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BMI For People Over 40 May Not Be Accurate

5 Minute Read
BMI is a measure of body mass divided by height

Key takeaways: 

  • A 2024 study suggests a body mass index of 27 (rather than the traditional cutoff of 30), is more accurate for identifying obesity in people over age 40. 
  • Research suggests that more middle-aged people may have obesity and might be missed in traditional BMI screening with a threshold of 30. 
  • Body fat may be a more accurate indicator of obesity in adults ages 40-80. 

New research from Italy suggests that body mass index (BMI) cutoffs for obesity may need to be lowered for people over age 40. 

Why might this be true? Everyone experiences changes in body composition with age. Starting at about 40 years old, we tend to gain body fat and lose lean mass

However, these changes in body composition may happen without changes in overall body weight. Therefore, an obesity diagnosis using BMI – a simplistic measure of body fat that’s calculated by height and weight – may not be optimal for middle-aged and older adults. 

Because obesity is defined as a disease of excess body fat, it’s important to include changes in body composition when diagnosing the condition. With this in mind, researchers aimed to test the traditional BMI cutoff of 30 as an indicator of obesity in middle-aged adults. 

How is BMI calculated? 

BMI is calculated by dividing a person’s weight (in kilograms) by the square of their height (in meters). 

While BMI can be helpful for assessing health at a population level, it greatly oversimplifies the relationship between a person’s body mass and their health. For instance, BMI does not account for variability in body fat based on race or sex. Some types of body fat are more harmful than others, and BMI does not distinguish between these different types of body fat. For example, athletes often have a higher BMI due to increased muscle mass, rather than increased body fat. 

Current recommendations from the World Health Organization (WHO) categorize obesity based on the following cutoffs for adults over 20 years old: 

  • Underweight: Below 18.5 
  • Normal weight: 18.5 to 24.9
  • Pre-obesity: 25 to 29.9
  • Obesity: 30 and above 

If you’re curious about your BMI (keeping in mind the measurement doesn’t reflect the exact amount of body fat you may be carrying) here’s how to calculate it.

What were the study’s key findings? 

The study included 4,800 adults in Italy who were between 40-80 years of age. Among the participants, 1,087 had a normal weight, 1,826 had overweight, and 1,887 had obesity, according to the WHO BMI standards. 

The researchers measured participants’ total body fat percentage using an X-ray imaging technique that provides an in-depth look at fat tissue, lean mass, and bone density. In addition to measuring body composition, these scans are commonly used to diagnose osteoporosis and other bone-related conditions.

After categorizing participants by body fat percentage, the researchers found that the optimal BMI cutoff to classify obesity in middle-aged adults was 27 rather than 30.

Using a BMI cutoff of 27 decreased the chance of false negatives (people with obesity being classified as normal weight). This cutoff also decreased the chance of false positives (people with normal weight being miscategorized as having obesity). 

This study was limited to people in Italy, so the results may not be applicable to other populations across the globe. Additional studies, especially those conducted over longer periods of time, may provide further insight into BMI cutoffs in middle-aged adults. 

Lower BMI cutoff may help people access treatment

With a lower BMI cutoff for middle-aged and older adults, more people would likely be identified as having obesity. This would allow people to receive treatment and start working towards a healthy weight earlier, thereby helping prevent complications of obesity like type 2 diabetes, some cancers, heart disease, and liver disease

Because obesity is a chronic disease that increases the risk of many other health conditions, it’s important to proactively diagnose and treat it. While BMI provides a general picture of health at a population level, we may need more specific criteria for different demographics, such as middle-aged adults. 

In the past decade, healthcare providers have recognized that lower BMI cutoffs may be needed for specific groups of people. In 2015, the American Diabetes Association (ADA) lowered the BMI cutoff for screening for type 2 diabetes to 23 instead of 25 for Asian and Asian American adults. This decision was based on research showing that Asian and Asian American people tend to store fat differently – primarily around the waist, which is associated with type 2 diabetes. 

The bottom line

As the field of obesity medicine continues to evolve, our understanding of what constitutes obesity may also change, especially for certain groups like older adults and people of color. Studies like this lay the foundation for developing more accurate ways to assess a person’s body mass and determine their specific health risks. 

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