By Dr Harry
The body mass index (BMI) can be calculated by using a formula which relates your weight to your height. The result, a number, can be interpreted to give you an idea of whether you are overweight or obese.
One flaw of the BMI is that all the work which has been done to define what is normal was originally done on middle class white Americans in the early 20th century.
Some research has questioned the validity of this approach, because it may not be safe to assume that the BMI ranges for normal, overweight and obese can be applied to all races in the same way.
Firstly, when measuring weight this includes both fat and non-fat (like muscle, bone, water and minerals). If some ethnic groups have higher bone density or greater muscle mass than other ethnic groups, this would affect their BMI but it would not mean they had more fat; so classifying them as obese or overweight may be inaccurate.
One study published in the American Journal of Clinical Nutrition relates measurements of body composition in black and white subjects and discovers that the non-fat content is greater in the black people studied compared to white subjects. This can be accounted for by an increased bone mineral density and/or protein content (muscle being the major protein).
The implication of this is that more black people may be classified as obese according to BMI than actually have excess body fat when body fat is measured directly. It is striking that according to US census bureau department statistics, a greater percentage of the black population are obese than the white population than perhaps would otherwise be the case.
Similarly you may find that at a particular BMI value, one ethnic group has more body fat compared to non-fat than another ethnic group. Yet the BMI interpretation currently does not factor race into the equation – there are no ethnicity-specific obesity classifications.
For example, a white person with a BMI of 25 has the same body fat content as a person from Singapore whose BMI value is 22. And for a white person with a BMI value of 30, the cut-off for obesity, a person from Singapore has the same body-fat content with a BMI value of just 27.
But some research shows the opposite effect in other Asian groups – e.g. those studied in rural Thailand . In this case the total body fat content which a white person has at BMI 25 would be the same body fat content a person from rural Thailand would have at a BMI of 27. In this case for a given BMI value, the white person has more body fat content. This most likely has a lot to do with the physical exercise which comes from working as a rural farmer in Thailand and has also been seen in young women in rural India.
In some countries where the population is not recognised as obese according to the World Health Organization classification, there is an increasing number of obesity related diseases such as diabetes and heart attacks. Although the WHO has considered this fact, their current advice is to keep the current BMI charts but to be aware of BMI values which should act as prompts for intervention. These include 23 (which is in the normal range but is approaching the start of being classified as overweight) and 27.5 (which is in the overweight range but is approaching the start of being classified as obese).
Researchers in some countries have therefore called for the WHO BMI cutoffs to be revised down to suit the needs of their own population and better act as a marker for risk to health.