Being in shape has a greater impact on heart health and longevity than the number on the scale, according to a recent study. 

People considered fit, regardless of their body mass index, had a similar risk of death as fit people with a normal BMI, researchers reported in the British Journal of Sports Medicine in November. In addition, overweight or obese individuals who were fit had a lower risk of death than unfit people with a normal weight.

“The risk of being unfit far exceeds the risks of carrying some extra pounds,” Siddhartha Angadi, PhD, senior author of the study and a cardiovascular exercise physiologist at the University of Virginia, told Health in an email. “Too many folks simply view exercise as a means to expending calories whereas exercise is a powerful tool for improving cardiorespiratory, muscle, and metabolic health, and often these benefits occur even in the absence of weight change.” 

The study’s results bolster claims by many researchers and medical providers that BMI—a calculation that compares someone’s weight to their height—is a flawed way to assess obesity and, by extension, health risk. In a report published Tuesday in the Lancet Diabetes & Endocrinology, an international commission argued that obesity should be assessed not by BMI alone but by taking into account various factors, such as a person’s medical conditions.

Angadi and his colleagues conducted a detailed review and combined results from multiple studies, allowing them to gather data from almost 400,000 people to see how BMI and fitness are linked to the risk of death. 

In most studies, people were considered fit if their VO₂ max was higher than 20% of others in their age group. VO₂ max doesn’t factor in BMI but instead evaluates how well the heart and lungs deliver oxygen to the muscles during exercise and how efficiently the muscles use that oxygen, Angadi said. 

To determine participant VO₂ max, researchers use an exercise test—something you can do at home. (Some fitness trackers also measure VO₂ max via heart rate data.)

Fit individuals, regardless of whether they were overweight or obese, had a similar risk of death as fit individuals with normal weight. On the other hand, unfit people, those with lower VO₂ max, had a two to three times higher risk of dying from any cause of heart disease, regardless of their weight. 

The findings show that being in shape, with a higher VO₂ max, may be more important for lowering the risk of death than just focusing on how slim you are.

“People should recognize that fitness is more than just a number on the scale (which isn’t even a measure of fitness),” Angadi said. “Nearly 100% of weight loss attempts fail after five years, and people may abandon exercise programs simply because they do not see the number on the scale change.” 

The study has some limitations, however, including that it didn’t break down the data by age or consider how long someone has been overweight or obese, which is increasingly common in younger patients, Diana Patterson, MD, orthopedic surgeon and sports medicine physician at Stony Brook Medicine, told Health. 

Furthermore, the study mainly included people from North America and Europe. Further research should explore whether this relationship is the same in populations like South Asia, China, and Africa, Angadi added.

Aerobic fitness reflects how efficient your body systems are. When you’re fit, your blood circulates more efficiently, your muscles are stronger, your heart and lungs work effectively, and your brain receives the nutrients it needs, Megan Wroe, MS, RD, wellness manager and Registered Dietitian at the Providence St. Jude Medical Center, told Health. 

Wroe said these factors provide a clearer picture of health than your weight. “To use weight and height as an indicator of health doesn’t make as much sense as how the physical systems are actually working,” she added.

BMI can also be misleading because people with significant muscle mass may have a higher BMI even though they’d still be considered fit, Patterson said. 

However, she added, BMI shouldn’t be completely discounted. Research shows that elevated BMI is a risk factor for specific health conditions that affect overall mortality, which is “still an important correlation without perhaps direct causation.”

Patterson said improving your aerobic fitness can be simpler than you think. It starts with small, manageable steps. She recommends setting easy, achievable goals. 

For example, begin with a 20-minute walk instead of aiming for a marathon. If joint pain is an issue, walk around in a pool instead of going for a run, which is easier on the knees but can still provide a good workout. 

Another way to build aerobic fitness is to stay consistent, Wroe said. “Start moving. Pick literally any movement that is feasible for your body and do it consistently every single day,” whether that’s multiple 10-minute walks or a few squats.

Once you get moving and stay consistent, you can gradually work toward the recommendations in the Physical Activity Guidelines for Americans, Wroe said.

The guidelines suggest that adults aim for at least 150 minutes of moderate-intensity cardiorespiratory exercise weekly or 75 minutes of vigorous-intensity exercise weekly.

“Even going for brisk walks most days of the week can impart health benefits,” Angadi said. “The sweet spot for moderate-to-vigorous intensity exercise is an intensity where you notice your breathing but are not out of breath.”