If price were no object and you were in the market for a vehicle that's going to perform better, for longer, which would you choose?
1. A Mercedes Benz G-Class with a 4.0-liter V8 engine
2. A Honda Civic with a 2.0-liter 4-cylinder engine
Most of us would choose the Mercedes, even though it's a bigger car. Because size doesn't tell you as much about a car’s performance as the engine does. And the same is true for us humans.
It’s entirely possible to have obesity and be “fit,” or to be at a healthy weight and “unfit,” according to Siddhartha S. Angadi, PhD, an exercise physiologist at the University of Virginia, who shared the car metaphor. In other words, you can have a bigger body with a strong engine or a smaller body with a weak engine.
That’s low fitness: Your body has a weak engine. That means, yes, you’ll probably have a hard time running a race or lifting a heavy weight, but “fit” also means your body functions in a healthy way, from normal cellular processes to fighting off illness.
Still, “I have low fitness” isn’t something you hear people say, while you commonly hear things like “I need to lose weight” or “I want to try Wegovy.” And yet low fitness has higher health risks than more famous threats. One notable JAMA study looked at 122,000 people and found low fitness to have a comparable or higher risk of death from any cause than heart disease, smoking, and diabetes.
It’s not often you find something worse for your health than smoking.
This isn’t just a story about raising your fitness levels and extending your life. It’s about understanding why having low fitness is so dangerous in the first place.
Shining a Light on Low Fitness
If you think of a spectrum with frailty on one end, “extreme” fitness, which means having almost zero health problems (rather than suggesting you necessarily exercise a ton), is on the other, says Olga Theou, PhD, professor of physiotherapy and geriatric medicine at Dalhousie University in Halifax, Nova Scotia.
Frailty is considered an older person’s problem, but data suggests frailty and pre-frailty are increasing in society, including among people younger than 65, which could be due to declining physical activity levels, Theou says.
Assess Your Fitness Level
Whether you’re just starting a fitness routine or already have a workout regimen, you should know your fitness level.
It’s hard to say how many people qualify as having low fitness because it’s not as well defined as frailty, which includes a loss of muscle, stamina, endurance, weight, and general fitness.
What we do know: About a quarter of U.S. adults don’t do any physical activity outside of work, according to the CDC, and nearly half don’t meet physical activity guidelines for strength training and aerobic activity. And cardiorespiratory fitness is the best way to assess where you stand for your age and gender, says Angadi.
“We recommend thinking about cardiorespiratory fitness as a vital sign,” he says, pointing to a 2016 scientific statement from the AHA that built this case.
Muscular strength also comes into play when assessing someone’s overall fitness, says Brad Schoenfeld, PhD, professor of exercise science at Lehman College in the Bronx. In general, though, people who have good cardiorespiratory fitness often will have sufficient strength to carry out activities, Shoenfeld acknowledges.
Those criteria are just the basics, however. The real story of low fitness happens on a cellular level.
The Biology of Fitness
So what does it mean to be “fit” in the context of the frailty/fitness spectrum?
“A fit individual has a large heart with greater capacity to pump blood, but they also have muscle that is much better at extracting oxygen and burning fuel, including glucose,” says Schoenfeld. “At the cellular level, that means your cardiomyocytes, [cells] in your heart, have a greater capacity to produce force. From a muscular perspective, the myocytes (muscle cells) are greater, [and] there are more capillaries so they're better supplied by blood, and they have more abundant mitochondria (the “powerhouses” of your cells).”
These things combine, he says, to make a highly fit individual.
“The inverse is what happens to a low fit individual,” Schoenfeld says. “You have a lower metabolic reserve,” which can impact your body in all sorts of not-so-great ways. Take all the positive things in the above paragraph and reverse them. But there’s more: Your body will have a tougher time fighting infections because your body can’t handle the metabolic cost, Schoenfeld says.
High cardiorespiratory fitness, though, means your body will be better able to deal with other conditions, too, including diabetes, organ transplants, and even cancer.
And fitness counts no matter your weight. People who are overweight or have obesity and have higher cardiorespiratory fitness may not be at a higher risk for all-cause or cardiovascular disease mortality compared with people considered “fit,” according to a 2024 BMJ study. But “unfit” people do have a much higher risk.
“Cardiorespiratory fitness goes all the way from the cellular level to the organ level to the organismal level,” Angadi emphasizes. “It doesn't just mean that you're going to be able to run a marathon. You want to have the level of fitness for basic health functions.”
The Unsung Vital Sign
While it’s clear how critical fitness is when painting a picture of overall health, you’re more likely to have a doctor flag that you have obesity — likely based on your body mass index (BMI) — than to tell you need to improve your VO2 max.
