Since the BMI was invented by a mathematician, it has been used over and over as an indication of health with very little modification to the category boundaries.
The BMI is merely a calculation of population health rather than an individual’s health. It works on the basis of dividing your weight in kilograms by your height in meters squared. Because this calculation can create a single number, it’s been used as a picture of health. BUT… it’s an estimate!
The BMI number does not show or take into account; race, age, gender, genetics, amount of body fat versus muscle mass, your lifestyle, or other more relevant measurements of health such as your cholesterol or blood sugar. It works solely on population correlation. While there are well-documented health risks to finding yourself too far on both ends of the weight spectrum, there is no evidence that if you are healthy and are slightly above or below the very narrow range that BMI considers "normal" and "ideal," it has any effect on your overall health.
For women, the “ideal” range is impossibly low for those above average height, a broader skeletal build, bigger muscle mass, or have conditions such as lipoedema. This is because it is based on the "ideal normal" weight of a woman in the 1830s when women were significantly shorter and almost never exercised or built-up muscle mass. Nowadays, women have much more active lifestyles from doing most of the household chores, working 40-hour weeks, caring for children and pets, going to exercise classes and gyms, etc. Add in that breast size has increased along with the average height all around the world, the BMI is not going to give an accurate representation.
Let’s use my numbers for reference.
Dress size: 16/18 (possibly 20 depending on the store)
Weight: 15.6 stone (99kgs)
Height: 5’6” (170.6 cm)
BMI: 34.7 (Obese)
Ideal weight for my height: 8.2st (52kg) – 11st (69kg)
Now I have never been a skinny woman, the smallest I have ever been was when I was suffering from anorexia, ate only one tiny meal a day, and had drop down to 11 stone which is just on the cusp of the healthy category for me. Even then I was a size 12 in clothing due to the fact I couldn’t shrink my bone structure. Once I started eating healthier and exercising properly, my weight instantly jumped straight back up to around 15 stone. This is where I sit naturally.
So, I decided to try one of those body-fat scales to see where most of my weight lies, and this is what I found:
Muscle mass: 51.9 kgs
Bone mass: 31.2kg
With just bone and muscle mass alone I am already out of the healthy category for my height. So, add in natural fat weights that all women carry such as belly fat, breasts, bum etc, and it is no wonder I am in the obese category on the BMI scale despite my good health and active lifestyle.
This is where the BMI needs to be improved, replaced, or just ignored. So many women and people of colour are told to lose to weight when they go to the doctors about any complaints just because their BMI is “too high” without questions about their lifestyle.
You can be fit and healthy and have a high BMI or low BMI, and you can be unhealthy with an “ideal” weight. It is all down to individual body compositions, which the BMI does not take into consideration.