A body mass index over 25 is normally considered unhealthy, but a study of more than 27,000 people in China suggests that may not be the case for older age groups
25 April 2022
People over 80 whose body mass index is higher than currently recommended have a lower mortality rate, a large-scale analysis in China has found. The findings suggest that weight guidelines should be changed for this age group.
BMI scores are used to estimate whether or not someone has a healthy weight. It is based on a person’s height and weight, and most guidelines suggest that someone with a score above 25 is overweight, while those with scores above 30 are considered obese.
These guidelines are largely based on measurements taken from younger age groups, says Xiaoming Shi at the Chinese Center for Disease Control and Prevention in Beijing. As the world’s population ages, it is important to ensure that these recommendations make sense for older age groups too, he says.
Shi and his colleagues studied mortality risk in more than 27,000 people over the age of 80 across China from 1998 onwards. The participants had an average age of 93 when they enrolled in the study and they were followed up until 2018 or their deaths.
Previous analyses have found a link between higher BMI scores in older age groups and a lower mortality rate, but this is the first study to look at this question with such a large sample size.
In this analysis, the researchers found that the optimal BMI for the over-80s was around 29. This was largely driven by a lower risk of death from non-cardiovascular causes such as cancer or respiratory disease. This group also had a lower risk of death from cardiovascular diseases, but the relationship was weaker.
Even those with a BMI in the “obese” range, between 30 and 35, had a lower mortality rate than those in the 20 to 25 range.
The team accounted for several factors, including socioeconomic status, education background and whether a person smoked.
Shi says it is unclear why a higher BMI is linked with a lower mortality rate, but speculates that it may be due to these people having a more nutritious diet. He also notes that, in general, the BMI scores of this population were lower than those found in the West. Over 40 per cent of the over-60s in the US are obese. “Our findings may not be generalisable to other age groups and ethnic groups,” he says.
“This study highlights the importance of taking age into account when considering the relation between BMI and mortality or other health risks,” says Louise Baur at the University of Sydney. She says this study can’t tell us exactly why being overweight may be linked to better health outcomes in the over-80s, but agrees that it may be due to better nutrition.
“While BMI is an accessible and affordable way to screen a person’s health, it shouldn’t be relied on as a single measure of health,” says Nicholas Fuller, also at the University of Sydney. “BMI is based on body weight, but a person’s disease risk is linked to body fat, not weight. It is more important to focus on measures that tell us more about fat in the body and where it’s distributed, such as waist circumference, to get a better understanding of health and risk.”
Journal reference: Nature Aging, DOI: 10.1038/s43587-022-00201-3
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