BMI - a true measure of obesity?

BMI - a true measure of obesity ?

New research from AUT University questions the appropriateness of Body Mass Index (BMI) as a measure for obesity for adolescent girls from different ethnic backgrounds.

It found that existing BMI definitions of overweight were relatively insensitive for predicting excess percentage body fat in South and East Asian girls. Conversely, low specificity was observed for Pacific Island and Maori girls.

AUT researcher Elizabeth Duncan says the association between BMI and body fatness is variable in children from different ethnic backgrounds.

"Children from Asian ethnicities tend to have more body fat at a given height and weight," she says "This means that our existing BMI scales could be underestimating obesity in these groups."

"While BMI is still the most practical estimate of obesity in population surveys, we may need to consider lowering our BMI action points for obesity in certain ethnic groups, particularly those from South Asian backgrounds."

Duncan went on to develop age-specific BMI cut-off points that correspond to an equivalent level of percentage body fat across all ethnicities. "If we wanted to standardise our definition of obesity we would need to lower the threshold by an average of nearly four BMI units in South Asian girls," she says "Such a change could have a dramatic effect on obesity rates in this population."

The study also revealed that many adolescent girls with excess body fat are unable to recognise their condition.

"Interventions and educational campaigns that assist girls in recognising the danger signs of obesity are a priority for all ethnic groups," says Duncan. "These will increase the likelihood that healthy behavioural changes necessary to combat widespread obesity are adopted."

Subsequent investigation revealed that physical activity levels of the girls were significantly lower in the weekends than on weekdays, and in older girls compared with younger girls. Furthermore, physical activity varied by ethnicity, with Maori girls the most active and South Asian girls the least active.

"It is clear that adolescent-aged girls and girls of Asian decent are priority groups for future physical activity interventions," concluded Duncan.

The main study involved a large-scale investigation of body composition in New Zealand's five major ethnic groups (European, Maori, Pacific Island, East Asian and South Asian). A total of 1,081 adolescent girls aged between11-16 years participated in the Girls' Activity and Body Consumption (ABC) Study. To extend the range, data from the Body-Size and Steps in Children (BASIC) Study which looked at 595 5-11-year-old New Zealand children was included.

Both studies measured BMI from height and weight, percentage body fat from bioimpedance measurements, and physical activity using sealed multiday memory pedometers over five consecutive days. A questionnaire was also used to gather data related to weight perceptions and practices.

ENDS

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