Every time I go to the gym, I see the same crowd of overweight people grinding away in their spin classes and treadmill, StairMaster, and elliptical sessions.

Every day they’re there, sweating on the same machines–probably reserved and named by now–and they’re just as fat as they ever were. Some are even fatter than when they started.

Now, cardio. For weight loss, the National Institutes of Health recommends at least 30 to 45 minutes of moderate-intensity exercise three to five days a week. But you can maximize your sweat sessions for efficiency if you alternate between high- and low-intensity workouts each day, says Forsythe. She suggests aiming to work out five days a week, alternating the intensities each day and working out no more than two consecutive days to allow your body time to recover.

For the record, high-intensity workouts are usually ones where you’re at 70 to 90 percent of your max heart rate, according to the American Heart Association. (That means it feels like you can barely say more than a few words at a time.) Moderate-intensity exercise is at about 50 to 70 percent of your max heart rate, and allows you to say a couple of sentences while you’re working.

Does Cardio Get in the Way of Building Muscle?

Many bodybuilders and fitness folk shun cardio, first because they just dislike it, and second because they believe it has an almost mystical power to shrivel up muscle and sap strength.

While we now know that excessive cardio is unhealthy, it’s pretty obvious that it’s also not conducive to muscle growth (just look at any marathon runner). But what about moderate or light cardio? Do they also interfere with muscle growth?

Should You Do Cardio or Weights First?

We’ve covered a lot of ground already but I wanted to address this final point before signing off, and I’ll keep it short and simple.

Lift weights first and then do your cardio.

Heavy weightlifting requires a lot of energy, both muscular and systemic, and if you do cardio first–especially high-intensity cardio–your lifts will suffer.

The Bottom Line on How Much Cardio You Should Do

Medicine has known the value of regular exercise for thousands of years but only recently have we gained a better understanding of how much is enough and how much is too much.

If you do at least a few hours of resistance training per week (and you should–it confers certain benefits you can’t get from cardio), you should view cardio as supportive, not essential, and you should do enough to reach your goals but not more.