Let me explain. According to BMI, someone under the range of 18.5 is considered underweight, 18.5-24.9 normal (or healthy) weight, 25-29.9 overweight, and 30 and over obese. But, of course, this isn't certain - or black and white - for every person who may be just a few pounds overweight or are on the high-end of healthy. For example, a woman who is 5'3-5'4 and 140-145 lbs is considered on the high end of a healthy BMI or a few pounds overweight. And while losing a few pounds wouldn't hurt - especially if she has a well-proportioned hourglass or pear figure with a bigger bone structure and moderate metabolism - technically, according to the BMI chart, she could lose almost 50 lbs and still be on the low-end of healthy at 100 lbs weight. But, is this necessarily healthy? No, of course not. Even though she would still be within a healthy BMI at 100 lbs, for that reason of a different body type, you wouldn't expect her to go from a size 10 to a size 0! It just wouldn't be necessary and with that different body type, it is probably more realistic for her to stay at 130-150 lbs then to be 100, 110, or even 120 lbs. Because she has a specific body type and bone structure, she may have to work harder to become really skinny, hence dangerous dieting habits that can lead to a severe eating disorder - all when she doesn't even have to!
It works the same way with someone who is naturally petite and no matter how hard they try, will not gain weight, because of their smaller bone structure and extremely high metabolism. While some of us may prefer to have their metabolism - as opposed to our own (who wouldn't want to eat whatever they want and still have "abs"?) - because their natural body type is healthier and more susceptible to staying at a lower weight, it's ok if they stay at the lower end of the BMI chart. It's ok if they stay at that weight, because they don't have to cut so many calories or resort to starvation to attain a certain weight loss or healthy body weight.
But, the other issue here is our society's obsession with a size 0 figure and the fitness industry's preoccupation with a ripped figure. Now, I'm NOT saying ALL personal trainers say this, but if you're - let's say - outside of your 18%-22% body fat range (as a woman), are you considered fat, even at a healthy weight? For example, there are many women who fit into the 5'5, 150 lbs, and have a high-end healthy BMI or a few pounds overweight, but with 30-ish percent body fat. Are they considered obese at this point? I don't think it's fair to put a medium-height, 150 lb person on the same level as a 300 pound person who has high body fat and is clearly obese. It's not right to call them obese; all it really means is that they have high body fat and need to tone up - and maybe lose a little weight.
The same goes for, let's say, a man who is 200 lbs at 6 feet tall and has a rough BMI of 27. In this case, they're considered "overweight," but even if their body fat is slightly higher than it should be and their muscle ratio is lower, I believe a man who is 6 feet tall and 200 lbs is at a standard weight for his height. He's taller and can get away with more weight, but even so, would that make him less healthy than someone who is all muscle and 300 pounds?
That's the thing. They say BMI is a measure of body fat, but I really think it's solely based on your height and weight. A 5'4, 145 lb woman isn't always going to have 25% body fat (at a BMI of 25). Not only that, but the average woman is 5'4, 164 lbs, and a size 12/14. While this means that the above average woman is overweight and some of the below average women are underweight, does this also mean that the average woman is a plus-size? While some plus-size models may not be considered overweight (or very overweight by BMI's rules) - because of their tall heights - there's a reason they call those women "plus size" or have "plus size" clothes. Plus-size is indicated by whether a woman is a size 14 or over (with potentially a larger bone structure or height), not to mention that in the modelling world, a size 8-12 (and sometimes even a size 4 or 6) is considered plus-size. But, if you're not a plus-size - but a size 6-12 - are you still considered overweight or fuller than the average weight? And if you are, then why aren't you considered plus size?
So many contradictions about weight in the fitness, media, and health industries. Should you really become paranoid with a chart that may not even be true - for you?
