What Body Mass Index is and its limitations
Body mass index (BMI) compares a person’s weight to his height, and is calculated by dividing his weight (in kilograms) by his height (in metres squared). The formula is BMI = kg/m2 where kg is a person’s weight in kilograms and m2 is their height in metres squared.
It gives a person an idea of whether he is 'underweight', a 'healthy' weight, 'overweight', or 'obese' for your height. BMI is one type of tool to help health professionals assess the risk of chronic disease. A BMI of 25.0 or more is overweight, while the healthy range is 18.5 to 24.9. BMI applies to most adults 18-65 years.
BMI is a relatively straightforward and convenient way of assessing someone's weight but it has some limitations. The BMI cannot tell the difference between excess fat, muscle or bone. The adult BMI does not take into account age, gender or muscle mass. This means:
• very muscular adults and athletes may be classed "overweight" or "obese" even though their body fat is low
• adults who lose muscle as they get older may fall into the "healthy weight" range even though they may be carrying excess fat
Pregnancy will also affect a woman's BMI result. A woman’s BMI will go up as her weight increases. She should use her pre-pregnancy weight when calculating her BMI. During pregnancy and lactation, a woman's body composition changes, so using BMI is not appropriate.
BMI is not used for muscle builders, long-distance athletes, pregnant women, the elderly or young children. This is because BMI does not take into account whether the weight is carried as muscle or fat. Those with a higher muscle mass, such as athletes, may have a high BMI but not be at greater health risk. Those with a lower muscle mass, such as children who have not completed their growth or the elderly who may be losing some muscle mass may have a lower BMI.
It is useful to consider BMI alongside waist circumference, as waist measurement helps to assess risk by measuring the amount of fat carried around a person’s waist. Waist circumference is a simple check to tell if you are carrying excess body fat around your stomach, which can raise a person’s risk of heart disease, diabetes, and stroke. Where your fat is on your body can be an important sign of your risk of developing ongoing health problems. Carrying excess body fat around your middle is more of a health risk than if weight is on your hips and thighs.
BMI is a useful measurement for most people over 18 years old. But it is only an estimate and it does not take into account age, ethnicity, gender, and body composition. It may be better to check one’s waist measurement and other risk factors.
Asians and other minority ethnic groups have a higher risk of developing some long-term (chronic) conditions, such as type 2 diabetes. These adults with a BMI of 23 or more are at increased risk; 27.5 or more are at high risk.
Healthy BMI, excess fat, and BMI for children
One can have a healthy BMI and still have excess tummy fat, meaning he is still at risk of developing those medical problems. To measure your waist:
1. Find the bottom of your ribs and the top of your hips.
2. Wrap a tape measure around your waist midway between these points.
3. Breathe out naturally before taking the measurement.
Regardless of one’s height or BMI, one should try to lose weight if the waist is:
• 94cm (37ins) or more for men
• 80cm (31.5ins) or more for women
For children and young people aged 2 to 18, the BMI calculator takes into account age and gender as well as height and weight. Overweight children are thought to be at increased risk of a variety of health conditions, and they are also more likely to be overweight as adults.
The BMI calculator works out if a child or young person is:
• underweight – on the 2nd centile or below
• healthy weight – between the 2nd and 91st centiles
• overweight – 91st centile or above
• very overweight – 98th centile or above
A child's BMI is expressed as a "centile" to show how their BMI compares with children who took part in national surveys. For example, a girl on the 75th centile is heavier than 75 out of 100 other girls her age. Measuring waist size is not routinely recommended for children because it does not take their height into account.
