Why BMI Isn't A Helpful Measurement

This month, the toxic combination of extreme heat and summer holidays will probably mean that you're going to expose more flesh than you would like to someone whose opinion you care about.

Couple picture from Shutterstock

January is the month of fat-phobia, when you regret that you allowed the gym membership you took out in August (the most popular month for gym sign-ons) to lapse in September. You're not alone: 67% of people with gym memberships no longer use them.

picture from Shutterstock

So let's cut to the bone on this fat business: How do we measure fatness? What is this BMI thing, and is it all it's cracked up to be? Is life just one long slide into adiposity, and what can I do about it?

Epidemiologists most commonly use body mass index (BMI) as a measure of fatness. BMI is calculated by dividing your weight in kilograms by the square of your height in metres.

Take me, for example. I weigh 81kg, and am 1.8m tall, so my BMI is exactly 25. BMIs less than 18.5 are considered to be dangerously low, 18.5-25 in the "normal" or "healthy" range (so I just scrape in, fasted, nude, fairly dehydrated, on a good day), 25-30 is considered overweight, and greater than 30 obese.

These cut-offs came from studies of relationships between BMI and all-cause mortality in non-smokers in the 1950s, when the risk of dying was lowest for BMIs of 20-25.

Now the limitations of BMI should be obvious to anyone. What if I've got a lot of muscle and not much fat? Michael Hooper (the current Wallabies' captain), boxing champion Mike Tyson and Rugby League great Mal Meninga in their prime would all be classified as obese, but I wouldn't describe them as fat, and certainly not to their faces.

What about Samoans and Fijians, who have a very large muscle mass, or Asians who tend to be much more lightly muscled? Some researchers recommend higher or lower cut-offs for specific ethnic groups.

There's some evidence that adults are getting fatter at the same BMI -- that is, someone with a BMI of 25 today is fatter than someone with a BMI of 25 a few decades ago. BMI is probably also biased against taller people due to some inappropriate scaling assumptions.

Some rather more surprising difficulties with BMI have emerged recently. While the risk of death may indeed have been lowest in the normal range in the 1950s, it certainly isn't now. A slew of recent studies have shown that the risk is now lowest for people right in the middle of the overweight range, with a BMI of about 27.

The problem is that BMI measures relative weight, at best a poor proxy for fatness, which may be the real metabolic culprit. Fatness may be better estimated from skinfolds (pinches of fat at embarrassing sites on the body), which the Exercise and Sports Science Australia-accredited exercise professional at your local gym should be able to measure for you.

Even more to the point is the distribution of body fat: being fat in the wrong places (around the abdomen) is worse than being fat in the right places (hips and thighs). In fact, thigh fat appears to be protective, leading to a healthier blood fat profile, but unfortunately less chance of attracting the desirable targets on the beach.

A simple measure is waist girth -- measure it with a tape half-way between the bottom of the ribs and the top of the hip bones. The Australian government recommends cut-offs of 94 cm (bad) and 102 cm (very bad) for men, and 80 cm and 88 cm for women.

Picture: Ed Yourdon

If you've really got some time and money to spare, universities and some radiologists can give you a DXA (dual-energy X-ray absorptiometry) scan, pretty much the gold standard these days, which will tell you just how much fat you have and where it is.

Does getting older inevitably mean getting fatter? Just about every longitudinal study shows weight gain with age. American and Australian studies report weight gains of 0.2 to 1.0 kg/year in adults aged 20 to 60, with slower rates of gain for people who are physically active, and richer.

There are good physiological reasons to expect increasing body fat with age: anabolic hormones responsible for converting excess calories into muscle rather than fat -- testosterone, human growth hormone -- decline rapidly with age, and physical activity becomes harder with age-related musculoskeletal and joint problems.

There's bad news on the weight gain front, I'm afraid: getting fat is worse than being fat. In a 16 year follow-up of almost 10,000 Americans aged 51 or more, those least likely to die were overweight people who remained overweight, while gaining weight led to higher risk.

Certain life events can trigger rapid weight gain. There is a sudden weight gain of a few kilograms when women move in with a partner -- perhaps it's the "I'm no longer on the market" effect. But there's still hope: divorce will reverse it.

