Can being overweight improve your life expectancy?

The Independent: "Recipe for a long life: overweight people have LOWER death risk"

The Telegraph: "Why being overweight could be a lifesaver: Being overweight could add years to a person's life, scientists suggest"

The Daily Mail: "Dreading your diet? Don't worry... plump people live LONGER than their skinnier counterparts (but only if they're a few pounds overweight)"

Ate too much over Christmas? Signing up to a gym as part of your New Year's resolution? According to some newspapers today you might be wasting your time, because being overweight could "add years to a person's life". But does that mean you should ditch the diet and reach for the leftover Christmas pudding instead?  

The newspapers' claims come from a study that featured in this week's Journal of the American Medical Association. Researchers analysed 97 reports that compared body mass index (BMI) to the risk of death from any cause. 

Altogether, the 97 studies provided a sample size of more than 2.88 million individuals internationally and more than 270,000 deaths. It concluded that those with a BMI of 25-30 (classed as overweight) were 'associated with significantly lower all-cause mortality' or in other words, had a lower risk of premature death than those with normal weight (BMI of 18.5-25).

In medical circles, this strange phenomenon is increasingly known as the 'obesity paradox'. But is there a causal link between putting on weight and longevity, as some papers suggest?

It should be noted that the study only compares BMI to the risk of death in general. As recognised by the study, it does not compare BMI to morbidity (the disease rate) or 'cause-specific' mortality (the risk of death from a particular cause). So could there be another explanation?

"When people become very ill, they often tend to become very thin", write Michael Blastland and Andrew Dilnot in their book 'The Tiger That Isn't'. "It is not, in their case, thinness that causes death, but impending death that causes thinness. Put their fates into the mix and it has a marked effect on the results."

A number of studies have explored this further, and some have concluded that it is actually unintended weight loss - often associated with illness - that actually increases the risk of death most severely. This would suggest that we should take the newspapers' advice to bulk up with a pinch of salt.

Furthermore, the study recognises two other factors that could help to explain the paradox: the "earlier presentation of heavier patients" and their "greater likelihood of receiving optimal medical treatment".

Developments in medical treatment have prolonged the lives of those who are overweight and obese. A report in 2010 indicated that the presence of obesity 'triggers prescribing' among doctors, which might extend the patient's life, if not the quality of it. 

Conclusion

So while there is a well-established link between higher BMIs and lower mortality rates, it is trickier to say that being overweight in itself enhances your life expectancy, as there are many other factors and explanations that could be in play.

Some experts have suggested that it is actually the ability to control and maintain a constant weight that is most important to a person's prognosis, a conclusion that is also often found in the medical literature.