Weight has three primary short to medium term factors:

1) Water weight -- which is primarily due to the body attempting to  
buffer itself from what it perceives as foreign substances; and

2) Muscle -- which can and will be broken down into glucose (and  
several nasty byproducts) via gluconeogenesis; and

3) Fat.

Fat is stored when insulin levels are high (read, you're eating  
carbohydrates) and is released to use as energy when insulin levels  
are normal.  Fat loss is directly correlated with carbohydrate intake.  
  Period.  Relatively high carb, low fat diets only work to the degree  
to which they reduce carbs from previously higher levels.

Jim