23 Aug 2013 Why BMI isn’t the best measure for weight or health
Researchers from the Perelman School of Medicine, University of Pennsylvania, believe the body mass index (BMI), based on the weight and height, is not an accurate measure of body fat content and does not account for critical factors that contribute to health or mortality, such as fat distribution, proportion of muscle to fat, and the sex and racial differences in body composition.
Obesity predisposes to diabetes, heart diseases, sleep apnea, cancer and other diseases. Although several studies have shown an increase in mortality in obese people, recent studies have suggested that obesity protects against death from all causes, as well as death due to chronic diseases such as diabetes, heart failure, and stroke.
This so-called “obesity-mortality” paradox suggesting a beneficial influence of obesity has generated much controversy.
In a new perspective article in the journal Science, Rexford Ahima, MD, PhD, Professor of Medicine and Director of the Obesity Unit in the Institute for Diabetes, Obesity and Metabolism, and Mitchell Lazar, MD, PhD, Professor of Medicine and Genetics and Director of the Institute of Diabetes, Obesity, and Metabolism, discuss the challenges of studying the health and mortality risks of obesity.
“There is an urgent need for accurate, practical and affordable tools to measure fat and skeletal muscle, and biomarkers that can better predict the risks of diseases and mortality,” said Ahima.
“Advances to improve the measurement of obesity and related factors will help determine the optimal weight for an individual, taking into account factors such as age, sex, genetics, fitness, pre-existing diseases, as well novel blood markers and metabolic parameters altered by obesity.”
Obese BMI is strongly associated with substantial increases in the risk of developing diabetes, cardiovascular diseases, cancer and other chronic diseases, leading to higher mortality.
However, studies have shown that some people with obese BMI have improved metabolic profile and reduced cardiovascular risk, whereas a subset of people with normal BMI are metabolically unhealthy and have increased mortality risk.
The researchers note that the true impact of obesity may not be appreciated because population studies often describe associations of BMI and health and mortality risks without assessing how intentional or unintentional weight loss or weight gain affect these outcomes.
“Future research should be focused more on molecular pathways, especially how metabolic factors altered by obesity change the development of diabetes, heart diseases, cancer and other ailments, and influence the health status and mortality,” noted Lazar.
Journal Reference:
- R. S. Ahima, M. A. Lazar. The Health Risk of Obesity–Better Metrics Imperative. Science, 2013; 341 (6148): 856 DOI: 10.1126/science.1241244
Why BMI isn’t the best measure for weight or health
The BMI provides an easy way to measure obesity, but more doctors are questioning its accuracy and usefulness.
BMI is supposed to estimate the amount of body fat a person carries based on height and weight, and categorises people based on what is appropriate for their size. BMI readings under 18.5 mean you are underweight, and could put on a few kilos. If you fall between 18.5 and 24.9, you’re considered normal, while a BMI of 30 or higher qualifies as overweight.
But in recent years, more researchers argue that it’s not the most accurate way to measure body weight. For years, scientists have said that BMI can’t distinguish between fat and muscle, which tends to be heavier and can tip more toned individuals into overweight status, even if their fat levels are low.
In the journal Science, the latest data from University of Pennsylvania shows that BMI also doesn’t tease apart different types of fat, each of which can have different metabolic effects on health. BMI cannot take into consideration, for example, where the body holds fat. Belly fat, which is known as visceral fat, is more harmful than fat that simply sitting under the skin. Visceral fat develops deep among muscles and around organs like the liver and by releasing certain hormones and other agents, it disrupts the body’s ability to balance its energy needs.
Even relatively thin people can have high levels of visceral fat, which means they might be considered healthy by BMI standards, but internally they may actually be at higher risk of developing health problems related to weigh gain.
In April of last year, a study published in the journal PLoS One documented such inconsistencies and questioned the accuracy of using BMI to classify weight status of 1,400 men and women. As TIME reported:
Among the study participants, about half of women who were not classified as obese according to their BMI actually were obese when their body fat percentage was taken into account. Among the men, in contrast, about a quarter of obese men had been missed by BMI. Further, a quarter who were categorized as obese by BMI were not considered obese based on their body fat percentage. Overall, about 39% of participants who were classified as overweight by their BMI were actually obese, according to their percent body fat.
So why is BMI still the preferred way to measure weight and evaluate obesity? For one, it’s a relatively easy measurement for doctors to take during an office visit. Taking a person’s height and weight and plugging it into an equation produces a number that informs doctors about whether their patients are at high, low or no risk when it comes to weight-related health problems.