BMI is something we hear about all the time in healthcare. Your height and weight are measured, your BMI is calculated, and suddenly you are placed into a category like underweight, normal weight, overweight, or obese.
But when we are talking about older adults, those standard BMI categories do not necessarily tell us the whole story.
In this episode of The Geriatric Dietitian Podcast, I am kicking off a five-part series all about BMI in older adults. We are starting with the basics: what BMI is, where it came from, what it actually tells us, and why we need to interpret this number differently as we age.
In This Episode
I cover:
- What BMI is and how it is calculated
- The history of BMI and why it was not originally created as an individual health assessment tool
- Some of the major limitations of BMI
- What research tells us about BMI and mortality in adults over age 65
- Why looking at the whole person matters more than simply placing someone into a BMI category
What Is BMI?
BMI stands for Body Mass Index. It is calculated using height and weight.
Using the standard adult BMI categories:
- A BMI below 18.5 is classified as underweight
- A BMI between 18.5 and 24.9 is considered normal or healthy weight
- A BMI between 25 and 29.9 is considered overweight
- A BMI of 30 or higher is classified as obese
The problem is that these categories do not necessarily mean the same thing for an older adult as they do for a younger adult.
The History and Limitations of BMI
BMI was originally known as Quetelet’s Index and was developed by a Belgian mathematician in the 1800s as a way to look at the distribution of height and weight among groups of people.
It was not originally designed as an individual health assessment tool.
BMI also only tells us two things: height and weight.
It does not tell us how much muscle someone has, how much body fat they have, the quality of their diet, their physical activity, their functional status, or their overall health.
That becomes especially important as we age because body composition changes over time.
Mortality and BMI in Older Adults
Research looking at adults over age 65 has found that the BMI associated with the lowest mortality risk may actually be higher than what we traditionally call the “normal” BMI range.
One large meta-analysis involving more than 200,000 adults over age 65 found the lowest mortality around a BMI of 27 to 27.9.
That is important because a BMI of 27 would traditionally be classified as overweight.
Research has also found that mortality risk rises more quickly at the lower end of BMI in older adults.
This does not mean that every older adult should try to reach a certain BMI. It means we cannot simply use the standard adult BMI chart and assume it applies the exact same way after age 65.
Look Beyond the BMI Number
When I am thinking about an older adult’s weight and nutrition status, I want to know much more than their BMI.
What is their usual body weight? Has their weight been stable? Are they maintaining muscle?
Additionally, are they eating enough and getting good nutrition? Are they strong and functionally independent? Are they experiencing unintended weight loss?
Those questions can often tell us much more than simply putting someone into a BMI category.
For a deeper look at BMI in older adults and the research behind these recommendations, read my article:
BMI in the Elderly: What You Need to Know
https://thegeriatricdietitian.com/bmi-in-the-elderly/
Listen to Episode 19- BMI in Older Adults
The biggest takeaway from this episode is that BMI is just one piece of information. It does not define someone’s health.
If we are going to use BMI in older adults, we need to interpret that number in the context of aging.
This is the first episode in my five-part series on BMI in older adults. In the next episode, we will take a closer look at optimal BMI for older adults and what the research actually shows.
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