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Defining obesity

Should excess muscle be included in the definition of obesity?

Obesity involves a combination of excess body weight comprising fat and muscle for a given height and not just excess body fat alone

Researchers have determined that obesity involves excessive body weight, including fat and muscle, for a given height and not just excess body fat as was formerly believed. The study was coordinated by Francisco B Ortega, a Ramón y Cajal researcher at the Faculty of Physical Activity and Sports Sciences at the University of Granada and published in the American journal Mayo Clinic Proceeding.

In the study, ‘Body Mass Index, the Most Widely Used But Also Widely Criticized Index’, researchers analysed data of more than 60,000 people who were examined over an average of 15 years. The goal was to study how factors such as obesity can predict the risk of dying from cardiovascular disease.

An international study led by the University of Granada has "redefined" the conception that modern science has of obesity. It has determined that the disease involves a combination of excess body weight comprising fat and muscle for a given height and not just excess body fat alone.

The concept of BMI was first proposed in 1832 by Adolphe Quetelet and is used internationally to define when a person is overweight (BMI≥25kg/m2) or obese (BMI≥30kg/m2). It has now been used in more than 100,000 published scientific articles, making it the most widely used anthropometric index in the world.

"Nevertheless, BMI is subject to a great deal of heavy criticism due to its inability to discriminate whether a high body weight is due to the person having an excess of fat, muscle or both. Many authors propose using a percentage of fat rather than the BMI, especially when studying with regard to cardiovascular disease," said Ortega.

In the study carried out at UGR, the authors considered whether an accurate measurement of body fat was a more powerful predictor of death from cardiovascular disease than the cheap, fast and simple BMI measurement. To the surprise of many, the result was just the opposite: BMI was significantly better than fat percentage in predicting future death from cardiovascular disease.

Led by Ortega’s team, they worked in collaboration with American researchers epidemiologist Steven N Blair and cardiologist Charles J Lavie. The researchers worked with data from the Aerobics Center Longitudinal Study (ACLS) carried out by the Cooper Institute in Texas, USA. The study, which began in the 1970s, tracked more than 60,000 participants over an average of 15 years. The objective was to study how factors like obesity can predict the risk of dying from cardiovascular disease.

Unlike most longitudinal studies of this kind, the researchers in the present study measured not only the weight and height but also the amount of fat and muscle of the participants. The weight and height measurements allowed them to calculate the body mass index (BMI=weight (kg) divided by height (m)2). To measure fat and muscle, they used skin fold measurements and, in a subsample of more than 30,000 participants, they used hydrostatic weight testing which is considered a gold-standard in the measurement of body fat.

They report that compared with a medium BMI, a very high BMI was associated with a hazard ratio (HR) of 2.7 for CD mortality, which was a stronger association than for body fat percentage (BF%) or fat mass index (FMI). Compared with a medium fat-free mass index (FFMI), a very high FFMI was associated with an HR of 2.2 for CVD mortality, with these estimates being markedly smaller for fat-free mass (FFM).

They also found that when the analyses were restricted only to the sample assessed with hydrostatic weighing (N=29,959, 51.7%), the results were similar, with even slightly larger differences in favour of BMI, compared with BF% and FMI.

“We estimated Harrell's c-index as an indicator of discriminating/predictive ability of these models and observed that the c-index for models including BMI was significantly higher than that for models including BF% or FMI (p<0.005 for all),” they write.

Even when the analysis was restricted to half of the sample (30,000 people), measuring body fat through hydrostatic weight testing, which is an extremely complex and expensive method, BMI was still the best predictor of mortality from cardiovascular causes.

"We considered that a possible hypothesis could be that not only are large amounts of fat associated with greater risk, but also great amounts of muscle or other weight unrelated to fat," said Ortega.

Scientists at UGR tested the hypothesis with data from the study and it was confirmed. This would explain that BMI, which is the sum of fat and muscle relative to height, is a better predictor, at an epidemiological level, of future cardiovascular disease than indicators of the amount of fat alone. In the study, the authors offer different physiological elements that can help to explain the results.

The study also roundly supports the use of BMI in large epidemiological studies and contributes to a better understanding of obesity and its relationship to cardiovascular disease. 

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