> BMI is really easy to measure, and is mostly accurate, that’s why it’s so pervasive.

BMI is easily misunderstood by people who know just enough to see that it’s imperfect, but not enough to understand why it’s still a valuable screening tool.

I’ve been in the “overweight” BMI range with low body fat before. It’s not too hard to get there if you’re lifting weights and paying attention to your diet consistently for years, but it takes a lot of work to get there. It doesn’t happen accidentally except for people who win some genetic lottery to build a lot of muscle and keep body fat low without trying.

Getting all the way to the obese BMI range while having healthy body fat is only happening for people with an extreme dedication to body building and diet (and let’s be honest, a lot of the people in this category are manipulating hormones too).

Yet whenever BMI comes up some people try to dismiss it as too flawed based on these possible edge cases. The edge cases for BMI exist, but that doesn’t mean it’s not useful. It’s a good general purpose screening tool with numbers that are available. If someone has more precise measurements available, those should be used instead. BMI is a really good first pass screener to determine if a closer look should happen.

Another common misconception is that one is still healthy with high BMI that comes from having lots of muscle. It's not clear that lots of muscle is healthy.

Yes, it is a pretty high level metric with good correlation a bunch of diseases, but really many of these is because it is ALSO correlated to percent of fat, which is often the more relevant metric. But as you said BMI is so much simpler to measure.

Many active gym people have pretty high BMIs but fairly low fat (because muscle is dense), and unsurprisingly have better outcomes than the average person (if you ignore the share that uses/overuses anabolic steroids and co)

How accurate is it, in practice, for a given individual? I'm not that out of the ordinary proportion wise. I have a slightly longer torso and arms versus my legs, a somewhat muscular-ish baseline and broad shoulders, but I accumulate fat almost exclusively abdominally. My teenage self, lifetime peak of my fitness, no visible body fat, hyperactive football player, qualified as solidly overweight. If I was to listen to it, I'd be called obese before I noticeably start to show body fat.

I often wonder far from the median I am in this regard. I was under the impression that it was pretty accurate for assessing populations, but fell apart very quickly at the individual level. How many "normal"/otherwise healthy people do fall outside BMI's numbers?

BMI is at best 66% accurate, so "mostly" is correct, but what is mostly good enough for?

BMI has known biases in gender, age, and race. It misclassified Asians, women, elderly w sarcopenia, and people with high body fat to lean tissue ratio.

Yeah BMI is sort of a 'good enough' technique but also has some very obvious areas that it can miss. If you fall outside the typical fat to muscle ratio is a good example.

Genuine question: What is the difference between overfat and overweight? Isn’t the extra weight in an overweight person comprised of fat?

But it's actually not just being "overfat".

It's specifically a high level of visceral body fat.

Genetics determines which parts of the body gain fat first as you gain fat overall, and some people have the unfortunate genetics to gain it first viscerally and some people have the fortune to gain it there last (and everything in between).

This means that you can have different people with the same body fat percentage but drastically different disease risks.

But yes this was also known already.

That's why it's been recommended to take a waist measurement alongside BMI to get a much more informative screening tool. Waist-to-height ratios are another alternative.