Yes, loss of muscle mass typically begins in one’s 30 or 40’s and continues, even for women who take hormone replacement. As muscles shrink, fat takes their place, and that process leads to a slowdown in metabolism and weight gain even if caloric intake and expenditure remain the same. Weight training can reverse this process, putting back muscle (which uses more calories and takes up less room than fat) and diminishing fat stores. So even if you lose no pounds by strengthening your muscles you can lose inches and sizes.