A version of this article appears in print on 12/06/2016, on page D 4 of the NewYork edition with the headline: Ask Well. Beyond The First Year. Muscle loss can occur from injury or periods of inactivity. According to Harvard Health Publishing, after age 30 you begin to lose as much as three to five per cent of muscle mass per decade. range. The insitute says that … This can occur by age 80, not 55. Young men have about two pounds more muscle mass in each leg than older men do. However, if you lost 10lbs of muscle, this may take a a month or so to recover, assuming you only eat in a calorie surplus of 500. If you were to eat more, you’ll gain it back quicker. So long story short, a lot happened in my life and I took a 4 month break from the gym where it resulted in a 15 pound weight decrease and a hit on my body composition. But, after two weeks of not moving at all the young men involved in the study lost 17 ounces of muscle… I had to take a year off for various reasons. If you’re still working out after your first year, you should be aware of the fact that muscle building decreases as you get bigger. Maybe a few days to a week. This can occur by age 80, not 55. An earlier version of this article misstated the age at which people can lose 30 percent to 40 percent of their muscle fibers. deep into the 200 lb. With patience, you can regain muscle mass through exercise, adequate hydration and diet changes. It’s partly genetics too. This kind of weight gain in one year can take a beginner’s bench press of 135 lb. How long does it take to regain lost muscle? “I’ve seen 70 years olds with incredible muscle mass because they take care of themselves. If you lost 2lbs of muscle, it won’t take long to overeat and gain this back. Additionally, your mitochondrial content in your muscle mass can decrease by 50% over the course of a week.