SYDNEY, NSW / The Sydney Morning Herald / Life & Style / Wellbeing / September 17, 2010
By Jane Brody
New York Times
"WHY, when I weigh the same as or less than I did when I was younger, does my waist keep getting bigger?" is a common lament by the middle-aged.
Put another way, the question could be: "Why, when my body mass index has not changed, am I fatter than I was?"
The simple answer is that the BMI is a crude measure of fatness. Calculated by dividing a person's weight in kilograms by the square of their height in metres, it doesn't differentiate between fatty and lean tissue.
Fat takes up about four times the space of muscle tissue, so it is quite possible to look and feel fatter even if your height and weight remain the same. This is particularly common among women past 50 and men past 60, and the results are likely to show around the middle.
For children and the elderly, body mass values can be especially misleading because the relationship of lean body mass to height changes as they get older.
Degree of body fatness is a better way than body mass to classify individuals. The World Health Organisation and the National Institutes of Health define obesity as more than 25 per cent body fat in men and more than 35 per cent body fat in women.
Dr Carl Lavie, a cardiologist at the Ochsner Heart and Vascular Institute in New Orleans, says therefore "a woman who is 165 centimetres and weighs 54.4 kilograms could be quite fat, even though her weight and BMI seem OK".
Among Americans in general, he said, "A 183-centimetre, 113.4-kilogram man will be obese, but if he were an NFL [football player] of 190 centimetres weighing 127 kilograms, he might be solid muscle with only 2 per cent body fat."
In an editorial in Mayo Clinic Proceedings, Lavie and colleagues discussed what has been called the "obesity paradox" among patients with heart failure. The paradox refers to the repeated finding that while overweight people are more prone to heart failure, patients with heart failure have lower mortality rates if they are obese. The reason for this paradox is far from clear, although Lavie suggested that one explanation could be that once people become ill, having more bodily "reserve" could be to their advantage.
The editorial was prepared in response to a report in the same journal by Antigone Oreopoulos, of the University of Alberta, and her colleagues. The authors compared the BMI of 140 heart failure patients with a more accurate, although more involved, measure of fat and lean body mass using a DEXA scan (for dual energy X-ray absorptiometry). They found that BMI value alone misclassified the degree of body fatness in 41 per cent of the patients.
The article said: "Although BMI is the most common method to define overweightness and obesity in epidemiological studies and major clinical trials, clearly this method does not necessarily reflect true body fatness, and BMI/body fatness may differ considerably among people of different age, race and sex."
A more reliable, but still relatively simple, assessment of fatness would rely on a skin-fold score based on measurements taken with a caliper at several areas (in men, the thigh, mid-chest and abdomen, and in women, the thigh, triceps and area above the hip bone).
Tale of the tape ... BMI is a crude measure of fatness.
Or, since abdominal fat is more hazardous, simply take a tape measure around the widest part of the abdomen and another at the hips and calculate the waist-to-hip ratio. For men it should be no higher than 0.90, and for women, 0.83.
An oversize abdomen is symptomatic of too much metabolically active visceral fat, which increases the risk of heart attack and premature death.
If just waist measurements are used, Lavie said, a man's waist should be less than 101.6 centimetres and a woman's less than 89 centimetres.
Exercise is the best way to minimise an age-related rise in body fat, the doctor said. Aerobic exercise is not enough. You must also do weight training to build and maintain muscle.
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