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• Posted March 22, 2023

When BMI Isn't Used as Measurement, Obesity's Health 'Benefit' Disappears

Much has been made of the so-called "obesity paradox"-- the observation that people with a heart condition seem less likely to die if they are overweight or obese.

But European researchers now say they've debunked that theory, which was based on earlier research that relied on body mass index (BMI, a measure based on weight and height) to judge whether a person carried excess weight.

The major new study shows the obesity paradox vanishes if other heart risk factors are considered along with a person's BMI.

Further, researchers found that another measure of obesity, the waist-to-height ratio, more accurately reflected the real heart health risk that comes from having too much body fat.

"Better measures of adiposity [excess fat] than BMI, such as waist-height ratio, eliminate the 'obesity survival paradox' and, indeed, show that greater adiposity is associated with a higher rate of hospital admission for worsening heart failure and worse symptoms and quality of life,"said senior researcher Dr. John McMurray, a professor of cardiology at the University of Glasgow.

For this study, McMurray and his colleagues analyzed data from nearly 8,400 heart failure patients suffering from reduced ejection fraction, a condition in which the heart is not able to pump a normal amount of blood throughout the body.

The patients were taking part in a clinical trial evaluating the safety and effectiveness of a heart medicine. As part of that study, extensive data was gathered regarding the people's weight, body size and shape, and other heart risk factors.

Looking at BMI alone, researchers found that people who qualified as overweight or obese did indeed appear to have lower death rates compared to those with a healthy weight.

But when they controlled that BMI measurement with other heart risk factors, the obesity paradox disappeared.

Those other risk factors included a patient's age, gender, blood pressure, heart function, and blood levels of natriuretic peptides -- hormones secreted by the heart when under pressure.

Another measurement of excess weight, in which waist circumference is compared to height, was also less likely to be associated with the obesity paradox, the researchers added.

All about the belly fat

This is probably because waist-to-height ratio is a better reflection of belly fat in and around key organs like the liver, McMurray said.

"Fat cells are not inert and are metabolically active, secreting a number of substances that are harmful, including mediators of inflammation,"McMurray said. "In addition to affecting vital organ function such as liver function, mediators secreted by fat cells may damage blood vessels, increase blood pressure, cause insulin resistance and lead to type 2 diabetes."

The flabbiest top 20% of participants, as measured by waist-to-height ratio, had a 39% increased risk of being hospitalized for heart failure compared to those with the lowest levels of belly fat, researchers found.

The findings were published March 22 in the European Heart Journal.

"Accumulation of excess fat around the midsection, including around and in the vital organs, is definitely worse for health,"said Dr. Jamy Ard, president-elect of the Obesity Society.

"This study suggests that there are other factors that might explain why individuals with higher BMIs were surviving longer in previous studies,"Ard continued. "It now means that we really need well-done studies to test if weight loss can be beneficial in those with heart failure and increased body fat, especially around the waist."

The study also adds more weight to the argument that BMI should be abandoned as a measure of obesity, McMurray said.

"BMI does not take into account the location of body fat or its amount, relative to muscle or the weight of the skeleton,"McMurray said.

Fat, muscle and bone weights also can vary based on gender, age and race, further confusing the issue, he added.

McMurray noted that heart failure could skew BMI even further, because the condition causes fluid retention that can increase body weight.

The National Institute for Health and Care Excellence in the United Kingdom recently suggested that waist/height ratio replace BMI in assessing excess fat, McMurray said.

Still, BMI is here to stay

Contemplating the effectiveness of BMI, an editorial accompanying the study compared the body type of a lean, muscle-bound professional wrestler against that of a huge Japanese sumo wrestler.

Even if both have the same BMI, it doesn't seem feasible to assume that both have a similar risk of heart disease, said editorial co-author Dr. Stephan von Haehling, a cardiologist with University of Gottingen Medical Center in Germany.

"BMI is a poor reflection of body composition because it does not adjust for muscle or fat but rather weight as a whole,"von Haehling said.

However, von Haehling doesn't think BMI will be abandoned so easily.

"BMI is a routine assessment everybody understands easily. I think it is there to stay,"von Haehling said. "We have to view it with a grain of salt and have to be aware of its deficiencies and of the fact that there are less easily accessible measures like [waist-to-height ratio] that may have their benefits in certain situations."

Ard also believes that BMI still can be a useful, if flawed, tool for assessing the health risks of average folks.

"BMI still has relevance at the population level. Most of us are not highly trained athletes with a BMI of 30 and single-digit-percentage body fat,"Ard said. "That means BMI is still useful as a screening tool, and then clinicians have to evaluate the person in front of them to determine if there is increased health risk."

McMurray agreed with Ard that research now needs to be focused on how to help people with heart failure lose weight and potentially improve their health.

"This is where we need research, urgently,"McMurray said. "More and more of our patients with heart failure are developing obesity and we need to treat this."

"Obviously, dietary intervention may be one approach but we need to know how to do this acceptably and safely and prove that it is effective,"he explained. "There are powerful new weight-loss drugs available, but again we need to show that these are effective and safe in people with heart failure."

More information

The U.S. Centers for Disease Control and Prevention has more on assessing your weight.

SOURCES: John McMurray, MD, MB, ChB, professor, cardiology, University of Glasgow, Scotland; Jamy Ard, MD, president-elect, Obesity Society; Stephan von Haehling, MD, PhD, cardiologist, University of Gottingen Medical Center, Germany; European Heart Journal, March 22, 2023