Why body compositiom matters

What does it mean to be physically fit? And how do we know if we are?

The easiest and the standard answer is the Body Mass Index (BMI) (weight in kilograms divided by the square of height in meters). It suggests the ideal body weight range given a person’s height. A BMI in the 18-25 range is considered normal weight. Above that you are overweight or obese; lower means underweight.

It’s a reasonably good metric for most people, but like all indices aiming to condense the answer of a complex question to a single number, it offers an imperfect and an incomplete picture. It doesn’t help after a point and is definitely not the ideal metric to track long-term fitness.

As I write this, my BMI is around 25. I got there in February 2021 after losing 20kgs over seven months. Before that, BMI categorised me as “obese”. So setting a goal for my fat loss phase was easy: lose weight. Weekly measurements indicated if I was on the right path.

But even during weight loss, I knew that progress is not just about body weight. What matters more is body composition: the percentage of fat, bone, and muscle in your body.

From WebMD:

‌‌‌Body composition is important because someone else can be the same height and weight as you, but they might have more body fat and less muscle. This could lead to different health issues in them. ‌The scale won’t tell you how much of your body is made up of fat and muscle mass. But assessing body composition can help you find that. Healthier body composition is less fat and more muscle mass. Too much body fat can lead to risks like cancer, diabetes, heart disease, and other health problems.

This is what BMI misses: it doesn’t measure body fat.

‌At the moment, I have between 18-20% body fat.

What does that mean? Here is a body fat chart from the American Council on Exercise (ACE):

So I am in an “acceptable” range. I want to hit the 15% mark. Some day.