BMI isn’t fatness. BMI is just weight / (height^2), which means that BMI grows linearly in proportion to a human’s scale. (Weight grows with the cube of dimensions but height^2 only grows with, well, the square.) BMI also fails to distinguish between muscle, bone mass, and fat. If, say, human heights had been increasing over time (while maintaining other bodily proportions), you’d expect BMI to increase over time.
That’s not to say humans aren’t getting fatter, but if you wanna hypothesize on the causes of trends in human fatness, you need data on human fatness. Not BMI.
Average BMI in the United States increased from 25.2 in 1975 to 28.9 in 2014, so a 3 point increase. Compare an average 1975 person with an average 2014 person. It’s far more likely that the 3 point increase is due to overeating, rather than other explanations like packing on muscle (3 whole points of muscle is a lot) or variation in bone mass (this is likely negligible). Overeating is the path of least resistance in wealthy Western countries. So yes, technically BMI is not the same thing as fatness, but they are highly correlated.
Also as Rockenots points out, the direction of your height claim is going in the wrong way. BMI is an underestimate for fatness for very tall people. For example, a healthy weight 6′2″ man’s BMI might be 17 or 18, which according to the standard BMI scale is underweight. That’s why measures like better BMI exist.
It turns out that in adults, BMI is negatively correlated with height. So, if human heights have been increasing over time, we’d actually expect BMI to decrease over time.
While this is technically true, in practice, this argument actually works in the opposite direction.
It is extremely rare for a human to have an obese BMI, while having low body fat %. This essentially only occurs in Bodybuilders. If you are in this group, noone will ever misidentify you as obese, because you will be fucking ripped. The book “Overcoming Fatlogic”, which I strongly recommend, has some images of extremely fit people with normal BMIs that really make that point emotionally/visually too, beyond the strong statistical support.
On the other hand, it is becoming increasingly common in our sedentary society for people to have a normal BMI, but be “skinnyfat”, that is, having such little muscle mass that their fat % still makes them sick.
Body fat percentage is also tricky to measure (body fat scales are unreliable, calipers are unreliable, visual assessments by pros are unreliable, and all of these require either expensive devices or professionals, or both); BMI is trivial to measure. For any really large study, it is the only thing that is feasible.
On a population level, BMI will under-, not overestimate obesity. It is a useful tool, and if BMI seems bad, the situation on a population level will currently be even worse.
BMI isn’t fatness. BMI is just weight / (height^2), which means that BMI grows linearly in proportion to a human’s scale. (Weight grows with the cube of dimensions but height^2 only grows with, well, the square.)
I used to believe this. I currently think this is aesthetically correct (ish?) but not biologically. Specifically I think there are greater health premiums for being thin when you’re taller.
BMI isn’t fatness. BMI is just weight / (height^2), which means that BMI grows linearly in proportion to a human’s scale. (Weight grows with the cube of dimensions but height^2 only grows with, well, the square.) BMI also fails to distinguish between muscle, bone mass, and fat. If, say, human heights had been increasing over time (while maintaining other bodily proportions), you’d expect BMI to increase over time.
That’s not to say humans aren’t getting fatter, but if you wanna hypothesize on the causes of trends in human fatness, you need data on human fatness. Not BMI.
Average BMI in the United States increased from 25.2 in 1975 to 28.9 in 2014, so a 3 point increase. Compare an average 1975 person with an average 2014 person. It’s far more likely that the 3 point increase is due to overeating, rather than other explanations like packing on muscle (3 whole points of muscle is a lot) or variation in bone mass (this is likely negligible). Overeating is the path of least resistance in wealthy Western countries. So yes, technically BMI is not the same thing as fatness, but they are highly correlated.
Also as Rockenots points out, the direction of your height claim is going in the wrong way. BMI is an underestimate for fatness for very tall people. For example, a healthy weight 6′2″ man’s BMI might be 17 or 18, which according to the standard BMI scale is underweight. That’s why measures like better BMI exist.
It turns out that in adults, BMI is negatively correlated with height. So, if human heights have been increasing over time, we’d actually expect BMI to decrease over time.
While this is technically true, in practice, this argument actually works in the opposite direction.
It is extremely rare for a human to have an obese BMI, while having low body fat %. This essentially only occurs in Bodybuilders. If you are in this group, noone will ever misidentify you as obese, because you will be fucking ripped. The book “Overcoming Fatlogic”, which I strongly recommend, has some images of extremely fit people with normal BMIs that really make that point emotionally/visually too, beyond the strong statistical support.
On the other hand, it is becoming increasingly common in our sedentary society for people to have a normal BMI, but be “skinnyfat”, that is, having such little muscle mass that their fat % still makes them sick.
Body fat percentage is also tricky to measure (body fat scales are unreliable, calipers are unreliable, visual assessments by pros are unreliable, and all of these require either expensive devices or professionals, or both); BMI is trivial to measure. For any really large study, it is the only thing that is feasible.
On a population level, BMI will under-, not overestimate obesity. It is a useful tool, and if BMI seems bad, the situation on a population level will currently be even worse.
How much of the data on increasing BMI is explained by increasing height, I wonder?
I used to believe this. I currently think this is aesthetically correct (ish?) but not biologically. Specifically I think there are greater health premiums for being thin when you’re taller.