Body mass index (BMI) is a measure of body size that is calculated by dividing a person’s weight by their height. A wealth of research has demonstrated that having a body mass index outside of the normal range (18.5 to 25) increases the risk of death. However, body mass index does not differentiate fat and muscle mass. Authors of a new report investigated the effects of body composition on risk of death.

Extra body fat has been shown to increase the risk of developing a number of chronic lifestyle diseases, while increased muscle (fat-free) mass has been shown to decrease disease risk. Previous research has demonstrated mixed results for the effect of body composition and risk of death, likely due to differences in study design.

The authors combined data from seven studies with over 16,000 participants between the ages of 20 and 93 years collected between 1994 and 2008. Researchers measured body composition using bioelectrical impedance and adjusted for age and sex. They also interviewed participants about their health, lifestyle, and socioeconomic factors and tracked them for an average of 14 years.

After adjusting for a number of demographic and lifestyle factors, the researchers found that having body fat below or above the normal range increased risk of death among the participants. Those with high body fat (37 kilograms) had a 56 percent greater risk of death, while those with highest fat-free mass had a 30 percent lower risk of death.

The authors concluded that fat mass and fat-free mass have opposite effects on the risk of death. They noted that their study included a large number of participants and a long follow-up period, which strengthened the quality of their results.

• Link to full report.