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Can being overweight be good for you?

by Ed Yong | Analysis

16 October 2009

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Earlier this week, the Telegraph reported that, “It is better to be fat than thin at age 40, according to a new study which shows that the overweight are likely to live longer than those who are slim.”

These claims were apparently based on a new Japanese study, although the article didn’t provide a reference and we haven’t been able to find the paper.

Even so, we have blogged about the alleged benefits of being overweight before, and this seems like a timely opportunity to revisit the evidence in this area, and to consider the new Japanese research in the context of other studies.

A bit of history

Over the last five years, there have been a few similar stories suggesting that the dangers of being overweight or obese are being oversold. And in a climate where the phrase “obesity epidemic” seems itself to be going viral, such stories prove to be very popular.

In 2005, a team from the US Centers of Disease Control reported that being obese (having a Body Mass Index (BMI) of 30 or over) does increase the odds of dying at a given age, but being overweight  (having a BMI of 25-30) actually reduces those odds. The same team later updated their statistical analysis to say that:

  • being overweight reduces the risk of dying early from causes other than heart disease and cancer, but not from these two groups of diseases
  • being obese was linked to higher death rates from heart disease and certain types of cancer
  • combined, obesity and being overweight actually led to a net reduction in the number of deaths, to the tune of 44,000 per year.

The analysis was hailed for its rigorous statistics, but critics noted a few problems (which we’ve blogged about before). For a start, it didn’t prove cause and effect. It suggested that lean people are more likely to die than overweight people. But sickness often makes people leaner and even conditions that result from obesity, like cancer and heart disease, eventually lead to weight loss.

It also represented a snapshot in time. Studies like these are often less reliable than forward-looking studies that follow people over their lives to see what happens.

Fortunately, several of these “prospective studies” have been carried out, looking at how body weight affects the risk of an early death. But as always, there are some disagreements.

There are studies like the most recent Japanese one, saying that being overweight increases lifespan or reduces the odds of dying early. However, there are others that show the opposite. A Swedish study that we blogged about before showed that being overweight as a teenager increasesd the risk of an early death by the same extent as smoking one to ten cigarettes a day.

The big picture

When some studies say one thing and others say another, it’s useful to look at the evidence as a whole and see what picture it paints.

And one group of scientists has done just that, pulling together the results of 57 different prospective studies across Europe and North America. Together, the studies looked at the health of over 894,000 people, making this the best evidence to date on the effect of obesity on death.

The researchers found that the optimal BMI was 22.5, which sits in the middle of the healthy range (18.5 to 25). At this point, the odds of dying prematurely are at their lowest, providing powerful support for the importance of maintaining a health body weight.

The results of this analysis are particularly strong because they’re based on such a large number of people. For comparison, the sample size in the most recent Japanese study was around 20 times smaller. Larger samples make it less likely that results are influenced by chance or biases that might skew the results.

Death and life

It’s also important to note that most of the studies we’ve discussed in this post have looked at the links between body weight and premature death. But it’s not just the quantity of life that matters but its quality.

Many of the conditions and diseases that are caused by obesity, including heart disease and diabetes, can often be successfully managed or treated. But that doesn’t mean that they should be ignored.

So it’s also important to look at whether being overweight or obese increase the risk of developing diseases rather than just dying from them. And certainly, when it comes to cancer, that evidence is very clear. After being a non-smoker, keeping a healthy body weight is one of the best ways of stacking the odds of avoiding cancer in your favour.

Being overweight and obese increases the risk of many different types of cancer, including some of the most common (breast and bowel cancers) and some of the most difficult to treat (cancers of the pancreas, gallbladder and food pipe (oesophagus) to name a few). In fact, as more large studies are completed, the list of cancers that are affected by body weight grows ever larger.