Hamilton, ON - A new analysis from the 52-country INTERHEART study suggests it is time to bid adieu to body-mass index (BMI) as a reliable predictor of MI risk [1]. Waist-to-hip ratio (WTHR) is a far better predictor of MI risk across diverse populations, INTERHEART investigators report. Their observations carry important implications for people and populations hitherto considered to be low risk on the basis of their BMIs.

"We need to redefine how we look at obesity," study coauthor Dr Arya M Sharma (McMaster University, Hamilton, ON) told heartwire. "Traditional definitions have been based on BMI, but what this paper very clearly shows is that, irrespective of your BMI, it's really your waist-to-hip ratio that's important.

The study appears in the November 5, 2005 issue of the Lancet.

Sharma, with lead author Dr Salim Yusuf and colleagues, examined rates of MI in relation to BMI, WTHR, waist circumference, and hip circumference in the 27 098 participants in the INTERHEART study. They found that while BMI showed a "modest and graded" association with MI, the strength of this association was substantially reduced after WTHR was taken into account and became statistically insignificant after adjustment for the other INTERHEART risk factors, namely smoking, lipid parameters, hypertension, diabetes, diet, activity, alcohol use, and psychosocial factors. By contrast, the relationship between increased WTHR and MI risk rose progressively with each increase in WTHR, with each quintile of WTHR carrying a significantly higher risk of MI, even after adjustment for other risk factors. When compared with the odds ratio for MI for the highest BMI quintile, the WTHR quintile was significantly more predictive of MI, with an odds ratio of 2.52 compared with 1.44. Waist circumference and hip circumference were also more predictive of MI risk than BMI, although not as predictive as WTHR.

Of note, Sharma told heartwire, WTHR may identify people at risk of MI who, on the basis of BMI, would appear to be low risk, and vice versa. "Even for individuals who would not traditionally be considered obese, because of a BMI in the range of 20 to 23, their risk of heart disease can be as high if they are in the highest WTHR quintile as someone whose BMI is greater than 30. As well, if their BMI is greater than 30 but they're in the lowest quintile of WTHR, then their risk might be much lower than someone who has a much lower BMI but a much higher waist-to-hip ratio."

By using a more precise tool like WTHR, physicians and public-health officials can identify not only individuals but also populations at risk of MI who might have previously been overlooked on the basis of "normal" BMIs.

"From a global perspective, one of the major public-health messages from this study is going to be that a lot of the populations we have so far not considered obese—people in South Asia and China, for example, who make up two fifths of the world's population—those populations may actually have a very substantial proportion of heart disease as determined by the fact that they have a few too many kilograms around the waist," Sharma said.

He also points out that BMI measurements have frequently prompted debate and confusion, since different cutoffs were needed for different populations or ethnic groups. "The beauty of the WTHR is that it appears to apply to everybody," he said.

In a Comment accompanying Yusuf et al's paper [2], Dr Charlotte Kragelund (Akershus University Hospital, Oslo, Norway) asks whether the study signals a "farewell to BMI."

This is not a farewell to BMI for every single health problem on the planet, but really for heart disease and probably for some of the metabolic conditions.

"The main message from the new INTERHEART report is that current practice with BMI as the measure of obesity is obsolete and results in considerable underestimation of the grave consequences of the overweight epidemic," she writes. "The INTERHEART investigators place what seems to be the final nail in the casket for BMI as an independent cardiovascular risk factor."

To heartwire, Sharma acknowledges that BMI, as a marker of excess body weight, still has a role to play for noncardiovascular diseases.

"We haven't thrown out BMI entirely, because, clearly, people who are heavier will have a number of other health problems, and we've focused only on heart disease. People who are heavier are more likely to have mechanical problems, like back pain, arthritis, and other problems. This is not a farewell to BMI for every single health problem on the planet, but really for heart disease and probably for some of the metabolic conditions."