For all we know about the complex relationship between obesity and health, experts still face a fundamental problem: The tools used to measure body fat can fail to give a true sense of a person's weight-related health risks.
A new review suggests that a simple measurement -- the ratio of one's waist circumference to height -- is significantly better at gauging cardio-metabolic risk than body mass index and waist circumference, two common measures.
In data being presented at the European Congress on Obesity in Lyon, France, researchers reviewed 31 scientific papers to determine the effectiveness of using BMI (a ratio of height to weight) versus measuring one's waist circumference and waist-to-height ratio, to gauge risk. Looking through the studies, they hoped to see which best detected problems like high blood pressure, type-2 diabetes, abnormal body fat levels and metabolic syndrome.
Compared with BMI, measuring waist circumference was considered superior in detecting adverse health outcomes. Waist-to-height ratios were even better at predicting diabetes, hypertension and cardiovascular disease, leading the researchers to determine that the measurement was an effective screening tool.
“What this is all trying to get at is the idea of central adiposity, or whether you’re an apple or a pear shape,” said Sai Das, a scientist in the Energy Metabolism Laboratory at Tufts University, who was not associated with the review. Research has suggested that excess abdominal fat puts people at greater risk for developing obesity-related conditions, like diabetes, high blood pressure and high cholesterol.
For years, experts have criticized BMI, saying it does not provide a clear measurement of one’s fatness and good health. For example, BMI does not distinguish between muscle and fat so muscular people may be mis-categorized as overweight or obese. Furthermore, people with normal BMIs may still carry a lot of fat around their waists, upping their risk for metabolic conditions.
These shortcomings explain why major health organizations like the Centers for Disease Control and Prevention suggest that waist circumference is a good way to assess risk. Men whose waist circumferences are more than 40 inches and non-pregnant women whose waists are more than 35 inches may be at greater risk of developing weight-related health problems, the CDC states. Having a large waist is linked with having fat around the internal organs, which can lead to inflammation and insulin resistance and increased cardio-metabolic risk.
"BMI measures total fat; waist circumference measures internal fat," said study researcher Margaret Ashwell, with the Oxford Brookes University in England.
And measurements that take into account both internal fat and height may be the best yet, she argued. “For the first time, robust statistical evidence from studies involving more than 300,000 adults in several ethnic groups shows the superiority of waist-to-height ratios over both waist circumference and BMI for detecting cardio-metabolic risk factors in both sexes,” she and her co-authors write in the abstract.
But one of the criticisms of waist circumference, said Das, has been that it does not discriminate at all according to height, meaning it does not provide a complete look at the body.
While Das called the new study a "step in the right direction" in terms of contributing to the discussion about what measurements are best, she cautioned that it is still preliminary and has not yet analyzed in a peer-reviewed journal. Other measurements still have value, she argued.
“BMI is still good at the [population level to let us know what the weight of the nation is, and where things are trending,” Das said.
But in an e-mail to The Huffington Post, Ashwell argued that the waist-to-height ratio should totally replace other measurements, both in public health applications as well as clinical use. “It’s always better,” Ashwell said.
And her takeaway for those wondering what waist-to-height means for them? “Keep waist circumference [to] less than half your height," she said.