What is BMI and why do we use it as a measurement?
BMI is a measurement which gives an indication of health risks such as being overweight, obese, diabetic and having high blood pressure and high cholesterol to name a few. It is not a measurement of how fat or how fit someone is, in fact it cannot determine the percentage of body fat. A normal BMI measurement should be between 18.5 and 24.9. A measurement of between 25 and 30 is regarded as overweight, and over 30 is regarded as obese. A measurement of under 18.5 is regarded as underweight.
Let’s take a look at the history of BMI and why it might be a little bit misleading.
It was produced by Lambert Adolphe Jacques Quetelet, a Belgian mathematician in the 1830s to determine the weight of the average person within the general population, however averages take the whole population into account and not the individual. It is a simple mathematics formula taking into account height and weight. It does not take into account what that weight is made up of – bone, muscle and fat, or where fat is stored in the body, neither does it account for the person’s age, race or frame size, all of which need to be taken into account when ascertaining how much fat a person is storing in their body.
It’s a popular misconception that BMI is used to measure fatness. BMI is simply a measure of size and does not take any health conditions into account. There are many people who have a normal BMI who are unhealthy and also many people who have a high BMI but who are healthy. A person who is overweight or obese will have a high BMI but it doesn’t work the other way around – a person with a high BMI doesn’t always mean that they are overweight – they could be very healthy with little fat.
In fact, someone who has a normal BMI but who is a heavy smoker, has a sedentary lifestyle and a family history of heart disease may be more at risk of certain health problems than someone who has a high BMI but doesn’t smoke, takes regular exercise and has a healthy lifestyle. This can be misleading as people with a normal BMI could have health implications that are being missed!
BMI on its own, cannot show any illness such as cardiovascular disease, high cholesterol or high blood pressure, however it is a useful guide that can indicate when a person might need to start looking more closely at their health, in order to reduce the risk of diseases such as high blood pressure, diabetes or heart disease, which are more likely to occur when a person is overweight.
Someone who has a high muscle mass such as a trained athlete or someone who does extensive strength training will most likely have a high BMI, putting them into the overweight or obese category when in fact they simply have a high muscle mass. BMI simply cannot tell the difference between fat and muscle – bone is denser than muscle and twice the density of fat.
So, why do we still use BMI as a measurement today?
BMI has been used as a measurement for so many years that it is a recognised measurement by everyone, and for many people who are not very active, it is still a good guide as to whether they are overweight, but this should not be relied on as the only measurement. A more accurate way to measure fat in the body is to use a bioelectrical impedance analysis machine which sends a tiny electrical current around the body to measure how much weight is fat and how much weight is muscle. A healthy woman should have between 21% and 33% body fat (any higher than this is unhealthy). The percentage of body fat in a person will steadily rise from the age of 30 and older people can have a third more fat than they did when they were younger. There will be more fat build up around the internal organs although the layer of fat just underneath the skin becomes smaller. If you don’t have a BIA machine, you can measure your waist and height – your waist should be less than half your height (BMI totally ignores waist size which is a true reflection of obesity).
For more information on the different types of fat stored in the body, read our blog on how to lose belly fat.
So, it’s fine to keep using BMI as a guide to keep a check on your health, but don’t let it be the only measurement that you rely on. Remember, it is not an accurate measurement of anything other than size! The best way to asses your health is to look at a variety of measurements for an overall picture ????
Caroline & Hannah xx