Sunday, March 27, 2016

In defense of the BMI

I've heard a few friends dissing the BMI recently, including being sent an article titled "BMI is a terrible measure of health", Katherine Hobson. I think the they are wrong to do so, and this is why.

The BMI has it's limitations, but it is certainly not terrible, and I think it can be very useful for helping people lead a healthy lifestyle, so here is a defense of the BMI. If you don't know what the BMI is, read this first.

One of the most common criticisms I hear is that the BMI will put athletic people into the overweight/obese categories because muscle is more dense than fat. BMI is very crude, and doesn't take these things into account. This seems a ridiculous limitation - if someone is athletic and sporty, are they actually in need of a metric to find out if they're a healthy weight? As Hobson says: "The goal of using any obesity indicator should be to identify people with excess fat, since that fat has been associated with bad health outcomes". I agree, and as such, it seems silly to use an obesity indicator on professional athletes.

Hobson goes on to summarise some research: "A study by researchers at UCLA published this month in the International Journal of Obesity looked at 40,420 adults in the most recent U.S. National Health and Nutrition Examination Survey and assessed their health as measured by six accepted metrics, including blood pressure, cholesterol and C-reactive protein (a gauge of inflammation). It found that 47 percent of people classified as overweight by BMI and 29 percent of those who qualified as obese were healthy as measured by at least five of those other metrics. Meanwhile, 31 percent of normal-weight people were unhealthy by two or more of the same measures. Using BMI alone as a measure of health would misclassify almost 75 million adults in the U.S., the authors concluded."

However, given the population of the U.S. at the time of the study was 322 million, it means that over 75% of the population are correctly classified. The research is behind a pay wall, so it's a shame to not see the full paper (I also wonder how those other metrics compare with each other).

On top of that, if the goal is to find people with excess fat, it is not fair to knock the BMI when it is being compared to tests with a different goal: to assess cardiometabolic health.  BMI might not be the best indicator of cardiometabolic health, but then that's not its goal. That said, with over a 75% success rate, it doesn't seem to be too shabby!

This study was commissioned because employers in the US wanted to penalise employees up to 30% of health insurance costs if they failed certain health metrics, including BMI. The study wanted to find out how BMI compared to six indicators of cardiometabolic health. If insurance companies, or employers, are misusing the BMI when it comes to health insurance payments, that's hardly the fault of the test, that's the fault of people not using it appropriately.

A better criticism of the "athletes get measured overweight" is that BMI doesn't take into account bone, fat or muscle, it just lumps the entire body together as a single unit. As stated, we are looking for a measure of excess fat - and the fact that BMI doesn't look at fat directly is a legitimate criticism. So what happens when we compare BMI to a more direct measure of body fat, our stated goal for this metric?

study compared BMI to skin fold thickness (which is a more direct measure of body fat), to assess the BMI cut offs (for healthy weight, obesity etc) in different ethnicities. They found those who were obese by BMI were obese by skin fold thickness 50 - 80% of the time depending on ethnicity and gender); and those who were not obese by BMI were not obese by skin fold thickness 85% to 99% of the time.

Clearly BMI is a crude, but useful, estimate of a person's body fat.

The BMI began as an epidemiological tool for populations, and so will always be a problem when applied to an individual. It's similar to the lottery - if enough people play, someone will win, but it won't be you.

In this case we're dealing with more realistic odds - if you're obese by BMI, it's not a certainty that you will get Type II diabetes for example, but it is much more likely. When looking at large populations, the effects of obesity can be seen, and they are not good for health.

So how does BMI compare when looking at other metrics for obesity? Is BMI a fair assessment of whether or not someone's weight is too high?

Measuring someone's waist circumference can be used to assess whther they are obese. The BMI compares against a measure of waist circumference pretty well. A study showed that 6% of people will have a healthy BMI, but an obese waist, and also, 6% of people will have an obese BMI, but a normal waist. The remaining 88% will have a waist circumference that matches their BMI.

An even better metric than waist circumference and BMI is,the hip to waist ratio, which takes into account where fat is stored in the body a little more - fat stored around the belly is associated with poorer health outcomes than fat stored elsewhere ("apples", those who store fat around the wait tend to do worse than "pears" who store more fat on the hips). The hip to waist ratio is a very good metric for assessing helath, and better than the BMI as far as I'm concerned. There is another metric that is better than BMI as well: the Surface-based Body Shape Index, lead to a more accurate predictor of mortality, compared to BMI.

So why am I still defending the BMI when other metrics are available, that also look to be better indicators of health? Well it can be harder to do accurately measure the hip to waist ratio, as there are different ways to do it. Likewise, the Surface-based Body Shape Index is again a little more complicated. BMI is very simple - you just need your weight and height, and if you're terrible at maths, it's very easy to calculate online.

Why is this important? Well, being overweight and obese greatly increases your risks for a number of diseases. However, biology is complex, and the effects of excess fat are not fully understood - whilst being overweight and obese can increase your chances of various maladies, once you have those maladies, you may be less likely to die from them than someone of a healthy weight.

Clearly, there is more to health than just measuring obesity - more than one metric should really be taken into account. However, obesity is still a significant risk to health. People severely underestimate the risks associated with lifestyle, and it's not a case of not knowing the risks. Simply knowing the risks isn't always enough. Let's take smoking, everyone knows it's a risk, however, not everyone fully appreciates this fact. Knowledge of smoking risk comes in four levels:

Level 1: having heard that smoking increases health risks.

Level 2: being aware that specific diseases are caused by smoking.

Level 3: accurately appreciating the meaning, severity, and probabilities of developing tobacco related diseases.

Level 4: personally accepting that the risks inherent in levels 1–3 apply to one’s own risk of contracting such diseases.

It's that level 4 - where you actually realise that the risks will apply, directly, to your own health, that many smokers seem not to get. I suspect that when it comes to other risk factors people don't have that fourth level of awareness.

Sadly, the fantastic  NHS tool "Atlas of Risk" is no longer running, however, it was based on a number of large data sets, and it would show you the leading causes of death in the UK, both nationally and locally, and would also show the biggest risks leading to those deaths. Thankfully, I got a screen grab of the tool for a lesson I taught. Here are the national risks, leading to death, in this country:

Being obese, not eating enough fruit and veg, and not getting enough exercise are the 4th, 5th and 6th most risky things that you can do if you don't want to die earlier than you otherwise would, from natural causes. The 1st and 3rd leading risks are also associated with obesity.

At the end of the day, the hardest bit about staying healthy is the effort you put in, because it's not that complex. You can ignore all those fad (and often expensive) diet plans - just follow the NHS EatWell plate. Get some regular exercise. It will make a difference.

That said, we will all die eventually, and people are free to be obese, smoke and do other activities that negatively impact their health.

But for those that want to improve or maintain their health, I don't think the BMI should be disregarded. Firstly, as we have seen, it's not perfect, but is still a useful indicator. However, secondly, and I think more importantly, it can help monitor your progress, in a way that the better waist to hip ratio may fails at.

Most people tend to lose excess fat evenly, they don't reduce fat in some areas and not others. Therefore, if people were using the hip to waist ratio, this would stay the same, even if significant changes to lifestyle were bringing about a reduction in over all excess fat. However, a person's BMI profile would improve in that time.

The BMI was initially developed as a tool for epidemiological studies - it was always going to have flaws when applied to individuals. But if you recognise those limitations, there's no reason why it can't be there in the tool kit available to assess one's health.

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