Ten countries. Twelve years. Half a million people. A new study on the link between health and weight is certainly wide-ranging.

According to the work, people who are overweight or obese are 28% more likely to develop heart disease than those at a normal weight. The researchers controlled for other factors like blood pressure and cholesterol, suggesting that excessive weight is unhealthy in itself. Their finding is echoed by a similar study reported in May (but still unpublishedThis means that the study's findings have not yet been "peer reviewed" - ie, assessed and declared scientifically sound by experts.).

This sounds like common sense - how often have we been told that thin is good and fat is bad? Last December, obesity was cited as one of the reasons why the life expectancy of Americans is fallingThe average life expectancy fell from 78.9 to 78.8 years between 2014 and 2015, according to the National Centre for Health Statistics.. But the science behind fatness is far from settled; this new study is only the latest twist in a debate that has been raging for over a decade.

Scientists tend to agree that severe obesity is a health risk. For the moderately fat, however, things are less clear. In 2013, a major study found that overweight people actually live a little longer than their slimmer peers.

The true marks of fitness, it suggested, are not determined by how heavy you are. In other words, you can be "fat but fit".

This study outraged many experts - one dismissed it as "a pile of rubbish". Others supported it with similar findings. Few subjects are more divisive in the world of health.

One reason is that we still know little about how fat relates to the body's functions. It seems to increase blood pressure and affect the production of insulin, leading to diabetes. On the other hand, it may help sustain the body through illness and cushion it against blows - but this is mostly speculation.

Another problem is that many studies - on both sides of the debate - rely on body mass index (BMI), a 185-year-old method for measuring weight adjusted for height. BMI has many shortcomings. For example, it cannot distinguish between fat and muscle, and the way it classifies people's weight is arbitraryThe boundaries of these categories (see above) have changed across time..

"Time for a change," say some. For too long, society has promoted slimness as an ideal, as if the science backed this up 100%. It does not. This ideal has more to do with old-fashioned notions of beauty than with health. Eating well and exercising are important, but nobody should be made to feel uncomfortable about their size.

"Don't get carried away," reply others. On the whole, the risks of being overweight have more basis in science than the benefits. The research may be inconclusive, and each individual is a different case, but that is no excuse to get as fat as we want. This new study is one of the biggest ever conducted. We should take it seriously.