Excess weight or obesity is more deadly than previously believed. According to a new study, it boosts the risk of death by anywhere from 22 per cent to 91 per cent.

The findings, published in the journal Population Studies, the counter prevailing wisdom that excess weight boosts mortality risk only in extreme cases.

The study showed that groups with higher body mass index (BMI) have higher mortality rates, contrary to numerous studies that show that heart disease, high blood pressure and diabetes (which are often associated with being overweight) elevate mortality risk.

"The conventional wisdom is that elevated BMI generally does not raise mortality risk until you get to very high levels and that there are actually some survival benefits to being overweight," said author Ryan Masters, Associate Professor of sociology at the University of Colorado Boulder.

He noted that BMI, which doctors and scientists often use as a health measure, is based on weight and height only and doesn't account for differences in body composition or how long a person has been overweight.

"It's a reflection of stature at a point in time. That's it," said Masters, noting that "it isn't fully capturing all of the nuances and different sizes and shapes the body comes in."

To see what happened when those nuances were considered, Masters looked at data from 17,784 people, including 4,468 deaths.

He discovered that a full 20 per cent of the sample characterised as "healthy" weight had been in the overweight or obese category in the decade prior. When set apart, this group had a substantially worse health profile than those in the category whose weight had been stable.

Collectively, the findings confirm that studies have been "significantly affected" by BMI-related bias.

According to most studies, the "overweight" category (BMI 25-30) surprisingly has the lowest mortality risk. Those in the "obese" category (30-35) have little or no increased risk over the so-called "healthy" category (18.5-25). And both the "underweight" (less than 18.5) and extremely obese (35 and higher) are at increased risk of death.

However, Masters found those with low BMI (18.5-22.5) have the lowest mortality risk. Contrary to previous research, the study also found no significant mortality risk increases for the "underweight" category.

Masters said he hopes the research will alert scientists to be "extremely cautious" when making conclusions based on BMI.