Measuring somebody’s Body Mass Index (BMI) is commonly used to decide whether that person is overweight. It is often used as an indicator of cardiovascular disease risk. However, BMI may not be such an accurate indicator as it lumps muscle and fat into one category, say researchers from the Mayo Clinic.
There are two ways you can calculate your BMI:
1. Metric system – Kilograms and Metres
[Your weight] divided by [Your height squared]
2. Imperial System – Pounds and Inches
[You weight] divided by [Your height squared] times 703
A person is healthy if his/her BMI is between 18.5 and 24.9.
The researchers looked at 40 studies involving 250,000 patients. Their analysis revealed that people with a BMI of 30-35 were at lower risk of cardiovascular disease than those whose BMI was below 20.
You can read about this study in the journal The Lancet, August 19 issue.
The team concluded “An explanation for the lack of a positive association with BMI and mortality in older ages is that, in older persons, BMI is a poor measure of body fat. The measurement of weight does not differentiate between fat and fat-free mass, and fat-free mass is progressively lost with increasing age.” Most of fat-free mass consists of muscle.
Dr. Francisco Lopez-Jimenez, lead researcher, said he and his team had suspected BMI was not a good indicator for quite some time. Dr. Lopez-Jimenez added “We presented a report at an American Heart Association meeting a few months ago showing that BMI did not correlate with fat. A better way to distinguish between fat and muscle is to take a cross-sectional view of the abdomen, to focus on the waist-hip ratio.”
A study carried out by researchers at the London School of Hygiene and Tropical Medicine concluded, after analyzing data on 15,000 patients, that BMI was not a good indicator of health risks for elderly people. They also favoured a calculation which looked at hip-waist ratio.
Hip-waist ratio gives a much better idea about how much fat a person has in the abdomen area (abdominal adiposity). If you have a ‘beer-gut’ the hip-waist ratio calculation will spot it more accurately than using BMI.
The Mayo Clinic team did find that BMI is a good calculator of risk for people with a BMI over 35. Dr. Lopez-Jimenez said “The group of people with BMI of 35 or higher had a bad outcome. Patients with BMIs of 35 to 40 naively believe that they have a lot of muscle. In extreme cases, fat determines the measurement.”
If BMI were the only calculation ever made, the whole of the English national rugby team would be classed as very-overweight to obese and at high risk of cardiovascular disease. The conclusion would have been similar for a fit, heavyweight boxer when he was at his peak. There is no way a professional boxer who is 1.88 metres tall (6ft 2 ins), weighing in at 104 kilograms (228 pounds) has the same cardiovascular disease risk as a sedentary man of same height and weight – they would both have a BMI of 29.42. However, their hip-waist ratio would be completely different.
Calculate your Hip-to-Waist Ratio
Association of bodyweight with total mortality and with cardiovascular events in coronary artery disease
Dr. Francisco Lopez-Jimenez. et al.
The Lancet
Written by: Christian Nordqvist
Editor: Medical News Today