A large-scale study carried out in China has found that people over the age of 80 with a body mass index (BMI) higher than the recommended guidelines may have a lower mortality rate.
BMI is a value that’s derived from measuring a person’s mass and height to calculate whether they’re a healthy weight or not. Typically, BMI guidelines suggest that those with a score of more than 25 are overweight, and having a score above 30 places someone in the obese category.
The study surveyed 27,000 people in China from 1998 onwards, with participants having an average age of 93 upon enrolment. They were followed right up until 2018 or until their death. Researchers found that the ideal BMI for people aged 80 and over was 29, a number which would typically put them in the “overweight” category.
Surprisingly, even those with a BMI that placed them in the “obese” category had a lower mortality rate than those with a “healthy” BMI between 20-25.
This higher optimal BMI was largely due to a lower risk of death from other ailments like cancer or respiratory illnesses, as well as a lower risk of death caused by cardiovascular diseases (however, this link was weaker). The study also took into account different factors such as socioeconomic status, education and smoking status.
Xiaoming Shi at the Chinese Center for Disease Control and Prevention in Beijing noted that it was unclear why a higher BMI in the elderly is linked to a lower mortality rate, but suggests it could be due to them having a nutritious diet.
The findings of this analysis suggest that BMI guidelines need to be adjusted for older age groups. However, BMI shouldn’t be relied on as a single measure of health as it doesn’t take into account important details such as age, sex and fat distribution.
Nicholas Fuller, of the University of Sydney commented: “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.”
Although the study evidences the link between BMI and mortality rate in people over 80, it’s important to note that general BMI scores among the Chinese population were lower than in the West. Therefore, the study's findings may not be applicable to other age and ethnic groups and more research is required.