A new U.S. study, recently published in The BMJ, was conducted to investigate the association of predicted fat mass and lean body mass, with all cause and cause specific mortality in men. The study included over 38,000 men (aged 40-75 years) from the Health Professionals Follow-up Study. The main outcome measures were all cause and specific cause mortality.
The researchers used anthropometric prediction equations, developed from the National Health and Nutrition Examination Survey, to calculate both lean body mass and fat mass for each participant. Using an average of 21 years for the follow up, there were 12,536 deaths recorded.
Fat Mass Predictions
Statistical models showed a strong positive correlation between predicted fat mass and all cause mortality. Men in the highest 20% of fat mass were approximately 35% more likely to die than men in the lowest 20% of fat mass.
Using a different model called the “cubic spline model,” the researchers showed that the risk of all cause mortality was relatively flat until men reached 46lbs of fat mass, and then the risk of death increased rapidly.
In terms of “specific cause mortality,” the study shows that men in the top 20% of predicted fat mass had are at a much higher risk for cardiovascular disease, cancer, and respiratory disease.
Lean Mass Predictions
Using the same statistical models, the researchers found that men in the lowest 20% of lean body mass had a higher probability of death than all other groups of approximately 8-10.
The “cubic spline model” showed that, with regards to lean body mass, there is a large reduction in mortality risk once lean mass gets to 123 lbs.
A strong inverse association existed between predicted lean body mass and mortality from respiratory disease, whereas there was a U-shaped curve for cardiovascular disease and cancer.
The study findings strongly support public health recommendations to maintain healthy, normal body weight defined by BMI of 18.5 to 24.9, in order to reduce the probability of chronic diseases and reduce the risk of mortality. More importantly, this is really the first large-scale study that shows that body composition (the distribution of lean mass vs. fat mass) really does matter, when it comes to longevity.
The study findings, with regards to being heavier and fatter, and thus having a higher risk of death, is likely not very surprising. However, the strong associations found with low predicted lean body mass and mortality, for men, are quite meaningful. It seems that low lean body mass, rather than low fat mass, may be driving the increased risk of mortality in the lower BMI range.
BMJ 2018; 362 doi: https://doi.org/10.1136/bmj.k2575 (Published 03 July 2018); Dong Hoon Lee, post-doctoral research fellow1, NaNa Keum, visiting scientist, assistant professor12, Frank B Hu, professor1 3 4, E John Orav, associate professor4 5, Eric B Rimm, professor1 3 4, Walter C Willett, professor1 3 4, Edward L Giovannucci, professor1 3 4