I had an "off" week in terms of my diet. The fair was in town, stress, you get the picture. I wasn't off the wagon every single day but several of my meals were off the charts. I watched the diet the rest of the time, but a few times I was not at my best. I didn't lose ANY weight this time, but I didn't gain any either. When I measured myself, though, I had lost SEVERAL inches off different parts of my body. I have been doing heavy aerobics (45 minutes 5 times per week that include weights). Is it possible that I really DID lose weight, but that muscle weighs more than fat? WHat is going on? 

 I think you're really lucky that your weight stayed the same after the week you described.  It's more likely that some of your fluid "shifted" because of the exercise -- especially if it's been more than a week since you started the classes.  It takes a long time of very, very strenuous exercise to put on even a pound or two of muscle.

I hope this helps.  Please let me know if you need anything else.

Pat Baird, MA, RD, registered dietitian