Is There An "Ideal" Body Mass Index?

Despite its limitations, the Body Mass Index (BMI) is still one of the easiest and must utilized tools for determing an individual's healthy body weight range.  Currently, a BMI ranging from 18.5 - 24.9 is considered normal or healthy.  A person with a BMI below 18.5 is considered to be underweight while a BMI of 25 - 29.9 is considered overweight and a BMI of 30 or more is defined as obese.  Numerous research studies have clearly shown us that a high BMI, particularly an obese BMI, is linked to poor health and increased risk of death from certain conditions.  However, the full relationship between BMI and overall mortality have remained unclear.

A group of investigators from around the world, including the National Institutes of Health, recently published the results of their examination of the link between BMI and death due to any cause.  In order to analyze this relationship, the researchers pooled data from 19 studies that included a total of 1.46 million adults.  During an average follow-up period of 10 years (ranging from 5 to 28 years), slightly more than 160,000 deaths were recorded.  After adjusting for multiple factors (age, exercise, alcohol consumption, etc.) and using a reference BMI of 22.5 - 24.9, the researchers revealed that among healthy, non-smoking women there were specific relationships between BMI and risk of overall death as outlined below:

• BMI of 15 - 18.4...  risk of death increased by 47%
• BMI of 18.5 - 19.9... risk of death increased by 14%
• BMI of 20 - 22.4... no increased risk of death was observed
• BMI of 25 - 29.9... risk of death increased by 13%
• BMI of 30 - 34.9... risk of death increased by 44%
• BMI of 35 - 39.9... risk of death increased by 88%
• BMI of 40 - 49.9... risk of death increased by 251%

The study investigators further reported that these relationships were also similar for the men in the study.  According to a related NIH press release, this study also showed that for all participants, every 5-unit increase in BMI was linked to a 31% increased risk of death due to any cause and that while a BMI of 25 or more was linked to an increase risk of death in all age groups, the risk was greatest for those who were overweight or obese before 50 years of age.

This is a truly fascinating and important study.  While it was probably not unexpected to see an increased risk of death for BMI levels over 30, the dramatic increase observed is astonishing.  The relatively high risk of death linked to being underweight is also an important observation.  Due to a variety of pressures (media, peers, etc.), many individuals strive for a body weight that is an unhealthy, excessively low body weight.  Hopefully, this observation will help these individuals realize the importance of staying within a healthy body weight range.

Interestingly, the risk of death is actually increased within the low part of what is currently considered a healthy BMI.  According to this study, individuals with a BMI between 18.5 - 19.9, while within the normal BMI range, are at a 14% increased risk of dying due to any cause.  While this level of risk is not on par with the risk observed with being obese, it clearly suggests that even within a normal BMI range, there appears to be an 'ideal' BMI range.  Based on these analyses, the risk of death is lowest for those with a BMI ranging from 20 - 24.9, which encompasses the mid- to high-range of the normal BMI range.  This is potentially important information for individuals looking to set weight loss goals or optimize their overall health.

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