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Why is it that medical assessments on obesity is based on BMI when BMI scales are inaccurate?

Take for example Phillip Heath (Mr. Olympia 2016)

Height: 5’9″
Competition Weight: 250 lbs

This gives a BMI of 36.9 when calculated using the formula (Weight in Pounds / (Height in inches x Height in inches)) x 703)

This is considered half way between obese and extremely obese and

a very high risk of BMI related diseases

Now you cannot say Phillip Heath is very obese so why do government organisations and health professions use BMI to rate people's level of obesity and look to reduce access to health care or introduce 'Fat Tax' to incentivise weight loss?

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    Because it's cheap and easy. Otherwise, it's sheer nonsense. – Carey Gregory Feb 27 '17 at 4:37
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All models are wrong, but some are useful.

The same is true for the body mass index. In general, people with a higher BMI have a higher chance of getting obesity-related illnesses than those with a BMI in the 18-25 range. But of course, there are exceptions like the one you mention. That's no reason to discard the entire BMI, because it does work in many cases. It's easier than other predictors (such as abdominal circumference).

Another example would be age, as a predictor of mortality. It's safe to assume that in general, a 90-year-old would die sooner than a 25-year-old. But it's not true that no 25-year-olds die before any 90-year-old.

For the second part of your question: obesity is defined as a BMI >30. In that sense, we can say that Heath would be obese. But once again, this "label" is not flawless. He's probably got a lower probability of getting obesity-related illnesses than someone with a BMI of 29.9 based entirely on a bad diet and lack of exercise.

TL:DR; BMI isn't a flawless predictor, but it's ease-of-use causes it to still be used.

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    A great answer, but it would be better yet with references. Can you please edit to add some? – Lucky Mar 1 '17 at 4:10
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Let's not conflate two different problems with BMI here. One the one hand you can have a low body fat percentage and still have a high BMI, but that only applies to body builders who exercise at a near professional level. Such a person will not get wrongly diagnosed by his or her doctor of being overweight. So, this shouldn't be a problem, however, as Carey Gregory points out, some insurance companies in the US make decisions about coverage and premiums purely based on BMI, which can lead to unjust decisions. A more practical problem with BMI is that it will underestimate the risk of cardiovascular problems and diabetes in people of certain ethnicities, particularly Asians. As recommended by the WHO, Asians should keep their BMI below 23 kg/m^2.

Then having cleared up these two real issues with the BMI, let's consider the context in which the argument for a fat tax and the denial of medical services are made in the sources quoted by the OP. The medical problem that at hand is that health problems caused by a poor diet combined with lack of exercise account for the lion's share of health care costs. The US is a special case where 50% of the health care costs is due to an inefficient system, in most other Western countries, the burden of health care costs to society is about 10% of GDP. But even that 10% of GDP is mostly due to people eating too much fat, salt and sugar and not doing enough exercise.

The people who choose to eat too many Big Macs are not only making me pay for my health care, they are actually limiting my health care options. The health care industry is hard at work at fixing the bodies of people who through their own faults, have wrecked their bodies, therefore I don't have to count on being treated in an optimal way. If at age 90 I would need a new kidney, I will be rejected because of my age despite having the fitness of a 60 year old by that time.

There are also costs for society outside of health care. Your car is significantly more expensive because the workers at the factories where cars are made are not making the right lifestyle choices. If you buy a plane ticket, you're paying more because transporting heavier people requires more fuel, the costs are shared by all passengers; people only pay for their extra checked luggage above their allowance, not your checked fat. This has led to calls for the system to be modified. The fat tax mentioned here is a fee that overweight people would have to pay. Contrast that with the notion of a "fat tax" in the source quoted by the OP. There it's about imposing a VAT on fat, which is a reasonable way to get the population to stick to a healthier lifestyle, thereby drastically reducing the costs incurred on society.

In conclusion, while it may look like the BMIs of very muscular athletic people poses a problem with using the BMI, a closer examination of the relevant facts reveals that this is not a major problem. Rather, it are obese people whose BMIs do actually reflect their miserable physical state (and also people with a normal weight who eat an unhealthy diet) who are making everyone pay a hefty fat tax, and they are making the quality of the health care available for you a lot worse. Bureaucratic problems with athletic people having high BMIs are trivial to fix, unfair as such problems may look like, let's not forget that at age 80 you are not eligible for a heart transplant, no matter how well your medical prognosis is. How unfair is that?

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    You don't have to exercise at a professional level to have a BMI that is completely misleading and incorrect. Any heavily-built athlete will easily have a high BMI even with low body fat. And BMIs get used by far more than just a personal physician who can see you and knows your actual status. They are also used by -- for example -- insurance companies to make coverage and rate decisions about you, and they most certainly don't know that you're not obese despite your obese BMI. It's a crap measure. – Carey Gregory Mar 1 '17 at 5:21
  • @CareyGregory I've added the problem you mentioned about insurance companies. – Count Iblis Mar 1 '17 at 5:59
  • I feel sorry for Asian athletes. – Carey Gregory Mar 1 '17 at 6:14
  • @CareyGregory I've added a bit more, the two sources quoted by the OP should be considered in more detail, and then you'll see that the actual proposals are quite reasonable. The problem exist at the macro-level, the health care system is strained because people don't eat a healthy diet. If people were to start eating healthy, everyone would eat 800 grams of fruits and vegetables, cut down on meat and fat, how many billions of dollars would that save society how much more resources would then be available to treat the remaining patients? – Count Iblis Mar 1 '17 at 6:35
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    If you happen to believe that added fat and lack of exercise are responsible for the obesity epidemic, your first added paragraph makes sense. I do not believe that. I disagree that athletes won't be rejected for certain medical procedures due to BMI requirements because such rejections can happen without the screener ever seeing the patient. – Carey Gregory Mar 1 '17 at 14:45
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As well as any other measurements it doesn't say much on it's own. You need to use it along with other personal details (and measurements) to get some meaning of it. One of the problems with BMI is that muscle weighs more than fat, so athletes (specially the ones that practise sports that rely on strength) tends to break the BMI meaning. It work well in regular people with a normal bone-muscle-fat ratio.

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