With so much conflicting information about fitness in the media, maybe you're wondering "how much cardio should I do?"

The short answer: it depends.

The amount of cardiovascular exercise you'll want to engage in should be predicated on your goals.

  • Are you training for a marathon or other race?
  • Is weight loss your goal?
  • Want to fit both strength training and cardio in your routine?
  • Don't have a lot of free time to squeeze in a 45-minute run?
  • Just want to get in better shape?
For general guidelines, the American College of Sports Medicine recommends 30 minutes or more of moderate-intensity exercise three to five times per week.

What does moderate intensity mean?

If you can't carry a conversation during a jog, swim, bike or other aerobic activity that gets your heart pumping for a sustained period of time, you're working out too hard. This is especially true if you're new to exercise or concerned about flooding your body with the stress hormone, cortisol. 

Moderate intensity is generally viewed as, after a proper warm up (think: brisk walk for five to 10 minutes), elevating your heart rate to about 50 to 65 percent of your maximum heart rate.

There are more scientifically precise ways of determining your maximum heart rate. The best method, especially for those who are around 40 years old or older and overweight is to do a treadmill stress test administered by a medical professional.

One formula that's often used for the general public is to take your age and subtract it from 220 and then multiply that by anywhere from .50 to .65, which will give you a heart rate guideline for moderate intensity.

The Karvonen formula is also cited as more reliable, though you'll have to know what your resting heart rate is to figure out your moderate intensity training range based on this formula. 

I'm training for a marathon. How much cardio should I do?

Before answering that question, first ask yourself why you want to train for a marathon. Is it just to prove that you can achieve a monumental task? Make sure you have a thorough understanding of sports nutrition and don't have any underlying health issues (an irregular heartbeat, for example).

If you're cleared by your physician and have studied sports nutrition extensively, you'll want to do cardio at least 5 days a week for several weeks if not months prior to a race. Each session should last well over an hour.

I lift weights and want to keep muscle. Won't too much cardio burn away my muscle mass?
If you're concerned about cardio exercise wasting away your muscle tissue, two to three moderate intensity cardiovascular sessions per week of 30 minutes should be enough.

Keep in mind that it's possible to sustain your heart rate at an aerobic capacity for 30 minutes or more during weight lifting. Full-body exercises like deadlifts and squats use your whole body and will tax your heart. To keep up your heart rate, consider focusing on muscular endurance by lowering the amount of weight lifted and increasing the amount of repetitions.

If you're concerned about staying as strong as possible, don't lift too light but do jump rope in between lifts to keep your heart rate up.

I don't have time to do 45 minutes of cardio at one time. What should I do?
Split up your routine. Performing two 20-minute sessions of cardio per day (jumping rope, climbing stairs or bleachers) a day has been proven to be just as effective, if not more so, than one continuous cardio session.

Cardio conclusion

Elite athletes and endurance exercisers thrive on doing high-intensity cardio for prolonged periods, provided that they supplement with adequate nutrition and rest. The average person would do well getting their heart rate up to at least a moderate intensity level five to six days per week. Striking a good balance between resistance and cardio exercise will be most beneficial. Pick an exercise program that accomplishes both to save time. Get clearance from your doctor before starting any exercise program.

Related on MNN: How much water should I drink in a day?

Judd Handler is a health writer in Encinitas, Calif.