A massive study spanning nearly 40 years may change everything we think we know about what defines a healthy body weight.

First invented by Adolphe Quetelet in the mid 1800s, the Body Mass Index (or BMI) identifies four basic weight categories; underweight, normal/healthy weight, overweight and obese. Individuals are placed into one of these categories based on a number determined by dividing one’s weight (kg) by one’s height (m) squared. The table below shows how the categories are identified:

Since the 1990s the BMI has been one of the most popular tools for measuring and determining the health of individuals and large populations. In fact it has become the standard measurement for body weight used by clinicians, researchers, governments and the general public all over the world. New research by Copenhagen University Hospital, however, may challenge our reliance upon this tool.

The study, led by Børge Nordestgaard, assessed the mortality rate of over 100,000 people split over 3 cohorts; 1976-1978, 1991-1994 and 2003-2013. Deaths within all three cohorts were recorded alongside BMI and the two data sets were compared. What the results found may be a game-changer for individual and public healthcare strategies.

Nordestgaard discovered that, across all cohorts and excluding individuals who died of cancer, the BMI category associated with the fewest deaths was the overweight category. What’s more, the specific BMI associated with the lowest mortality rate has been steadily increasing, from 23.7 to 24.6 to 27. A similar pattern was identified for deaths caused specifically by cardiovascular disease, the biggest killer in the world and one of the most common diseases caused by excess body fat. What we thought was healthy was completely wrong.

100,000 people is a huge sample size, no matter who you ask. With all research, however, we need to look a bit deeper. The vast majority of participants were white Danes, meaning that this data probably can’t be extrapolated to other ethnic groups. Furthermore, we must consider how advances in medicines may have influenced the results. As the years have progressed, our ability to treat conditions such as cardiovascular disease has increased immensely; maybe, thanks to medical advances, people are alive today from the 2003 cohort who would have died if they were part of the 1976 cohort.

We cannot say for sure, but what we do know is is that overweight individuals are more likely to survive than ‘healthy’ people. It is unlikely that this is the case due to treatment for CVD, Type 2 Diabetes or other conditions associated with ‘excessive weight’.

Before we go any further we must say that this research does not vindicate a life of fridge-picking and lazy evenings in front of the TV. It does, however, suggest that how we define and measure a healthy body needs a drastic rethink.

Despite its use in so many circles, the BMI has actually been under scrutiny for a long time. For a start, it cannot distinguish between muscle and fat; according to the BMI a 20 stone professional rugby player would be overweight if not obese, as would a pregnant woman. The categorical nature of measurements is also difficult to swallow – there is no scientific reason why a BMI of 24.99 is normal but a BMI of 25.00 is overweight. Furthermore, the categories are very unreliable across different ethnic groups.

In fact, aside from it’s simplicity and lack of expense, there are no compelling reasons as to why the BMI should be used as a global measure of health, and yet it is. No matter one’s personal circumstances, ethnic background or body composition, you can be in no doubt that a doctor assessing your body weight will use the BMI.

The research by Nordestgaard et al may be both a blessing and a curse. Years of trying to shed those last few pounds that just won’t budge could be at an end, but realising that we’ve been measuring weight incorrectly for decades puts us in a kind of limbo. We have our cake, but are we allowed to eat it?

With a world becoming more and more interested in the individual, this may be the perfect opportunity to develop a measuring tool which looks at data from YOU, rather than an entire populace, to determine whether you are healthy. Plenty of businesses are already using an individualised approach to improve your life (and theirs as well) – what to advertise to you, what to sell in your local shop, what news stories to show you and what you’re likely to complain about. Why not use the same approach with our health?

The mechanism by which this tailored approach may be achieved is up for debate, but with giant companies like Google creating healthcare products using personal data, there may be a number of options at our disposal.

For the time being the old standby “mind what you eat and get plenty of exercise” is still as effective as ever.

To find out more about the study, check out the original research paper at JAMA.