Body mass index — or BMI — can be a useful measure of disease risk.
You have a greater risk of heart disease, type 2 diabetes, and some cancers if your BMI is too high.
Conversely, if your BMI is too low, you could be at risk for malnutrition and osteoporosis.
However, there are several limitations to relying solely on BMI for measuring disease risk and overall health status, especially for older adults.
This article explains what BMI is and why current BMI ranges may not be appropriate for assessing disease risk or nutritional status in older adults.
What is BMI?
BMI is a measure that relates body weight to height.
It was introduced in the early 19th century by a Belgian mathematician named Adolphe Quetelet. Today it’s used as a public health tool for recording obesity statistics (1).
Your BMI is a good indicator of whether you’re underweight, overweight, obese, or at a healthy weight for your height.
You can calculate BMI using a metric or imperial formula:
- Metric: BMI = weight (kg) / [height (m)]2
- Imperial: BMI = 703 × weight (lbs) / [height (in)]2
For adults 20 years and older, BMI is interpreted using weight status categories, which are the same for men and women of all body types and ages.
The weight status categories with BMI ranges for adults are highlighted below:
- Underweight: Below 18.5
- Normal or healthy weight: 18.5-24.9
- Overweight: 25-29.9
- Obese: 30 and above
These ranges are based on the relationship among body weight, disease, and death.
People classified as overweight or obese are at an increased risk of (2):
- Heart disease
- Type 2 diabetes
- High blood pressure
- Sleep apnea
- Some cancers including, breast and colon cancers
There are also several health risks linked with being underweight such as malnutrition, osteoporosis, and poor immune function.
The limitations of BMI
While BMI is a useful indicator of disease risk, there are several limitations associated with relying solely on BMI.
The largest limitation of BMI is that it does not distinguish between fat, muscle, or bone mass.
For example, older adults generally have more body fat than younger people of the same BMI, and women generally have more body fat than men of the same BMI (3).
Moreover, people with higher amounts of muscle mass — such as athletes — may be considered overweight or obese by classification but have a low percentage of body fat.
Due to differences in body proportions, BMI may also not correspond to the same degree of body fat in different populations since at the same BMI, blacks have less body fat than whites, and Asians have more body fat than whites (4, 5).
BMI ranges for older adults
Although there are no official recommendations on BMI ranges for older adults, the current cut-offs may not be appropriate.
A review of 32 studies involving nearly 200,000 older adults age 65 or older found those with a BMI less than 23 had the greatest risk of death, whereas those with a BMI of 24-31 had the lowest risk of death (6).
These results held even when researchers adjusted for factors that may affect risk of death such as smoking status, pre-existing disease, and geographical location.
In another review, this time involving more than 370,000 older adults, researchers found a reduced risk of death for those with a BMI between 23 and 29 and an increase risk of death when BMI fell below 22 (7).
BMI values above 30 in older adults, however, have been linked with an increased risk of disease and a lower quality of life (8).
Intentional weight loss in older adults
Because BMI alone is an imperfect tool for measuring health risk, encouraging weight loss in older adults who are classified as overweight or obese should be cautioned.
For older adults with a BMI of 30 or greater and who have health problems, weight loss can be beneficial, but special attention must be given to diet and exercise to negate the harmful effects of sarcopenia.
Sarcopenia is the progressive loss of muscle mass and strength that is linked with physical disability, poor quality of life, and death. It affects 5-13% of adults aged 60-70 years, and 11-50% of adults older than 80 years (12).
But for those older than 80 years with a BMI of 30 or greater, it’s generally recommended to maintain weight rather than lose weight given the lack of information about the risks and benefits (14).
The bottom line
BMI is a screening tool widely used to determine whether a person is at a healthy weight for their height.
Research has increasingly shown that current BMI weight status categories are not appropriate for use in older adults. A BMI goal of 25-29.9 is likely a better “normal” or “healthy weight” for older adults.
While weight loss may be beneficial in some older adults, it should only be considered on an individual basis keeping their preferences, goals, and overall health status in mind.