A pot belly increases the risk of a Heart Attack - even if the rest of the physique is as skinny as a beanpole.

The person with that build would have a low body mass index (BMI). Based on that, the person would be classified as having a low Heart Attack risk. But, insist researchers here, it’s really the ratio between the distance around that pot belly and the circumference of the hips that tells the tale of risk.

If the waist-to-hip ratio is high, irrespective of the BMI, the risk of a Heart Attack is increased, Arya Sharma, M.D., and Salim Yusuf, M.D., of McMaster University here and colleagues reported in the Nov. 5 issue of The Lancet, based on a new analysis of the Interheart study data.

“If you have more of a waist line than a hip line, then you’re at risk,” Dr. Sharma said in an interview. He is director of the Canadian Obesity Network.

In general, he said, people are regarded as being at low risk if their waist-to-hip ratio is less than 0.85 for women and 0.9 for men. The waist is measured between the top of the hip bone and the lower edge of the rib cage without clothing; the hips are measured over light clothing at the widest part of the buttocks.

Dr. Sharma and colleagues showed last year with Interheart data that abdominal Obesity accounts for about 90% of the risk for myocardial infarction around the world, with eight other risk factors - including cigarette smoking, diabetes, poor diet, and lack of exercise - accounting for the rest.

Now, Dr. Sharma and colleagues reported that of several measures of Obesity, including BMI, the best predictor of Heart Attack risk is the waist-to-hip ratio.

“It just reinforces what we’ve known for a long time - the whole apple-shaped, pear-shaped thing,” he said. “What’s new here is that BMI falls right out of the equation.”

“If you just look at crude data, there is some association, but that’s just because people with high BMI tend to have a high waist-to-hip ratio,” he said. “If you correct for other risk factors, BMI essentially gives you no information.”

On the other hand, he said, there’s a smooth increase in risk with increasing waist-to-hip ratio, regardless of BMI.

The Interheart study - a case-control analysis of acute myocardial infarction - involved 27,098 participants in 52 countries, the investigators reported.

The study had three key findings, he said:

 
• That BMI does not predict Heart Attack risk, while waist-to-hip ratio does.  
• That looking at Obesity through this new measure triples the number of people who should be regarded as at risk.  
• And that the link between increased waist-to-hip ratio and increased Heart Attack risk is consistently found in all countries and cultures.

“The same rules apply to everybody,” he said.

The Interheart study puts “what seems to be the final nail in the casket for BMI as an independent cardiovascular risk factor,” said Charlotte Kragelund, M.D., of Akershus University Hospital, in Lorenskog, Norway, and Torbjorn Omland, M.D., of the University of Oslo.

“Current practice with body-mass index as the measure of Obesity is obsolete,” they wrote in an accompanying editorial.

According to the investigators, the odds ratios for every successive quintile of waist-to-hip ratio were significantly greater, compared to the lowest, even when adjusted for age, sex, smoking, and region of the world:

 
• For the second quintile, compared to the lowest quintile, the odds ratio for Heart Attack was 1.15, with a 95% confidence interval ranging from 1.05 to 1.26.  
• For the third quintile, the odds ratio was 1.39, with a 95% confidence interval ranging from 1.28 to 1.52.  
• For the fourth quintile, the odds ratio was 1.9, with a confidence interval of 1.74 to 2.07.  
• And for the fifth quintile, it was 2.52, with the confidence interval ranging from 2.31 to 2.74.

On the other hand, when other risk factors were taken into account, the odds ratio for BMI and Heart Attack - even between the highest and lowest quintiles - was non-significant. It was 0.98, with the confidence interval ranging from 0.88 to 1.09.

There was also marked geographic variation - western Europe and North America predictably had the highest waist-to-hip ratios and China and South Asia had the lowest. However, Dr. Sharma noted, in every region, cases had higher ratios than controls.

The population attributable risk for waist-to-hip ratio was 33.7% overall (with a 95% confidence interval ranging from 31% to 36.5%) although it was slightly higher for women (35.9%) and slightly lower for men (32.1%).

The population attributable risk for body mass index, however was only 10.8% - implying, Dr. Sharma said, that estimates of Obesity based on BMI are sharply under-estimating the cardiovascular risk associated with overweight.

Dr. Sharma said the finding should change how physicians approach Obesity and the risk of Heart Attack.

“Before this paper, your doctor would have put you on the scale and measured your height and calculated your BMI to give you some idea of your risk category,” he said. “After this paper, she’s going to forget about the scale and take out her tape measure.”

Primary source: The Lancet