New Delhi, Aug. 22: The body mass index (BMI), a concept invented about 180 years ago and used for decades as a measure of health, is in danger of losing its own certificate of fitness.
Sections of the medical community are questioning the reliability of the BMI as a predictor of ill health and mortality after several studies indicated, paradoxically, that a high BMI may at times lower the risk of premature death.
The BMI -- calculated by dividing the weight in kilograms by the square of the height in metres -- is widely used by doctors and the public as a measure of fitness and health. Air India earlier this year prescribed a BMI test to its cabin crew above 40 years of age.
No one doubts that obesity is a key risk factor for cardiovascular disease, diabetes, cancer, and non-alcoholic fatty liver disease among other illnesses.
But the sanctity of the BMI has come into question amid studies that suggest that being overweight or mildly obese, measured through standard BMI values, may in certain situations be associated with lower risk of premature death than having normal weight.
“The BMI should not be treated as a single, reliable predictor -- we need to move away from the BMI and look for alternative predictive tools,” said Rexford Ahima, an endocrinologist at the Perelman School of Medicine at the University of Pennsylvania in the US.
Ahima and his colleague Mitchell Lazar today called for a reassessment of how obesity is measured. In a report published in the US journal Science, the researchers have reviewed some puzzling studies that show the protective effect of what would conventionally be labelled as unhealthy BMI values.
A research team in the US found last year that death rates among newly diagnosed patients with diabetes in the US were higher for patients who had normal BMI than for patients who were overweight or obese.
The study by preventive medicine specialist Mercedes Carnethon and her colleagues at the Northwestern University and published in the Journal of the American Medical Asssociation, indicated that thinner patients with diabetes had higher death rates.
Two other independent medical studies conducted over the past six years have also shown similar associations -- overweight and obese patients appear to have lower death rates from cardiovascular mortality than patients with normal BMI values.
“The BMI doesn’t tell you anything about how fat is distributed in the body, or about the fat-muscle ratio, and doesn't take into account racial differences or differences between male and female,” Ahima told The Telegraph.
Endocrinologists say the distribution of fat in the body may be as relevant to health as the amount of fat. They say excessive abdominal fat in obesity predisposes an individual to insulin resistance, abnormal lipid or cholesterol levels, and cardiovascular diseases.
“It is possible that subcutaneous depots provide a safe harbour for potentially toxic lipids in obese individuals, thereby improving (their) cardiovascular health," Ahima and Lazar wrote in their report.
Given the deficiencies of the BMI as a marker of obesity-associated ill health and mortality, the researchers said there is a need to intensify the search for new, more reliable biomarkers.
“We don’t have anything to replace it with at this time,” Lazar, director of the Institute for Diabetes Obesity and Metabolism at the University of Pennsylvania, told this newspaper.
The researchers have suggested that certain hormones and chemicals called cytokines may be candidate biomarkers.
A Belgian statistician-sociologist Adolphe Quetelet had introduced the concept of the BMI in 1832 as a way to evaluate obesity taking into account a person’s weight and height.
A senior Indian endocrinologist said the debate over BMI has been under way for nearly a decade. “But everything considered, it is still the simplest and most reproducible measure, apart from waist circumference,” said Anoop Misra, director of the National Foundation for Diabetes, Obesity, and Cholesterol.
“If we combine BMI, waist circumference, body fat distribution, glucose, insulin, and new things such as a measure of the C-reactive protein, we’ll get better assessment of cardiovascular risk,” Misra said. “Many of the expensive biochemical tests, however, are mostly not required nor are cost-effective for Indian population.”
Under standard global classification guidelines, people with BMI values between 18 and 25 are considered as having normal, or healthy, weight, while those whose values fall between 25 and 30 are overweight and values above 30 represent obesity.
Five years ago, relying on patterns of obesity-related illnesses among Indians, Misra and his colleagues had recommended revisions in the BMI guidelines for Indians -- BMI values between 23 and 25 should be labeled as overweight and values above 25 imply obesity.