Using a tape measure and a scale together help predict which teens have potentially dangerous levels of body fat, Canadian researchers have found.

The commonly used body mass index is based on someone's weight and height but it doesn't tell the difference between lean muscle and fat, or properly distinguish between different body frames, particularly in teens who are growing quickly.

To test if adding waist circumference is a better indicator, Dr. Michael Khoury of the University of Toronto and the Hospital for Sick Children and his co-authors analyzed data on 4,104 students in Grade 9 in the Niagara Region.

The students' heights, weights and waistlines were measured, their blood pressure and cholesterol levels were checked.

"Waist measures appear to be important discriminating measurements when assessing lipid and blood pressure measurements in adolescents with high BMI and should be included when screening" overweight and obese adolescents, the study's authors concluded in this week’s issue of the Archives of Pediatrics & Adolescent Medicine.

Obese and overweight children with moderate waistlines had slightly elevated fat levels, the researchers found. Those with BMIs in the normal to overweight category with normal waist measurements tended to have healthy blood pressure and no cholesterol problems.

The study's authors said they were unable to take multiple blood pressure measurements.

The race, ethnicity and stage of puberty of the subjects weren't recorded.

A second study appearing in the journal PLoS One concluded that in adults, waist circumference consistently predicted risk of death.

Dr. Nirav Shah and Eric Braverman of Weill Cornell Medical College in New York estimated about 39 per cent of Americans who are classified as slightly overweight by BMI are probably obese when body fat is measured directly using X-ray imaging.

"Obesity, body fat and increased adiposity are more prevalent than the American public and American physicians are aware of," the pair concluded.