHR and waist circumference are inexpensive and easy to measure, and better proxies for abdominal fat deposition. WHR is increasingly identified as the most useful marker for patients at risk for cardiovascular disease and other metabolic conditions.
Clinicians can easily identify patients with the classic "apple" shape who have more abdominal fat and then can quickly measure WHR. As the above studies indicate, in the elderly and patients with known cardiovascular disease, that ratio is a better means to stratify risk.
But since primary care clinicians routinely measure a patient's height and
weight during an office visit, BMI is an accessible starting point for
discussing lifestyle modification and nutrition counseling. It can help
identify and appropriately treat those who fall into the extremes, and most
people are familiar with this measure. 
Footnotes
To assess waist-to-hip ratio (WHR), measure the waist in inches at the smallest point while the patient is standing relaxed (not holding breath or pulling in stomach). Measure hips in inches at the widest point, usually at the buttocks. The result is calculated by dividing the waist measurement by the hip measurement. A WHR is considered risky if >0.80 for women and >0.95 for men.
A body mass index (BMI) calculator is available from the National Institutes of Health at http://nhlbisupport.com/bmi/. Weight is categorized by the following BMIs:
References
2. Romero-Corral A, Montori VM, Somers VK, et al.: Association of bodyweight with total mortality and with cardiovascular events in coronary artery disease: A systematic review of cohort studies. Lancet 368:666–678, 2006.[Medline]
3. Carey VJ, Walters EE, Colditz GA, et al.: Body fat distribution and
risk of non-insulin-dependent diabetes mellitus in women: The Nurses' Health
Study. Am J Epidemiology 145:614–619, 1997.
4. Willett WC, Dietz WH, Colditz GA: Guidelines for healthy weight.
N Engl J Med 341:427–434, 1999.
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