“It never fails,” says my friend, Beth. “Every time I have an annual check-up my doctor wants to talk about my weight.” And at each appointment she dutifully answers his questions about her lifestyle. But Beth feels he’s only half-listening. Distracted crunching numbers on a calculator, he determines her body mass index (BMI) and how many pounds she’s over her ideal weight.
Health care providers use BMI to assess whether a patient’s weight is in a healthy range for their height. BMI is calculated by dividing our weight in kilograms by the square of our height in metres.
According to the World Health Organization and Health Canada, BMI is classified into ranges of health risk. Adults with BMIs less than18.5 are labeled underweight, which is associated with osteoporosis, undernutrition and eating disorders. BMIs between 18.5 to 24.9, are considered to be within the normal weight range and carry the least amount of health risk. BMIs from 25.0 to 29.9 are interpreted as overweight, while greater than or equal to 30, as obese. These latter values are associated with an increased risk for diabetes, cardiovascular disease and certain cancers – health conditions linked to having a high percentage of body fat.
But BMI doesn’t directly measure body fat. According to Scott Lear, an associate professor and researcher at Simon Fraser University, the problem with BMI is it tracks our weight, which consists of bone, lean tissue, water and fat.
Lear, who studies the relationship between body fat and risk for diabetes and heart disease, says: “From a health perspective, we are mainly interested in body fat not weight, per se.”
“BMI doesn’t tell us where a person may be carrying excess body fat,” adds Lear. “And we know that fat around our waists carries a greater risk for diabetes and heart disease than body fat elsewhere.”
A waist circumference (WC) measurement is a useful indicator of abdominal fat. WC measures equal to or greater than 102 centimetres (40 inches) for men and 88 centimetres (35 inches) for women, are associated with an increased risk of the aforementioned health problems. In general, risk increases as WC increases, even if our BMI is in the normal weight range.
BMI isn’t a reflection of lean body mass either. Lear emphasizes that BMI may mislabel muscular adults. Endurance athletes may be classified as overweight; heavyweight boxers, as obese. Their large muscle mass inflates their weight, which raises their BMI and, thus, overestimates their health risks.
When determining a person’s ideal weight range, health care providers can’t hang their hat on body mass index and waist circumference alone. Genetics, bone structure, body type, existing health issues and lifestyle need to be considered.
We come in all shapes and sizes. An ideal weight range shouldn’t be dictated by our unrealistic cultural norms. Instead, it should be defined by our ability to exercise on a regular basis without feeling fatigued; meet our nutritional needs by moderately eating a wide variety of foods; and use restraint with booze, drugs and cigarettes.
published in Eyes On B.C., March 1, 2012
