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Most of us are all too familiar with how much we weigh — but how many of us know how fit we are? And which matters more?

When it comes to lowering our overall risk of death and dying from heart disease, fitness may be just as important, if not more so, than weight. That’s what researchers concluded after studying fitness, weight and mortality among 14,345 middle-aged men in an 11-year study.

Most studies that have previously linked weight gain, overweight and obesity to higher mortality risk have focused only on BMI, or body mass index, a ratio of height and weight. That’s because weight can indirectly affect a number of different metabolic processes that contribute to mortality, such as how we burn calories or process sugars, and how high our blood pressure is. But weight may also be masking the effect of another factor that could protect or propel us to an early death: how efficiently our hearts and lungs are working, or, in other words, how fit we are.

Duck-Chul Lee, a postdoctoral fellow in the department of exercise science at the University of South Carolina School of Public Health, and his colleagues decided to focus specifically on the role of fitness in overall mortality rates as well as deaths due to heart disease. So they recruited a group of men aged 20 to 100 and had them run on a treadmill to measure their heart and lung function. The researchers compared the participants’ maximum fitness levels — how long the participants could run at increasingly steeper inclines — taken at two points during the 11.5-year study, to death rates among the group, and factored in changes in the participants’ weight as well.

The men who maintained their fitness levels between the two measurements lowered their risk of dying from heart-related or any other causes by up to 30%, compared with those who lost fitness. Those who actually improved their fitness lowered their risk even more, by up to 44%. In fact, for every unit improvement in fitness, measured as metabolic equivalents (METs), there was a 15% decrease in death from any cause, and a 19% decrease in dying from heart-related events.

All of these changes occurred regardless of how much weight the men gained or lost. When it came to BMI, fluctuations during the study period weren’t linked to any changes in all-cause mortality, though men whose BMI went up had an increased risk of dying from a heart event compared with those whose BMIs went down.

“Regardless of weight change — some lost weight, and some gained, while some remained stable — loss of fitness was associated with a higher risk of mortality,” says Lee.

It’s a confusing concept — after all, isn’t weight a reflection of how fit we are? Well, yes and no. To a certain extent, yes, the more weight we gain, the less fit we tend to be. In fact, when Lee and his team looked at who in the study lost fitness, they were the sedentary men who started smoking and developed conditions such as diabetes and hypertension. And those who were the least physically active also lost the most fitness.

But that assumes that a high BMI is primarily due to extra fat tissue, which in many cases it is. But muscle also contributes to a person’s weight, and people who are more active may also develop more muscle tone, which may add to their weight — and their BMI — without necessarily harming their health. That’s why the researches wanted to tease apart fitness from weight to pinpoint how each contributes to mortality. “When you change your body weight, you have to consider whether you become more fit or not,” says Lee. “If you gain weight, but become more fit, then that might be okay regarding your mortality risk. We have to start considering other factors when we talk about weight change and health outcomes.”

Lee stresses, however, that the results don’t completely absolve weight as a potentially health-harming factor. He notes that the study included white upper-middle class men who were close to normal weight or only slightly overweight. Previous studies have shown that among the obese, weight loss can have a much more dramatic effect in lowering risk of dying from heart events of other causes.

The findings do suggest, however, that lowering your risk of early death may be more complicated than simply watching the scale. If you’re trying to stay healthy and to lower your risk of dying from a chronic condition, you might not need to shed pounds, but you’ll still have to exercise — the best way to stay fit, says Lee, is to be physically active.

Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.