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Being underweight as dangerous as being obese

People who are underweight are more likely to die early than people who are obese, according to a major review of research on body mass index (BMI).
By Grant Stewart

BMJ Group News

What do we know already?


Medical journals and the general media are full of studies and stories about the dangers of obesity (being very overweight). We’ve all heard a lot about this ‘obesity epidemic’ and how it’s affecting our health. But, apart from occasional stories about super-thin supermodels, we don’t hear much about the dangers of being underweight.

Studies about body weight usually refer to a measurement called BMI (body mass index). This is worked out by dividing someone’s weight in kilograms by their height in metres squared. The result is a number that doctors use as a guide to which of several categories someone falls into: underweight, healthy weight, overweight, obese, or severely obese.

A healthy weight supposedly falls between a BMI of 18.5 and 25. More than 25 is considered overweight, and less than 18.5 is classed as underweight. But most research into how BMI affects health looks at the negative health effects of being too heavy. You’d be forgiven for thinking that being underweight just means being a bit skinny. As Wallis Simpson supposedly said, you can never be too thin. Except that, as far as your health goes, you really can.

Now researchers in the US have examined whether being underweight can shorten people’s lives.

How was the new study done?

The researchers had planned to use data from a review of 97 studies that looked at the link between BMI and length of life. But only 51 of the studies even looked at underweight as a category. The previous research was mainly focused on overweight. Half of the studies completely ignored people who were underweight.

The researchers pooled the results of the 51 studies that included underweight people. The studies ran for varying lengths of time, with many lasting longer than five years. The researchers looked to see how many people in the various underweight and overweight categories died during the years of the studies, compared with people of a healthy weight.

What does the new study say?

People in the underweight category were the most likely to die during the various studies’ follow-up periods, compared with people of a healthy weight.

  • Obese people were 20 percent more likely to die compared with people of a healthy weight
  • Severely obese people were 30 percent more likely to die
  • Underweight people were 80 percent more likely to die early.

How reliable is the research?

This was a large, careful study, which gives it a good level of reliability. But a study of this type can’t actually prove that one thing causes another. It can only suggest a strong link. So we can’t say for certain that people’s weight affected their length of life.

It’s possible that some thin people were thin because they were already unwell. An illness such as cancer can cause people to lose weight.

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