 [ Symptoms ] i had gallstones fatty liver now blood in stools and blood in mouth [ Disease ] if you are training for an endurance event like a marathon when you might run 60 miles or more per week you will almost always see a decrease in your muscle mass for most of us who do more moderate amounts of physical activity there will be minimal if any loss in muscle mass from aerobic exercise    so there is nothing to worry about if you do plan on lots of aerobic exercise and are concerned about losing muscle try starting with 20 30 minutes of moderate aerobic exercise at 50 to 70 of your maximum heart rate two to three days per week and see how it goes