SODIUM!!!

Sodium has nothing to do with fat-loss because sodium has zero calories. It's an electrolyte like what's in Gatorade to help an athlete maintain a water balance. If you have too much sodium and don't drink enough water you will hold extra water-weight. Personally I have high amounts of sodium everyday but I also drink lots of water so it really is not an issue for me.
Great question!

Unfortunatle a lot of pre-packaged foods that are supposedly "healthy" are still not that great for your other that being low fat and calories sometimes. They are still made with a lot of fake additives. Eating all natural as much as you can is your best bet.

And here's something I found on why too much sodium is bad:
"If you do not drink enough water throughout the day, your body will go into survival mode. This means that your body's natural tendency is to hold onto water and store it because it is not getting enough from outside sources. When your body does this, it stores the water beneath your skin and on top of your muscles.

This water retention can then obscure your muscle definition and make you look softer than you really are. It can also easily be mistaken for fat. And yes, it does add unecessary weight to your body.

Sodium has a tendency to cause a need for more water being retained within your body. The only way to combat this is to either take in less sodium and/or drink an adequate supply of water throughout each day to rid your body of any sort of water retention.

The most important thing is to drink enough water...you do this and you won't have to worry so much about water retention and you'll keep that unecessary water weight off your body."

Also, here's a good article to read:
http://www.mayoclinic.com/health/sodium/NU00284

sodium makes you retain water, but it is essential for brain function, so it shouldn't be reduced too much- just not had in excess )

Also, disregard any "sudden weight gain" after eating high doses of sodium, or "overnight weight gain" it's all just temporary.

Thanks for saying that! I needed to hear it. I am beating myself up for seeing a 1 -1/2 lb weight increase when i woke up this morning. I knew it was sodium and I was beating myself up! I feel better and am now re-focused!!!

When I started my quest for health, I started making a lot of fresh foods and cooking. After tracking with MFP, I found that the 6 days I didn't eat manufactured foods, I was only getting 1500-1900mg of sodium, but I eat out once per week with people from work and occasionally on weekends, those days I blow the 2500 limit out of the water. I found though that when I was getting low sodium levels, I got more frustrated with work, couldn't think clearly, and would tire more quickly when exercising. I had to start adding a little salt to my foods that I was cooking, either with salt, or a salty sauce of some kind (Worcestershire, Soy, A1, etc).

@JesaGrace - One thing about soups is that if you get the low sodium soups, they replace the usual salt with a potassium based salt, most people never get the level of potassium they're supposed to get, so watch for those soups. It does change the flavor a little, but shouldn't be too much of a change.

If you want to cut down on sodium in your diet, you can do like I did and start hunting and eating the wild game, start a garden, and cut out the frozen dinners. It doesn't take much time to cook some awesome meals for the week and split them up into individual serving containers and freeze them. That way YOU control the portions, YOU control the ingredients, and YOU know what you're eating. MFP makes it easier too because you can enter your recipes and track your meals. I entered my soups, venison burgers, curry dishes, and stir-fry dishes that I enjoy most.

Your mileage may vary, but this worked for me, 310 down to 230 between 12/2006 and 06/2007, back up to 260 in 11/2009 because of two knee surgeries in a year, and working 100 hour weeks between two jobs, and now down to 210 pounds, and counting.