It is a bit more complicated in that yes you can mess with your metabolism. If you are insulin resistant your metabolism will be slower than someone who is not. Resolving it can increase your metabolism. And if you tend to be insulin resistant a bingy meal may slow you for a couple days.
There can be variations in your body from day to day with hormonal changes as well. Being sick can sometimes raise your basal metabolic rate - however most of that is probably taken back with the fact that you probably wont exercise as hard on a day you have a high fever.
So if you are looking at some calorie counter (like fitday) and over the course of 3 months the calorie counter says you have a cumulative deficit of 35000 calories so you "should" have lost 10 lbs and you only lost 6 you might consider that your metabolism is in general slower than the calculators predict.
But absolutely if you went off plan by 300 calories and you get on the scale the next day and you are up 2 lbs it can not be fat. Absolutely can not.
Yesterday I weighed 155.4, today I weighed 152.2 If I did exactly the same workout today and ate exactly the same foods I would not lose another 3 lbs today. They were a bit of water and undigested Xmas cookies from sunday.
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Before dieting, a slip of a few pieces of pizza would result in a tiny, percentage of a pound gain. Now, I can gain 2lbs from one night of splurging. This leaves me feeling a tiny bit panicked!
Before dieting you probably had a fairly steady stream of carbohydrates and sodium in your diet. You probably also had an amount of fat and low fiber in your diet that kept your intestines sluggish. You were always retaining excess water and excess waste material so small binges weren't noticeable to the body . When you are dieting you have entered a different balance than before. The body is retaining less water and waste and so the whammy of sodium and carbs and slow to digest greasy cheese has a greater TEMPORARY effect on weight.