When BMI Doesn’t Quite Measure Up

Last month, we did a “Friday Mythbusters” edition of 10almonds, tackling many of the misconceptions surrounding obesity. Amongst them, we took a brief look at the usefulness (or lack thereof) of the Body Mass Index (BMI) scale of weight-related health for individuals. By popular subscriber request, we’re now going to dive a little deeper into that today!

The wrong tool for the job

BMI was developed as a tool to look at large-scale demographic trends, stemming from a population study of white European men, who were for the purpose of the study (the widescale health of the working class in that geographic area in that era), considered a reasonable default demographic.

In other words: as a system, it’s now being used in a way it was never made for, and the results of that misappropriation of an epidemiological tool for individual health are predictably unhelpful.

If you want to know yours…

Here’s the magic formula for calculating your BMI:

• Metric: divide your weight in kilograms by your height in square meters
• Imperial: divide your weight in pounds by your height in square inches and then multiply by 703

“What if my height doesn’t come in square meters or square inches, because it’s a height, not an area?”

We know. Take your height and square it anyway. If this seems convoluted and arbitrary, yes, it is.

But!

While on the one hand it’s convoluted and arbitrary… On the other hand, it’s also a gross oversimplification. So, yay for the worst of both worlds?

If you don’t want to grab a calculator, here’s a quick online tool to calculate it for you.

So, how did you score?

According to the CDC, a BMI score…

• Under 18.5 is underweight
• 18.5 to 24.9 is normal
• 25 to 29.9 is overweight
• 30 and over is obese

And, if we’re looking at a representative sample of the population, where the representation is average white European men of working age, that’s not a bad general rule of thumb.

For the rest of us, not so representative

BMI is a great and accurate tool as a rule of thumb, except for…

Women

An easily forgotten demographic, due to being a mere 51% of the world’s population, women generally have a higher percentage of body fat than men, and this throws out BMI’s usefulness.

If pregnant or nursing

A much higher body weight and body fat percentage—note that these are two things, not one. Some of the extra weight will be fat to nourish the baby; some will be water weight, and if pregnant, some will be the baby (or babies!). BMI neither knows nor cares about any of these things. And, this is a big deal, because BMI gets used by healthcare providers to judge health risks and guide medical advice.

People under the age of 16 or over the age of 65

Not only do people below and above those ages (respectively) tend to be shorter—which throws out the calculations and mean health risks may increase before the BMI qualifies as overweight—but also:

• BMI under 23 in people over the age of 65 is associated with a higher health risk
• A meta-analysis showed that a BMI of 27 was the best in terms of decreased mortality risk for the over-65 age group

This obviously flies in the face of conventional standards regards BMI—as you’ll recall from the BMI brackets we listed above.

Read the science: BMI and all-cause mortality in older adults: a meta-analysis

Athletic people

A demographic often described in scientific literature as “athletes”, but that can be misleading. When we say “athletes”, what comes to mind? Probably Olympians, or other professional sportspeople.

But also athletic, when it comes to body composition, are such people as fitness enthusiasts and manual laborers. Which makes for a lot more people affected by this!

Athletic people tend to have more lean muscle mass (muscle weighs more than fat), and heavier bones (can’t build strong muscles on weak bones, so the bones get stronger too, which means denser)… But that lean muscle mass can actually increase metabolism and help ward off many of the very same things that BMI is used as a risk indicator for (e.g. heart disease, and diabetes). So people in this category will actually be at lower risk, while (by BMI) getting told they are at higher risk.

If not white

Physical characteristics of race can vary by more than skin color, relevant considerations in this case include, for example:

• Black people, on average, not only have more lean muscle mass and less fat than white people, but also, have completely different risk factors for diseases such as diabetes.
• Asian people, on average, are shorter than white people, and as such may see increased health risks before BMI qualifies as overweight.
• Hispanic people, on average, again have different physical characteristics that throw out the results, in a manner that would need lower cutoffs to be even as “useful” as it is for white people.

