It’s a scientific rule of thumb so widespread, so deeply embedded in a million magazine articles and diet plans, that it’s probably better known than all four laws of thermodynamics put together.
Your health can be gauged by calculating your body mass index (BMI), a simple process that involves dividing your weight by the square of your height. It’s a formula used by everyone from gym instructors to nutritionists to paleo hucksters to your local GP.
There’s just one problem. It doesn’t work.
“I think partly there is a misconception that you can pick someone who has a heart health risk when they walk into the room, and that’s just wrong,” Professor Garry Jennings, chief executive of the Heart Foundation, said. “You can’t tell what somebody’s blood pressure or cholesterol is. Obesity is something you can see, so people tend to put a judgment on that rather than the other factors that might be contributing.”
Building on a body of evidence that over the past few years has increasingly thrown the reliability of BMI measurement as a health indicator into question, two recent studies look likely to prompt a wholesale reassessment of its worth.
And in the US, at least, the need to do so is now urgent. Late last year that country’s Equal Employment Opportunity Commission (EEOC) proposed new regulations governing employer-provided health insurance. The new scheme will allow employers to force workers to pay up to 30 per cent of their own costs if they fail to meet particular health targets – namely a BMI within the “normal” range.
BMI categories are expressed as numbers, and approved by the World Health Organisation. The “normal” range is between 18.5 and 24.99. Anything lower is classified as “underweight”. A BMI between 25 and 29.99 is called “overweight”, and 30 or above is “obese”.
The EEOC’s proposals are based on the assumption that a normal BMI is synonymous with good health, while scores in the ranges above or below are proportionally associated with poorer prospects.
A study published this month in the International Journal of Obesity pretty much torpedoes that idea. The research team, led by Dr Janet Tomiyama, of the University of California, Los Angeles, checked individual BMI results against other indicators, including cholesterol levels, blood pressure and insulin resistance. The results were gobsmacking.
The study found that 30 per cent of people in the normal BMI range were at above-average risk of heart disease. Conversely, 48 per cent of “overweight” and 29 per cent of “obese” people were happily heart-healthy.
“Using BMI categories as the main indicator of health, an estimated 74,936,678 US adults are misclassified as cardiometabolically unhealthy or cardiometabolically healthy,” the study concluded.
A second study, published in January in the journal Obesity, also tested whether the standard application of BMI measurements accurately reflected individual health. Led by Dr Howard Karloff, of the Georgia Tech College of Computing, the research looked at the lives of 566,000 US pensioners who had filled out a health and diet survey in 1996-97.
The study tracked theirs lives, or deaths, since then, to see whether standard BMI measurement correctly predicted mortality. It also ran the numbers on variations of the BMI formula to see if any were better at estimating the chances of an individual’s death.
The researchers concluded: “The average personalised optimal BMI is approximately 26, which is in the current ‘overweight’ category.”
In other words, being a bit tubby, at least if you’re at retirement age, might actually be a good thing. Interestingly enough, it’s a conclusion that goes against common wisdom but doesn’t surprise other researchers in the field.
“If you are in your 60s, there’s no point in trying to lose weight because you’ve already got there, and the evidence is that you would be better off staying where you are – overweight – than losing weight,” Dr Leon Flicker, professor of geriatric medicine at the University of WA, said.
In 2010 Flicker published the results of a decade-long study into the relationship between health and BMI among more than 9000 people aged between 70 and 75. The study found that through the 10-year period overweight oldies were significantly less likely to die than their normal-weight colleagues.
Flicker said he and his team were initially surprised by the result. Further refinements, however, such as adjusting the calculations to account for chronic illness, smoking and other lifestyle factors, left the conclusion unchanged. The best BMI for older people was 26.3.
“The overweight range was better than being so-called normal weight when you’re over 70 as far as survival goes,” he said. “We simply see that surviving 10 years is a good thing, and not surviving is a bad thing. And you’re more likely to have a good thing if you’re overweight.”
Identifying a statistical tendency and figuring out the cause of it, however, are two very different things. Just why carrying a few extra kilos is good for you when you reach a certain age remains an open question.
“There is a body of evidence that suggests that as you get older there are associations between slightly high BMI and health,” the Heart Foundation’s Jennings said.
“And we don’t quite understand that yet. It might be that it gives you a bit of a reserve if you get hit with something else. It might be a statistical flaw, and that the people who are a little bit overweight and have survived to old age have got some sort of resistance to heart disease. ‘Survivor effect’, that’s called.”
Flicker rejected the notion of a statistics glitch, but agreed the relationship between being overweight and living longer might simply be because other overweight people die earlier, leaving the metabolically stronger ones to keep going. “The people who have reached their 60s and 70s might be different from the ones who have died off,” he said.
The failures of standard BMI measures, however, are not restricted to the elderly. Karloff at Georgia Tech might have used pensioners for his source material, but believes his findings extend across all age groups.
His team’s primary finding was that a “personalised optimal BMI” – that is, a BMI measurement modified by a range of other factors such as diet and education – not only changed what was statistically “normal” but was also a better predictor of longevity.
Speaking to Fairfax Media, Karloff said part of the problem was that the whole concept of body mass index was first formulated in the 19th century. It was developed as a method of measuring populations, not as a pointer to individual health.
“Not only is it an issue of using a tool designed for a purpose for which it was not intended,” he said. “It’s the result of using a tool designed around 1840, in a period lacking big data sets and computers, when better data analyses and tools exist today.”
Moving to a personalised BMI could significantly improve health outcomes, Karloff said.
“On average, over all individuals, the change would be significant. More importantly, for some individuals the change would be substantial.”
It’s a sentiment that was echoed in the findings of Tomiyama’s study. Her team suggested that using the current BMI to assess health could actually put people at risk.
“Misclassifying individual health on the basis of high BMI has numerous potential consequences,” the study warned.
“Not only do these types of punitive [insurance] policies exacerbate the well-established economic consequences of being heavy, but they are also perceived as stigmatising by heavier individuals, which can have a host of negative mental and physical health consequences.
“Furthermore, individuals with an overweight or obese BMI are often instructed by their physicians to lose weight. If these individuals are otherwise healthy, however, intentional weight loss may actually increase risk for mortality.”
So is it time to ditch the BMI?
Karloff thinks it is. “There is enough evidence now that the BMI recommendations should change. We feel that major health organisations should revisit the recommendations. Rather than object, many would welcome the change as overdue.”
Jennings at the Heart Foundation was more equivocal, but agreed that BMI measurements, on their own, are not sufficient gauges of health.
“While I’m not going to jump rabidly to the defence of BMI, I don’t think it’s time to ditch it,” he said.
“It is a measure that has some value and, just like cholesterol or blood pressure, there’s no single variable or measurement that you can make that tells you everything about your cardiac and metabolic health. But if you put it all together then you can tell a story.”