A study spanning almost four decades and involving more than 100,000 adults in Denmark has found that those with an ‘overweight' body mass index (or BMI) were more likely to live longer than those in the 'healthy', 'underweight', and 'obese' categories.

The results bring into question one of the fundamental assumptions we have about our health right now - that a 'healthy' BMI equals a longer life. And it’s not the first time - a number of studies in the past have found that packing on a few extra pounds might not be so bad after all.

Before we go into any of the details, these results are definitely not an excuse to cancel your gym membership and have ice cream for breakfast. No one’s arguing that forgoing exercise and eating crap is your best shot at living a long, happy life. (Damn.)

What these results are suggesting is that we might need to rethink our definition of what the term "overweight" actually means.

The study, led by clinical biochemist Børge Nordestgaard from Copenhagen University Hospital, analysed the medical data of more than 100,000 adults in Denmark, recruited in three groups about 15 years apart.

They found that during the four decades of analysis - from 1976 to 2013 - the BMI associated with lowest risk of death increased from 23.7 to 27.

If your BMI is between 18.5 and 24.9, you're considered normal or 'healthy', and if your BMI is between 25 and 29.9, you’re considered 'overweight'. A BMI of 30 or more is classified as 'obese'.

The study also found that those in the 'obese' category ended up having the same risk of death as those in the 'normal' range, even when factors such as age, sex, family history of disease, socio-economic status, and smoking were taken into account.

This means that in the past 40 years, the weight category associated with the longest lifespan has gone from 'normal' to squarely in the 'overweight' camp, which suggests that either our classification for 'normal' weight is wrong, or the link between our weight and our overall health is far more complicated than we thought.

"BMI as a number alone may not be sufficient to predict health and risk of death," physician Rexford Ahima from the University of Pennsylvania School of Medicine, who wasn’t involved in the study, told Science News. "It has to be taken within context."

The case against BMI as being the go-to metic for a healthy weight has been brewing for some time, with NFL star Tom Brady and NBA veteran Paul Pearce famously nudging the 'obese' category, and a study from earlier this year finding that 34.4 million 'overweight' and 19.8 million 'obese' Americans are actually healthy, based on a range of cardio-metabolic health markers.

Quite simply, it looks like the body mass index is pretty much bullshit, and we should stop using it as a horribly generalised indication of what an individual 'should' weigh.

"In the overweight BMI category, 47 percent are perfectly healthy," said Jeffrey Hunger from the University of California, Santa Barbara, who was part of the findings from back in February. "So to be using BMI as a health proxy - particularly for everyone within that category - is simply incorrect. Our study should be the final nail in the coffin for BMI."

The shift that Nordestgaard and his team found in this latest study could signify how many of the health risks associated with a higher weight, such as high cholesterol and high blood pressure, are more effectively diagnosed and treated now than they were 40 years ago.

"So maybe you can be overweight if you have [these conditions] treated," he told Esther Landhuis at Science News.

The study does have its limitations - while the 100,000 people analysed are considered to be a good representation of the population of Copenhagen, it consisted of mostly white people, so we can’t say what these results could mean for people with other backgrounds.

"A substantial fraction of Asians, for instance, develop type 2 diabetes and heart disease despite having BMIs lower than the existing cut-off point for being overweight," Landhuis points out.

But it does agree with what a number of studies have been suggesting - how long a person lives is far more complicated than the ratio of kilos to cm on their frame.

As Landhuis explains:

"In one study of type 2 diabetes patients, those with normal weight when diagnosed were more likely to die than those who were overweight or obese. And a 2013 meta-analysis of 97 studies found that being overweight was associated with lower risk of death than having a normal BMI - a surprising finding that echoed a 2005 study by the same researchers."

Hopefully, as more studies like these are published, we’ll take a more personalised approach to health in the coming decades, so we don’t end up focusing on the wrong things when it comes to what’s best for an individual.

The results of the most recent study have been published in JAMA.