Body Mass Index Needs Two New Friends But It Only Gets One For Now

fat-skinnyby David Greenwalt

As a wellness coach I’m sick to death of the same old crap when body mass index (BMI) is demonized as a measure of fatness when the following reason is given; WHAT ABOUT ALL THAT MUSCLE MASS PEOPLE HAVE?! People don’t have too much muscle. If anything they have too little. Truth be told? Most who demonize BMI as worthless for the muscle excuse, are usually just carrying too many pounds of extra fluff.

More times than not people who are assessed as being too fat based on BMI are exactly that – too fat. When 66 percent of the public is overweight and 35 percent are obese it’s not all that hard to be right when we presume a high BMI to correctly indicate someone with a high BMI is overfat.

But BMI does need some help. BMI gives skinny-fat people a pass and it doesn’t consider other aspects of overall fitness. Specifically BMI needs three new friends but, as you’ll see, it’s likely to only get one anytime soon.

What Is BMI?

BMI is a measure of body fat based on height and weight. To calculate BMI you only need two things: height and weight. This is the reason it’s so universally used in wellness and medicine as a baseline indicator of fatness – it’s easy and completely non-invasive. BMI is a person’s weight in kilograms divided by the square of height in meters. Hundreds of websites offer BMI calculators and charts.

Healthy BMI

Yes, it’s true, that BMI will miss the mark when we’re evaluating a thickly muscled person. Again, however, I’m going to stand my ground and say it’s just such a TINY fraction of the public I barely care.

I’m Skinny Fat and a Couch Potato BUT I Have a Healthy BMI

A class of individuals that BMI misses far more often are the middle-aged and elderly who, by BMI, are assessed as being at a healthy weight but who are under-muscled and over-fat. Yep, the good ol’ “skinny fat” people. And it’s far more likely to occur as people age when they aren’t engaged in strength training. Unless you’re engaged in strength training you WILL lose muscle as you age. It’s called sarcopenia.

If you’re at a “healthy” weight by BMI but your body fat is equal to or greater than 25% or 32% as a man or woman respectively are you okay? No way. You’re obese.

Previous research has highlighted the so-called “obesity paradox” – overweight and moderately obese people with heart disease or other chronic ills tend to outlive thinner people with those same conditions.

It seems counterintuitive until you consider the “skinny fat” couch potatoes are considered a “healthy” weight by BMI if their height and weight fit the formula. What if some of the slightly overweight people have a lower body fat than their “healthy-weight” counterparts? Also what about overall fitness?

If your BMI is a healthy 24 but you are skinny fat AND you engage in nothing that improves fitness (vigorous physical activity) are you healthier than someone with a BMI of 27 (overweight) but who has a lower body fat and engages in a solid exercise routine five days a week? Not likely. Not likely at all.

So BMI misses the mark not just by punishing the thickly muscled (rare) but also by giving too much credit to the skinny fat and/or couch potatoes.

BMI Needs Two Friends But, For Now, It Only Gets One

As sick as I am of BMI being demonized because of these, supposedly, thickly muscled freaks throwing off the BMI calculation I absolutely believe BMI gives TOO much credit to all skinny fats but especially the middle-aged and elderly. And, because BMI wasn’t designed to measure fitness, only fatness, BMI completely misses MOST aspects of fitness (i.e., muscle strength, muscle endurance, flexibility, aerobic capacity, anaerobic capacity, agility).

So BMI needs some help. It’s time BMI got two friends to help with the heavy lifting of quick-fatness assessment and another friend to help with assessing fitness beyond body fat (body fat IS a part of overall fitness).

The two fat-assessment friends BMI needs are:

1. A body fat measurement
2. A waist-to-hip ratio (requiring a waist and hip measurement)

More and more research finds more correlation to premature death and disease with a high body fat and/or a poor waist-to-hip ratio than BMI.

Body Fat

A healthy body fat for a woman will range between 14 and 25 percent. For a man a healthy body fat will range between 6 and 18 percent. You’re obese as a woman if your body fat is above 32 percent and obese as a man if your body fat is greater than 25 percent.

Great, so let’s just check body fat and help BMI out. Not so fast.