Further reading on this: BMI and the BIPOC Community

In summary:

If you’re an average white European working-age man, BMI can sometimes be a useful general guide. If however you fall into one or more of the above categories, it is likely to be inaccurate at best, if not outright telling the opposite of the truth.

What’s more useful, then?

For heart disease risk and diabetes risk both, waist circumference is a much more universally reliable indicator. And since those two things tend to affect a lot of other health risks, it becomes an excellent starting point for being aware of many aspects of health.

Pregnancy will still throw off waist circumference a little (measure below the bump, not around it!), but it will nevertheless be more helpful than BMI even then, as it becomes necessary to just increase the numbers a little, according to gestational month and any confounding factors e.g. twins, triplets, etc. Ask your obstetrician about this, as it’s beyond the scope of today’s newsletter!

As to what’s considered a risk:

• Waist circumference of more than 35 inches for women
• Waist circumference of more than 40 inches for men

These numbers are considered applicable across demographics of age, sex, ethnicity, and lifestyle.

Source: Waist circumference as a vital sign in clinical practice: a Consensus Statement from the IAS and ICCR Working Group on Visceral Obesity

There are more options than just waist circumference though; we delve into them in detail here:

Better Than BMI

Take care!

When it comes to dairy products, generally speaking, fermented ones (such as most cheeses and yogurts) are considered healthy in moderation, and unfermented ones have their pros and cons that can be argued and quibbled “until the cows come home”. We gave a broad overview, here:

Is Dairy Scary?

Furthermore, you may recall that there’s some controversy/dissent about when human babies can have cows’ milk:

When can my baby drink cow’s milk? It’s sooner than you think

So, what about bird flu now?

Earlier this year, the information from the dairy industry was that it was nothing to be worried about for the time being:

Bird Flu Is Bad for Poultry and Dairy Cows. It’s Not a Dire Threat for Most of Us — Yet.

More recently, the latest science has found:

❝We found a first-order decay rate constant of −2.05 day^–1 equivalent to a T₉₉ of 2.3 days. Viral RNA remained detectable for at least 57 days with no degradation. Pasteurization (63 °C for 30 min) reduced infectious virus to undetectable levels and reduced viral RNA concentrations, but reduction was less than 1 log₁₀.

The prolonged persistence of viral RNA in both raw and pasteurized milk has implications for food safety assessments and environmental surveillance❞

You can find the study here:

Infectivity and Persistence of Influenza A Virus in Raw Milk

In short: raw milk keeps the infectious virus; pasteurization appears to render it uninfectious, though viral RNA remains present.

This is relevant, because of the bird flu virus being found in milk:

World Health Organization | H5N1 strain of bird flu found in milk

To this end, a moratorium has been placed on the sale of raw milk, first by the California Dept of Public Health (following an outbreak in California):

California halts sales of raw milk due to bird flu virus contamination

And then, functionally, by the USDA, though rather than an outright ban, it’s requiring testing for the virus:

USDA orders testing of milk supply for presence of bird flu virus

So, is pasteurized milk safe?

The official answer to this, per the FDA, is… Honestly, a lot of hand-wringing and shrugging. What we do know is:

• the bird flu virus has been found in pasteurized milk too
• the test for this is very sensitive, and has the extra strength/weakness that viral fragments will flag it as a positive
• it is assumed that the virus was inactivated by the pasteurization process
• it could, however, have been the entire virus, the test simply does not tell us which

In the FDA’s own words:

❝The pasteurization process has served public health well for more than 100 years. Even if the virus is detected in raw milk, pasteurization is generally expected to eliminate pathogens to a level that does not pose a risk to consumer health❞

So, there we have it: the FDA does not have a reassurance exactly, but it does have a general expectation.

Source: US Officials: Bird flu viral fragments found in pasteurized milk

Want to know more?

You might like this mythbusting edition we did a little while back:

Pasteurization: What It Does And Doesn’t Do ← this is about its effect on risks and nutrients

Take care!