Most doctors use BMI to assess health, which is overlooking a major vital sign – muscle mass.

For decades, our society has been using a person’s body mass index, commonly known as BMI, to determine a person’s health. Yet for many, this simple math equation – dividing a person’s weight by the square of their height – offers an incomplete picture of health because it doesn’t take important health factors like muscle mass into account. In fact, nearly 1 in 5 people have a misleading BMI, according to the CDC’s National Health and Nutrition Examination Survey.

BMI fails to account for differences in muscle mass

BMI first became popular in the 1970s as a way to assess the prevalence of obesity across large, diverse populations for tracking long term trends. It has since then become the go-to way to assess people’s health. Yet, BMI ignores major individual health factors like age, chronic conditions, and physical activity level.

Additionally, BMI is just based on body weight and does not differentiate between fat mass and muscle mass (also known as body composition). Having more muscle mass is associated with better health and greater longevity. Because BMI doesn’t account for someone’s muscle mass, it overlooks this important health indicator entirely.

Since muscle weighs more than fat, it’s common for people to get wrongly characterized as either healthy or unhealthy based on their BMI. For example, athletes whose muscular physique misleads the scale could end up being erroneously classified as overweight. Conversely, others who have a normal BMI with very little muscle could appear in the “healthy” range, but actually have low muscle mass putting their long-term health at risk.

In both cases, body composition is being ignored by BMI. For the athlete, it’s a simple flaw of the measurement. For others, research has found that ignoring a person’s body composition, especially their muscle mass, can lead to confusion about a person’s health status, and can be a matter of life or death in some cases.

Why muscle mass matters

Muscle mass represents the weight of skeletal muscle and can make up to 40 percent of your body weight. Adding muscle mass into a person’s health assessment can give us a better idea of a person’s health status.

Research confirms as much. One study of nearly 900 older adults published in the Journal of the American Geriatrics Society found no link between BMI and mortality. On the other hand, when these researchers looked at the link between muscle mass and mortality, they found that those with the highest level of lean mass — a term often used interchangeably to describe muscle mass — were far less likely to die than those with the lowest.

A new review published in Annals of Medicine reached similar conclusions. It examined 140 recent studies in inpatient, outpatient, and long-term care settings. Patients ranged from trauma patients to individuals with chronic obstructive pulmonary disease. The researchers  found one common theme:

muscle mass plays a huge role in health outcomes.

The review showed that people with less muscle mass had more surgical and post-operative complications, longer hospital stays, lower physical function, poorer quality of life and overall lower survival.

Muscle mass predicts health outcomes in cancer patients

Muscle mass has also proven to be a strong predictor of health outcomes in cancer patients. Studies have shown that cancer patients who lose muscle mass struggle to perform daily activities and don’t respond as well to chemotherapy.

A 2018 study published in JAMA Oncology reported that breast cancer patients with sarcopenia, the medical term for advanced loss of muscle mass and function, had a 41 percent greater risk of death than those without the condition. Muscle loss is literally a killer. So this research should compel doctors to make measuring muscle mass a priority when formulating their treatment plans for cancer patients.

Tools to measure muscle mass

The good news is that technology is enabling us to better and more easily understand our body composition. It’s now time for more of the medical community to adopt it for patient care.

Measuring muscle mass is currently more difficult than calculating BMI. But it’s becoming easier. Tools already exist and are being used in clinical practice for other applications.

Consider one type of imaging technology, generally known as the DEXA scan, that’s already commonly used to measure bone mass. This technology also has the ability to break down one’s body composition into fat and lean mass in a matter of minutes. In addition, both CT scans and MRIs can provide information on body composition, which may be helpful for people with cancer, cardiac disease and other conditions that require them to routinely undergo these types of imaging studies.

More portable tools such as handheld ultrasounds and bioelectrical impedance (BIA) devices may also be useful for measuring body composition and are currently under various stages of development or validation.

The hand grip test

Assessing a patients’ muscle health could even be as simple as shaking their hand. A recent study published in BMJ found that a stronger hand grip strength was a better predictor than measurements like blood pressure or BMI in assessing a person’s risk of developing heart disease and overall mortality.

There are other, simple ways to get a sense of someone’s muscle health. Low energy, slower walking speed, decreased strength, unintentional weight loss, fatigue, and general weakness are all signs of muscle loss.

There are a number of questionnaires that can be used that are specifically designed to help evaluate the physical function of aging adults. Health professionals can also perform simple physical function tests like a walking test or chair stand tests to get a sense of patients’ muscle health and mobility.

Adding muscle mass as a vital sign

Keeping a pulse on muscle health is an easy way for healthcare professionals to help improve their patients’ health. The nice thing about muscle is it’s a forgiving organ – adults can easily replenish lost muscle through a proper diet with adequate protein and physical activity.

It’s time that the way we measure health caught up to the advancements that are being made in technology and research. BMI can only tell people so much about their health and is missing the full picture. It is time to consider adding an assessment of muscle mass as a vital sign.

The more we know about our body composition, the more we can take simple, educated steps to maintain a healthy and active life at any age. Understanding muscle mass is one way to begin closing this gap. New technology and innovations are making it easier to measure and the information is more accessible than ever.