In the field of design they’ll tell any prospective customer you can have any two of the following:


Unfortunately for body fat it sort of works the same.

Body fat evaluation type:

Dual-energy X-ray absorptiometry (DXA) | GOOD

Considered the new, gold standard of body-fat analysis DXA is reliable but not practical or cheap for most.

Lying on a table for 10-15 minutes is easy but most DXA tests are over $100. What if you wanted to keep an eye on your body fat each month? Good luck. And if primary care docs started offering in-office DXA insurance would probably be billed $67,900 (okay I exaggerated – a little).

Skinfold | FAST, CHEAP

Skinfold is fast, cheap and completely UNreliable if you are dealing with someone seriously obese and/or you don’t have a super skilled person holding the calipers. Skinfold is nicely reliable if you are measuring someone with a relatively low body fat AND the person holding the calipers is highly skilled. I’ve administered no less than 2,000 skinfold tests and it’s IMPOSSIBLE to use skinfold on the severely obese.

Click HERE to see separate videos of someone doing a NICE job with 3-site skinfold measuring on both male and female.

Hydrostatic (under-water weighing) | GOOD

Hydrostatic body-fat analysis is among the best of the best. But it’s completely impractical for a physician’s office or quick assessment from home. It’s also not the cheapest – usually costing between $100 and $200 per test.

Bioelectrical impedance analysis | FAST, CHEAP

BIA_measuresMost home scales that measure body fat use bioelectrical impedance analysis (BIA).

BIA circulates a small electric signal throughout the body. It’s safe and cannot be felt by adult or child. BIA measures the impedance or resistance to the signal as it travels through the water that is found in muscle and fat. The more muscle a person has, the more water their body can hold. The greater the amount of water in a person’s body, the easier it is for the current to pass through it. The more fat, the more resistance to the current.

But is it accurate? It depends. If you’re dehydrated when you get measured it’ll throw things off and may indicate you are fatter than you really are and vice versa. And that’s why you’re not going to see it in physician offices anytime soon. It’s not universally reliable.

Waist-To-Hip Ratio

Waist–to-hip ratio (WHR) is the ratio of the circumference of the waist to that of the hips. This is calculated as waist measurement divided by hip measurement. For example, a person with a 25″ (64 cm) waist and 38″ (97 cm) hips has a WHR of about 0.66.

You’re normal weight, overweight and obese as a woman if your WHR is < 0.80, 0.81-0.84, and >= 0.85 respectively. Men are normal weight, overweight and obese if their WHR is < 0.90, 0.91-0.99 and >=1.00 respectively.

And this, WHR, is the only friend BMI is going to reasonably get if we’re wanting our medical system to improve their evaluation of fatness anytime soon. WHR is easy, fast, cheap and reasonably reliable, and especially when used in conjunction with BMI.

What About Measuring Fitness?

Although completely anti-climatic to this article measuring true fitness is hardest and most expensive of all. And are we talking about “performance-related fitness” or “health-related fitness?”

With respect to performance-related fitness there is no one-size-fits-all-easy test that’ll tell us how strong you are, your muscular endurance, how flexible you are or whether you have reasonable balance and agility.

With respect to health-related fitness it’s just as important that, at a minimum, you know your cholesterol (good, bad and the newer particle-based evaluations), blood pressure, fasting glucose and triglycerides.

See my article HERE where I debunk the theory that Crossfit was the first to define fitness and I delve deeper into what fitness really means.

David Says

As a whole people need to shut their yappers when whining about how bad BMI is at evaluating the overweight because of all that muscle so many supposedly have. But BMI fails more often at giving a pass to the skinny fats (especially middle-aged and elderly who don’t do strength training and have lost a lot of muscle). Although it’d be nice if physician offices could give BMI a body-fat friend what’s more likely to happen, and needs to happen, is people should be evaluated for the waist-to-hip ratio in addition to BMI. Finally, BMI doesn’t give any account for overall fitness (neither performance- or health-based) beyond a crude measure of fatness. Performance-based screenings are impractical but health-based biometric screenings are possible via primary care docs. For those who get regular checkups these screenings are nothing new.