Yes and no. Has what you have eaten resulted in two pounds of fat? No absolutely not. When you go on a weight loss diet/plan. Your body adjusts to continuously eating a calorific deficit. It does this initially by eating up its stores of glycogen, the notoriously high losses you get in your first week or two, the 'water weight'. Dropping off plan sees your body replenish these sources of energy pretty quickly.
They will drop out again when you kick yourself back into plan.
There are also a few other reasons why eating Junk food can mess up your weight. That's the salt content. Junk foods are very high in salt as a rule, following the SW plan your eating will be salt free except for what you add yourself. The food won't be saturated in the stuff. Increased salt causes you to retain large amounts of water. If you flush your body with even more water the salts are quickly cleaned out.
the other one is just that having high fat foods can cause your digestive system to stall. I've seen my intestines just come to a complete stop for a week following some take out food.
.. Never being naughty again haha
