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The skinny on cardio

It’s a common blunder a lot of women do: performing frequent, drawn-out cardio sessions in an attempt to burn more body fat, faster. I mean you keep reading how cardio is good for you and your health, and your sweat-soaked clothes after those workouts have to be a surefire sign that you’re stripping off that unwanted fat. Don’t be so sure of that. Your endless treks on the treadmill could actually be undermining your fat-loss efforts.

Get up to speed

While it’s true that cardio will have you burning through calories, it will also cost you valuable muscle. Long bouts on the treadmill for example cause oxidative stress and spur the release of free radicals, which play a role in tired, sore muscles during and after cardio sessions. That’s because your metabolic rate when you exercise rises 10 to 15 times above what it is at rest, which increases oxygen delivery to your cells, thus increasing free radical production at a rate that exceeds your body’s ability to maximize its own defense. So even though you’re blazing calories, the resulting muscle damage, fatigue and compromised immune function far outweigh the benefit.

Too much cardio also causes your body to churn out cortisol in excess, which brings forth another problem. Since cortisol inhibits the uptake of amino acids into the muscle cells, as well as decreasing calcium absorption, this means there’s no bone growth and no muscle growth when cortisol levels are too high for too long. This could send you spiraling into a state in which lean muscle is broken down, you gain fat, compromise your immune system and reduce your ability to repair tissue damage following a workout. Plus, chronic cortisol exposure often leads to increased appetite and cravings for certain foods — especially sweets — so you’ll constantly feel hungry.

The result: You’ll set off physiological alarms that cause your metabolism to slow, so you’ll end up burning less body fat in the long haul, not more. Many experts argue that you incinerate more pounds using this method, but scientific evidence confirms you’ll burn significantly more body fat by doing shorter runs while bumping up the intensity.

A study from the University of New South Wales in Australia tracked the effects of high-intensity cardio in 45 women over a 15-week period. The women were divided into two groups: One group did 20 minutes of eight-second sprints followed by 12 seconds of rest, while the other did 40 minutes at a steady rate. At the end of the study, researchers found that the women who did the high-intensity-interval-training protocol lost significantly more body fat than their lower-intensity counterparts. More interestingly, researchers noted that the women who performed HIIT burned more abdominal and lower-body fat (in the trunk and legs) while gaining more lower-body muscle. The women who performed steady-state cardio actually gained more lower-body fat.

Unlike the slow-and-steady approach, cardio at a faster, harder pace also can boost metabolism even after you jump off the treadmill, ensuring you’re continuing to burn fat long after the workout is over. If you’re looking to get lean fast without burning through or compromising muscle function, it’s an easy choice. Go hard, not long.

So forget walking on the treadmill at a slow to moderate speed for 45 to 60 minutes daily. Instead opt to do 20 minutes of high-intensity interval training using a 2:1 ratio of work to active rest, every two to three days. (For example, if you do a 20-second interval, do 10 seconds of active rest.)