BMI: We know it's flawed, so why do we still use it? - Deepstash

The population-wide BMI statistics show that nearly 60 per cent of people in the UK and US are overweight.

BMI is the ratio of body weight in kilograms divided by the square of one's height in metres. It is represented as kg/m2. 

  • A 'normal' BMI is 20-25kg/m2. 
  • A BMI of 18kg/m2 and lower is underweight.
  • A BMI of 25-30kg/m2 is considered overweight. 
  • Over 30kg/m2 is classed as obese.

BMI is flawed as it only uses weight and height. It cannot differentiate between muscle and fat.

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BMI is easy to calculate and easily scalable. For most of us, the higher our BMI, the more fat we tend to have. 

But BMI shouldn't be used by health care professionals as an accurate measure to inform treatment or advice. It has to be used with other information, such as fasting insulin and glucose levels, blood pressure, and a family history of metabolic disease.

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DUEL-energy X-ray absorptiometry or DEXA is the gold-standard method of calculating fat percentages. It is very accurate but also expensive and not suitable for use in population-wide studies.

Another approach is the use of body-fat scales, where an undetectable electrical current is passed through your body. Since muscle contains more water, it conducts electricity better than fat. A greater electrical resistance will mean you have more body fat. But they are often inaccurate.

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Fat cells get bigger when you gain weight and smaller when you lose weight. But the number of cells stays about the same.

Your fat cells are designed to store fat. When cells become full, the fat ends up in the wrong places, such as our muscles or liver, and that's when we can suffer from diseases such as type 2 diabetes. 

Our fat cells can expand to different sizes depending on our genes, meaning that health varies depending on ethnicity. If you go past your safe fat-carrying capacity, you will get ill.

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