According to estimates, we consume around an extra 3000-3500 calories on Christmas day. I would suggest that this number is highly variable and may not apply to everyone, especially some women. This may sound like a huge amount of calories but before you worry, hear me out.
According to calorie estimates, around 3500 calories of stored energy would theoretically create 1 lb / 400g of fat. This means that it takes quite a lot of excess food to create even a small amount of fat tissue. In addition, this fat tissue will be spread out across the body generally, therefore being undetectable to the naked eye. Even if you did manage to add on this unimpressive 400g of fat tissue, simply eating your normal diet for around a week or so will probably see that fat disappear. However, you won’t even notice if it is there or not, even if you did put it on.
What you are more likely to notice however, is a vast amount of water retention that occurs due a sudden increase in food, salt and carbohydrates. A significant level of bloating can also occur from the food solids being processed in your digestive system for a day or two. You may even see your scale weight fly up in response to all this extra food inside of you. By the way, this is highly not recommended as it may ruin your christmas spirit!
This has been scientifically proven. In one study, participants were given an excess (extra food on top of their normal diet) of 1500 calories a day, for 3 days (1). This is totally possible over the Christmas period, right? They found that whilst everyone increased weight by about 1 Kg, it was entirely through increased body water and food solids in the digestive system. There was no increase in fat tissue and their body weight returned to normal after 5 days.