I am posting this question for my dad...He is 46 5'10 200 lb's.  When he lifts weights he will gain weight.  When i say lift weights i mean throw around weights for maybe 15 to 20 mins 3 times a week.  The other days he walks on a treadmill for 40 mins.  If he doesn't do any of this he starts to drop weight..( while eating sweets at night)...??  I know that muscle can add weight, but there is no way his little bit of lifting can put on 2 pounds in a day.  Any ideas on why exercise has the opposite effect on him?