The maximum amount of cardio you can do without losing muscle mass depends on two main factors:
1. The type of cardio you do:
You can do a lot more low intensity cardio work such as walking without losing muscle as compared to higher intensity training such as distance running. Interval training, if done for no more than 15 minutes, can actually support muscle mass while burning fat effectively.
2. Your state of nutrition:
If you are eating a lot of carbs, your body will preferentially burn those for energy rather than eating up your muscle mass. The downside to this is that you also burn less fat. Thus, if you are dieting to lose fat, you stand a greater chance of your body eating up muscle mass to provide energy.
As it is, most people do cardio to lose fat, in which case strive to keep your protein intake high and make sure to do weight training to help preserve your muscle.
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