Yes, you absolutely can work out too much. If you're dong weight training, you need to give your muscles at least 24 to 36 hours to recover, or all you'll do is break them down more. If you're doing cardio, you generally don't need to do more than an hour of it.

Remember, it's all about energy balance. You want to have a caloric deficit, but not one so large that your body can't repair itself after exercise, and not one so large that your body has to use lean muscle mass just to exist.

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