Body Mass Index, Lean Mass and Fat Mass
Among the criteria used to assess your level of fitness, you read everywhere that it’s crucial to reach your ideal weight. But you may ask yourself, ideal for whom and compared with what?
Your BMI is the answer.
Forget about aesthetics – they’re often the source of serious misconceptions in the definition of “ideal weight”. A universal way to get a general idea of an adult’s physical condition is the famous BMI, or Body Mass Index.
Basically, it’s the number you get by calculating the ratio between a person’s weight (in kilos) and the square of their height (in meters):
BMI = body weight (kg) / height (m2)
For instance, the BMI of a person who weighs 75 kg and is 1.80 m tall is:
BMI = 75 / (1.80 x 1.80) = 75 / 3.24 = 23.1
According to the numbers you get, in broad strokes you can establish if you are:
- Underweight, if your BMI is less than 19
- Normal, if your BMI is from 19 to 24
- Overweight, if your BMI is from 25 to 30
- Obese, if your BMI is greater than 30
Quick and convenient. But in order to correctly assess your possible status as overweight or underweight, this information is not enough. It has to be related to other factors, including one very important thing: the percentage of fat mass and lean mass you carry around with you.
Tell me how much mass you’ve got and I’ll tell you if you’re in shape.
A provocative example: if you weight 5 kg more than a friend your same height, it doesn’t necessarily mean that you need to go on a diet or that your friend should gain weight. It might simply mean that your bodies are composed differently and you may both be in shape (or both overweight or underweight).
How is this possible?
Because more or less, total weight is the sum of the muscles, fatty tissue, and bones, and for the same weight, these three components can constitute different percentages from one person to the next.
In addition to the bone structure, which remains more static (and has often led to the excuse “I weigh so much because I’m big boned”), the other two components are variable and are referred to as “fat mass” and “lean mass”: in other words, fat, and everything else (principally muscles and water).
It’s the percentage of these two elements that tells us what kind of shape you’re in, not your total weight: since fat weighs less than muscles, supposing that you feel guilty for weighing a few kilos more than your friend, theoretically you could be the one bursting with health, thanks to a solid, well-trained muscular structure, and your friend might have a higher percentage of fat.
If you want to look like you’re in shape, and more importantly be in shape, you need to know and monitor your body fat percentage, and not just for aesthetic reasons. Fat makes up our energy reserves, but when there is too much of it (and especially too much belly fat) it becomes bad for your health, making the onset of problems like hypertension, hyperglycemia, and diabetes more likely, and in general leading to unpleasant consequences for the cardiovascular system.
How is body fat measured?
There are a number ways. Some are fairly expensive and can only be done using specialized equipment. Other methods are affordable and “portable”, such as measuring skin folds using a simple plicometer (a type of pincers with a calibrated spring).
But because you can’t get a plicometer from your neighborhood pharmacist, something that everyone can get their hands on is a bioelectrical impedance scale.
It’s a regular scale with two electrodes on the surface that conduct a weak electrical signal through your body and determine the percentage of fat and water in your body as a function of sex, age, and height.
The more modern scales can store a history of your weigh-ins, showing calories burned, for example.