There are five key areas to consider when you are trying to get fit and they aren't all focused on losing weight.
There is not a single definition for physical fitness.
Aerobic exercise seems to be the most important for improving health, followed by strength training and flexibility work.
New guidelines recommend two and a half hours a week of moderate to vigorous aerobic exercise.
With the onset of another year approaching, many people plan to renew their annual resolve to get in better shape. From there, the details get murky.
For some people, efforts to get fit involve eating fewer sweets. Others focus on adding more weight to the bench press, running further on the treadmill or getting to yoga class more often.
Science, it turns out, is equally undecided about what it actually means to be physically fit, with studies pointing to a variety of measures that can affect health outcomes in many ways.
The good news, according to a growing body of research, is that you don't need to get skinny or run an ultra to improve your health. Even if your BMI is on the high side, a simple walking program can drastically reduce your risk of heart disease and other ills. And people who exercise the least have the most to gain by simply getting off the couch.
"There is a huge return on a small investment when it comes to exercise and health," said Tim Church, director of preventative medicine research at Louisiana State University's Pennington Biomedical Research Center in Baton Rouge.
"When someone goes from running 10Ks to running marathons, they haven't improved their health," he said. "But when you go from being a full-blown couch potato to being a regular walker, you have really reduced the risk of bad things happening to you -- I would say on the order of 35 to 50 percent, maybe more."
Researchers have been narrowing in on these five key areas to consider as you plan your yearly lifestyle makeover:
The word fitness didn't enter the lexicon of scientific research until 1970, when scientists at the Cooper Institute in Dallas began testing people to see how long they could last on treadmills as the grade of the running surface was set to steeper and steeper angles.
When people do well on the treadmill test, they are said to have a high level of cardiorespiratory fitness, which refers to the ability of their heart and lungs to withstand exertion without getting winded.
That term was quickly shortened to just "fitness," Church said. And hundreds of studies in the intervening decades have linked high cardiorespiratory scores with positive consequences, including lower rates of heart disease, stronger bones, lower risk of certain types of cancers, lower blood pressure readings, and a smaller chance of dying early.
In a study of more than 14,000 men, published earlier this month in the journal Circulation, Church and colleagues found that people who maintained or improved their level of fitness over the course of six years were much less likely to die in the next 11 years compared to men who let their fitness levels decline. For every step gained in fitness (measured in units called metabolic equivalents, or METs), the men's risk of dying from cardiovascular disease dropped by 19 percent.
Those benefits were the same regardless of whether men lost or gained weight during the study period, said lead researcher Duck-chul Lee, a physical activity epidemiologist at the University of South Carolina's Arnold School of Public Health in Columbia.
"People worry a lot about weight gain and they try really hard to lose weight," Lee said. "But in terms of mortality risk, changes in fitness are more important than changes in body weight."
Today, elite athletes use the same kind of treadmill test to measure VO2 max, which shows how efficiently their bodies convert oxygen to fuel. But for the average resolution-maker, Church said, it's simply worth doing enough aerobic exercise that you don't get winded when walking up stairs or attempting other everyday tasks.
Specifically, the American College of Sports Medicine's new guidelines recommend that most adults do at least 30 minutes of moderate-intensity aerobic activity at least five days a week, and three 10-minute bouts of movement are just as good as one that lasts three times as long. For 20 minutes, three days a week, the activity should be vigorous. Good cardiorespiratory choices include running, biking, swimming, basketball, dancing and walking.
While decades of fitness research have focused on aerobic exercise, only recently have scientists begun to investigate the benefits of weight lifting and other forms of strength training. These activities appear to be particularly important as we age, because adults naturally begin to lose muscle mass once they reach their 40s.
Studies show that strength training can help combat metabolic conditions like diabetes. Working muscles adds bone mass, and it's good for the heart and joints. Weight training can also preserve a high quality of life into old age, allowing people to do simple things, like getting on and off the toilet or carrying bags of groceries in from the car.
Two or three days a week, the ACSM recommends that adults put major muscle groups through resistance exercises with free weights, machines that use stacked weights or even resistance bands that allow joints to move through a full range of motion.
Stretching alone won't make anyone fitter, and flexibility is a distant third in importance compared to cardiorespiratory and muscular fitness, Church said. But staying or becoming more limber can be an important part of an overall fitness regimen, especially for older people.
Yoga, stretching and related activities that fall into a category called "neuromotor exercise" can enhance balance, agility and coordination. That reduces the risk of falls, which can lead to debilitating injuries.
The ACSM recommends holding stretches for 10 to 30 seconds at the point of tightness or slight discomfort -- at least two days a week and as often as every day.
Before intense exercise, however, research shows that static stretching can lead to injury. Instead, use gradual and dynamic movements to warm up.
As much as people like to set weight-loss goals, there is no such thing as an ideal body weight for anyone, said Arthur Frank, medical director of the George Washington University Weight Management Program in Washington, D.C.
And even though many scientists now argue that fitness neutralizes many of the dangers of being overweight (see above), losing small amounts of weight in gradual stages can be hugely beneficial for people who are obese.
One reason is that intra-abdominal fat tends to come off first and that kind of fat is linked to the worst health consequences. Losing weight also increases mobility, making it easier to exercise. In turn, exercising makes it easier to lose weight.
"You can normalize your weight but still be a couch potato, and you've accomplished something but not enough," Frank said. "You're going to have a much more difficult time maintaining your weight unless you get fit."
Muscles, bones and hearts aren't the only body parts the benefit from exercise. In study after study, even people with severe medical conditions report feeling dramatically better after starting an exercise program. For everyone, exercise has the potential to lower levels of depression, stress and anxiety.
"I jokingly say that 90 percent of aerobic exercise is above the shoulders," Church said.
"Whereas smoking is bad for every single cell in the body, exercise is pretty much good for every single cell in the body," he added. "Everything we look at, every little molecule, I'm always surprised by the benefit that exercise has."