Mothers' weight increases permanently by up to 4 kg after the birth of their first child, for reasons which any mother will be happy to tell you about. After about age 60, weight starts to fall, usually marking the beginning of the slow decline to frailty and death.

Picture: Ed Yourdon

So what can you do to avoid weight creep, or at least minimise its effects? I have nothing new to offer in this department: eat lots of fruit and veges and wholegrains, get lots of exercise, get a good sleep, get rich.

Remember that being lean and unfit is worse than being fat and fit. In one study, men who were lean but unfit had almost twice the risk of dying compared to men who were overweight but fit. And all the rest, from crossfit to core training, from carbs to quinoa, all the rest, as Verlaine says, is just literature.The Conversation

Tim Olds is Professor of Health Sciences at University of South Australia.

This article was originally published on The Conversation. Read the original article.


Comments

    I bet 99% of people making this argument don't have a high muscle to fat ratio, and are actually just overweight.

      Dunno. Mine seems to be a bit off. But the calculators say I'm good 22. But I reckon I could probably do to loose a wee bit off the stomach.
      Though that's the opposite to the people complaining.
      Althetes and alike naturally have bmi that don't match. Your example for instance I have a female friend of islander background who is short but an amateur power lifter and her bmi says she needs to loose heaps of weight according to her. But she's an extreme example.
      In conclusion I mostly agree you.

      Last edited 10/01/15 8:26 pm

        I'm not saying it's not wrong for that type of person, just that a lot of people find a lot of excuses to do nothing about their health.

          People will tell themselves literally anything.

          My brother's a radiographer and on more than one occasion has had to get a supervisor to have a discussion with the patient about how they are too big to be measured by equipment designed for humans and will need to be referred to a facility which treats livestock, or a zoo. This is a real thing that has happened.

          How does it get that far? People tell themselves the pretty little lies that excuse their inaction.

          (Edit: In my own life, I refer to it as the 'buy new pants' decision. If you're feeling a bit tight in the pants, the solution is to panic, backpedal, and make dramatic changes to get back in the black, then institute more sustainable changes than what you've been doing. The solution is NOT to buy new fucking pants. If you do that, then you've accepted defeat. Because you will always buy new pants.)

          Last edited 12/01/15 5:37 pm

      I used to use this argument when I was a bit healthier and had some muscle mass.

      In fairness though, anyone that exercises regularly shouldn't pay any attention to BMI. The only people that should be looking at BMI are those that are sitting on their couches eating pizzas and doing 0 exercise per week. Then the BMI is about 95% accurate in my opinion.

        When I was at my physical peak bmi said i was overweight. But now I'm my lazier days my bmi says I'm a healthy weight. There are always exceptions and it's only meant to be a quick general guide.

          Yeah. The article is coming at it from the context of asking, "Am I fat?" And frankly before you even ask, you really do already know the answer. You've been living in that body all your life... you know if you're fat.

          But as a quick general guide, it's useful. Especially in the medical industry, where a lot of places have it up on quick-reference charts.

          And even then, they usually don't care so much about the 25-26s worriers ("Oh god, am I 'overweight' or normal? How close am I? So if I were just half a kilo lighter I wouldn't be overweight?! HOW IS THAT FAIR?!"), they're more trying to gauge whether someone's over or under BMI 40.

          Because after a certain point the raw majesty of the morbidly obese can distort a resonable person's ability to estimate. Also, underweight is good to know, too. Quickly checking to make sure someone healthy'ish-looking but lean, in flattering clothes, isn't secretly so light that they might not actually wake up from the anaesthetic they're about to take.

    We need a 3d graph: height vs weight vs fat percentage. Is. For this height and weight I need to be within this fat percentage range or I am not healthy.

    Bmi is very easy to measure and very hard to measure wrong. Waist measurement is harder to get the right location to measure and has other variables that can make measuring it difficult.

    That's why bmi is useful even if it is not as accurate.

    Last edited 11/01/15 2:02 am

    MeasureUp runs a fantastic affordable DXA (DEXA) service. I typically go to their office on Pitt St in Sydney but they also have a mobile service which runs all over the country.

    I thought this was common knowledge by now? I havent trusted BMI since it told me I was underweight simply due to a .1 change in weight normally it was perfect.

    I just like to give myself a score of 1 to 10 based on how many toes I can count when I look down.

    Hmmm - Today's not a good day.

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