VO2 max (how well your body uses oxygen) is the gold standard for cardio fitness measurement. Not only does VO2 max correlate with risk of cardiovascular disease, it gives you a “full-body snapshot” of health and fitness, per Angadi.
Some fitness trackers assess VO2 max, but Angadi warns that no over-the-counter tracker or online calculator is as accurate as a lab-based test. And that’s the problem with VO2 max as an everyday metric.
“If you want to measure cardiorespiratory fitness, you need to have a treadmill or a bike and a metabolic cart in every doctor's office,” says Angadi. “When we look at patients that are at a high cardiovascular risk, then we start doing these tests all the time, but to operationalize it on a population scale could be tougher.”
A preoccupation with BMI could also be casting a shadow on cardiovascular fitness: “We're obsessed with BMI,” Theou says, although there’s been a push to shift away from this metric in the scientific community.
Fitness Dos and Don'ts
The problem: BMI doesn’t look at body composition (muscle versus fat) or tell you much about your body’s ability to function. That means people with a lot of muscle and less fat can end up in the obesity category.
“American football linebackers would be considered obese [by this measure], and I would say they have quite good levels of fitness to do what they do,” Schoenfeld jokes.
Being “thin” could also be a health mirage.
“Low BMI potentially could mean low muscle mass, too,” Theou says, so people who don’t have enough muscle mass and are at a high risk of becoming frail can fall in the “healthy weight” category. With the prevalence of obesity medications and people rapidly losing muscle mass in addition to fat, this distinction could become even more critical.
That said, body composition is one factor. “At the end of the day, you need to get a better idea as to what's really going on under the hood,” says Angadi, which is what cardiovascular fitness tells you.
Schoenfeld agrees: “Obesity is a risk factor for low fitness, but there are certainly people who are obese and they would not have what I would [call] low fitness.”
And that distinction is important when it comes to assessing your overall health: Higher cardiorespiratory fitness reduces the risk for cardiovascular disease and all-cause mortality associated with overweight and obesity, per a 2024 BMJ study coauthored by Angadi.
How “Fit” Are You?
To assess your overall health, you want to look at body composition, cardiovascular fitness, and muscular strength. Here’s how to get a quick snapshot:
Cardiovascular fitness: A proper VO2 max test in a lab or fitness center is the gold standard. For quick-and-dirty, Angadi suggests walking 1 mile as fast as you can, then entering your data in an online calculator (Google “VO2 calculator” and you’ll have many options).
Muscular strength: Measure your grip strength over time. “There is quite a bit of evidence that hand grip strength is positively correlated with greater longevity,” says Schoenfeld. You’ll need an instrument called a dynamometer, which cost less than $20 online. Find all the how-to info here.
Body composition: The simplest way to get a sense of your body composition is to measure your waist circumference, Angadi says, since having a majority of fat around your waist (rather than your hips) is riskiest. All you need is a tape measure. Exhale, then wrap it just above your hip bones, per the National Institutes of Health. Less than 35 inches is best for women, less than 40 for men.
What Should Your Fitness Goal Be?
The short answer: Improvement.
Just get better from wherever you’re starting.
And it’s never too late to start, says Schoenfeld, who points to classic research showing nursing home patients in their 90s increased their strength by 150% and their functional capacity (including being able to walk without a cane) by 50% in eight weeks. (If 90somethings can do it …)
“Most people unfortunately just think of exercise as a means of losing calories, and that's a terrible way of thinking about it,” says Angadi. “Exercise is just damn good medicine for your heart, lungs, muscles — name an organ system, and it's affected favorably by exercise — no matter what size you are.”
“I think resistance training is the single most important activity that people can do for their overall health.”Brad Schoenfeld, PhD
Any exercise will do, but if you’re serious about building to a good fitness level, Schoenfeld and Angadi emphasize at least two days per week of “muscle-strengthening activity.”
“People might want to debate this, but I think resistance training is the single most important activity that people can do for their overall health,” Schoenfeld says. “I'm not dismissing that cardio is very important, it is, but I think people tend to neglect resistance training, and that is a serious mistake. You can't outrun frailty.”
Angadi agrees that strength training is critical, and perhaps even more so for women who tend to have lower bone and muscle mass than men. Weightlifting is great, but Schoenfeld says bodyweight exercises like squats and lunges can work, too. “Your body weight is a weight,” he says.
If that sounds intimidating, Theou adds that something is better than nothing, especially in the beginning.
Outside of exercise, you’ll want to consider the other tenets of good health including nutrition, sleep, and social health. Theou points to the acronym AVOID for frailty which stands Activity, Vaccinate, Optimize medication, Interact, and Diet and Nutrition. In particular, consuming enough food and particularly protein is important, especially as people get older, per Schoenfeld.
But really, these are the tenets of good health that we’d all be wise to follow, whether we’re driving Civics or that sweet Mercedes.