Welcome to FitDay. Contrary to popular belief, muscle doesn't weigh more than fat. A pound of muscle and a pound of fat both weigh a pound. Muscle will take up less space per pound though. It is also very difficult to gain muscle, it's much easier to actually lose fat.
With that being said, you will not notice a big change in weight due to muscle loss or gain within a couple days, it just won't happen. If you were taking anabolic steroids and eating 5000cals a day, like a teenage boy, then you MAY notice a change in muscle growth. Most people are lucky if they can actually gain 10 pounds of muscle during a year.
Chances are, if you are seeing a decent change in your weight over a very short period of time, it's water weight. The biggest cause of this for most people is because of sodium intake. Another factor in fast weight changes is if you are weighing at different times. Most of us will probably go to bed weighing a couple pounds heavier than we work up, it's normal. We will also look thinner in the morning. Hell, you may even look a tad taller when you 1st wake up b/c your spine has been getting compressed through out the day.
I will say that if you are losing so much muscle that you notice it on the scale after a couple days then you have some crazy medical thing going on.
Another thing that confuses me is that you said that you work out 5 days a week and when you do, your weight goes up, yet your weight went down 7 pounds in 2 weeks. You are working out more days than not... and your weight over the 2 weeks went DOWN not up.
Last edited by 01gt4.6; 02-04-2011 at 10:13 PM.